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Crop Physiology-Applications in Agronomy

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Page 1: Crop Physiology-Applications in Agronomy

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MANUAL

ON

MEENA SEWHAGAssistant Professor

PARVEEN KUMARAssistant Professor

SURESH KUMARAssistant Professor

A.S. DHINDWALSr. Agronomist & Head

DEPARTMENT OF AGRONOMYCCS HARYANA AGRICULTURAL UNIVERSITY

HISAR-125004

iii

MANUAL

ON

MEENA SEWHAGAssistant Professor

PARVEEN KUMARAssistant Professor

SURESH KUMARAssistant Professor

A.S. DHINDWALSr. Agronomist & Head

DEPARTMENT OF AGRONOMYCCS HARYANA AGRICULTURAL UNIVERSITY

HISAR-125004

iii

MANUAL

ON

MEENA SEWHAGAssistant Professor

PARVEEN KUMARAssistant Professor

SURESH KUMARAssistant Professor

A.S. DHINDWALSr. Agronomist & Head

DEPARTMENT OF AGRONOMYCCS HARYANA AGRICULTURAL UNIVERSITY

HISAR-125004

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FOREWORD

The field of crop physiology includes the study of all the internal activities of cropplants, which are studied at many levels in the scale of size and time. At the smallestscale are molecular interactions of photosynthesis and internal diffusion of water,minerals, and nutrients. At the largest scale are the processes of plant development,seasonality, dormancy, and reproductive control. Crop physiology is the investigationof the plant processes driving growth, development, and economic production bycrop plants. Hence, these processes are the foundation for understanding theconcepts of crop production for an Agronomist.

The manual contains very basic and practically useful information on processes suchas photosynthesis, respiration, transpiration, evapo-transpiration, growth anddevelopment responsible for crop production with the techniques to quantify andoptimization of these processes for final economic yield production. Picturesillustrating the processes and techniques are very educative. It seems to be ofimmense help to the under-graduate students of agriculture in understanding thebasics of crop physiology and post-graduate students in carrying out their researchstudies.

I congratulate the authors for bringing out this manual in a simple and easilyunderstandable language. The efforts made by them for compilation of informationon the aspects of ‘Crop Physiology-Applications in Agronomy’ are commendable.The financial assistance extended by Indian Council of Agricultural Research, NewDelhi for printing this manuscript is thankfully acknowledged.

(Dr. O.P. Toky)Dean,Post-Graduate Studies,CCSHAU, Hisar.

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PREFACEThere has been many fold increase in the food grain production due to the greenrevolution but the yield level in most of the food grains has reached the platue.Further increase in production without increasing the cultivated area would bedifficult without understanding the whole crop system at plant community level. Thiswould also require a thorough understanding of basic principles of crop productionwhich, involves the study of the plant processes responsible for the growth,development and production of economic yield by crop plants, focusing on wholeplants and plant communities and not individual plant parts, organs, or cells becausemost of the processes that control yield operate at the whole plant-plant communitylevel.

From the Agronomic point of view, it is necessary to understand how the chemicaland physical processes interact within a whole plant grown from seed to maturityunder natural conditions. This manual describes the fundamental plant processesresponsible for yield production and the techniques used for quantification of thegrowth and ultimately maximizing the economic yield through translocation ofassimilates in the desirable plant parts. This manual on Agronomic aspects of cropphysiology is an attempt to fulfil the urgent need of understanding the cropproduction system, as it covers all major aspects of crop physiology related to cropproduction. Although, it is meant primarily for the under-graduates students ofagricultural universities, yet it will provide valuable basic information and techniquesfor the post-graduate students for their research work.

We are very grateful to worthy Vice-Chancellor Dr. K.S. Khokhar for motivating tobring out this publication. We are highly thankful to Dr. (Mrs.) Sucheta Khokhar,Dean, College of Agriculture, CCS Haryana Agricultural University, Hisar for her everencouraging attitude and all possible help and Dr. O.P. Toky, Dean Post-GraduateStudies for his precious advice and writing foreword. Our sincere thanks are due toDr. K.D. Sharma, Dr. R.K. Pannu and Dr. Satish Kumar for their valuablesuggestions. The financial assistance extended by Indian Council of AgriculturalResearch, for printing this manuscript is thankfully acknowledged. We are deeplyindebted to the authors of various books and manuals for getting useful informationfor inclusion in this manuscript.

March, 2011Hisar Authors

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CONTENTSUnit Sub-

unitParticulars Page

No.Introduction 1

1 Photosynthesis 2-111.1 Factors affecting photosynthesis 21.2 Measurements of photosynthetic rate with photosynthesis system 31.3 Different experiments related to photosynthesis 5

2 Respiration 12-182.1 Tricarboxylic Acid Cycle 132.2 Photorespiration 152.3 Different experiments related to respiration 16

3 Soil- Plant- Water Relations 19-293.1 Measurement of plant water status 203.2 Processes responsible for water movement in the system 213.3 Rooting characteristics and moisture use 27

4 Transpiration 30-36

4.1 Distribution of stomata in monocot and dicot leaves 324.2 Different experiments related to transpiration 33

5 Morphogenesis 37-505.1 Growth 375.2 Development 395.3 Factors affecting growth and development 425.4 Measurement of crop growth 445.5 Measurement of plant growth with the help of Arc Auxanometer 475.6 Measurement of root growth 485.7 Measurement of root density 495.8 Measurement of root growth with soil profile scan system 49

6 Evapotranspiration 51-586.1 Reference crop evapotranspiration 516.2 Crop evapotranspiration under standard conditions 536.3 Crop evapotranspiration under non-standard conditions 536.4 Measurement of evapotranspiration 53

Appendices I - III 59-61

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INTRODUCTIONThe Green Revolution resulted in many fold increase in the food grainproduction. Crop yields have increased steadily over time, but continuing theseincreases above the current high levels would be difficult. But in order to meet thedemands of the increasing population, the crop production must increase in asustainable manner. Sustainable increase in the total production without increasingcultivated area represents a huge challenge that can only be met by integrativeinvestigations at the whole plant-plant community level. Improvement of cropproduction systems will require a thorough understanding of basic principles of yieldproduction. This involves the study of the plant processes responsible for the growth,development and production of economic yield by crop plants, focusing on wholeplants and plant communities and not individual plant parts, organs, or cells becausemost of the processes that control yield operate at the whole plant-plant communitylevel.

The processes responsible for the primary productivity of crop communities arephotosynthesis, respiration, transpiration, nutrient utilization and how the products ofthese processes are converted to economic yield and morphogenetic processes.

Crop physiology is the study of plants at the whole-plant and plant-community levelof organization and Agronomy is the study of crops and cropping systems at the farmlevel over a period of time. At this level of organization, the complexity of the systemis so large that comprehensive attempts are to be made to examine the mechanismsunderlying the crop responses to management or climate.

The scientific approach to agriculture has been to isolate (i.e., the experiments inwhich everything else is equal except the studied phenomenon) and to reduce (i.e.,the process by which, a phenomenon is quantified in terms of one or more essentialmolecular processes), because the production system is too complex. Whereas theunraveling of crop production systems to simple chemical and physical phenomenahas improved our general understanding of the system, it is necessary to understandhow these chemical and physically processes interact within a whole plant grownfrom seed to maturity under natural conditions for this information to be useful inagronomy.

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Chapter 1

PHOTOSYNTHESISPhotosynthesis is the process by which the green plants convert atmospheric CO2

into organic compounds using the energy from sunlight. Photosynthesis is the mainmeans by which plants, algae and many bacteria produce organic compounds andoxygen from carbon dioxide and water. Photosynthetic organisms are calledphotoautotroph, since they can create their own food. In plants, algae, andcyanobacteria, photosynthesis uses carbon dioxide and water, releasing oxygen as awaste product. Photosynthesis is vital for all aerobic life on Earth. As well asmaintaining the normal level of oxygen in the atmosphere, nearly all life eitherdepends on it directly as a source of energy, or indirectly as the ultimate source ofthe energy in their food.

Photosynthesis changes the energy from the sun into chemical energy, splits waterto release Oxygen (O2), and fixes Carbon dioxide (CO2) into sugar. Carbon dioxide isconverted into sugars in a process called carbon fixation, which is a redox reaction.So photosynthesis needs to supply both a source of energy to drive this process,and the electrons needed to convert CO2 into carbohydrate, which is a reductionreaction. In general, photosynthesis is the opposite of cellular respiration, whereglucose and other compounds are oxidized to produce carbon dioxide, water, andrelease chemical energy.

The general equation for photosynthesis is therefore:

2n CO2 + 2n H2O + photons → 2(CH2O)n + n O2 + 2n e-

Carbon dioxide + electron donor + light energy → carbohydrate + oxygen + oxidizedelectron donor

Since, water is used as the electron donor in oxygenic photosynthesis, the equationfor this process is:

2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2

Carbon dioxide + water + light energy → carbohydrate + oxygen

1.1. Factors Affecting Photosynthesis

1.1.1. Light

There is a linear relationship between incident light and CO2 fixation rates at low lightintensities. At higher light intensities, gradually the rate does not show furtherincrease as other factors become limiting. What is interesting to note is that lightsaturation occurs at 10 per cent of the full sunlight. Hence, except for plants in shadeor in dense forests, light is rarely a limiting factor in nature. Increase in incident lightbeyond a point causes the breakdown of chlorophyll and a decrease inphotosynthesis.

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1.1.2. Carbon dioxide Concentration

Carbon dioxide is the major limiting factor for photosynthesis. The concentration ofCO2 is very low in the atmosphere (between 0.03 and 0.04 per cent). Increase inconcentration upto 0.05 per cent can cause an increase in CO2 fixation rates; beyondthis the levels can become damaging over longer periods.

The C3 and C4 plants respond differently to CO2 concentrations. At low lightconditions neither group responds to high CO2 conditions. At high light intensities,both C3 and C4 plants show increase in the rates of photosynthesis. What isimportant to note is that the C4 plants show saturation at about 360 μl/L while C3

responds to increased CO2 concentration and saturation is seen only beyond 450μl/L. Thus, current availability of CO2 levels is limiting to the C3 plants. The fact thatC3 plants respond to higher CO2 concentration by showing increased rates ofphotosynthesis leading to higher productivity has been used for some greenhousecrops such as tomatoes and bell pepper. They are allowed to grow in carbon dioxideenriched atmosphere that leads to higher yields.

1.1.3. Temperature

The dark reactions being enzymatic are temperature controlled. Though the lightreactions are also temperature sensitive they are affected to a much lesser extent.The C4 plants respond to higher temperatures and show higher rate ofphotosynthesis while C3 plants have a much lower temperature optimum.

The temperature optimum for photosynthesis of different plants also depends on thehabitat that they are adapted to. Tropical plants have a higher temperature optimumthan the plants adapted to temperate climates.

1.1.4. Water

Even though water is one of the reactants in the light reaction, the effect of water asa factor is more through its effect on the plant, rather than directly on photosynthesis.Water stress causes the stomata to close hence reducing the CO2 availability.Besides, water stress also makes leaves wilt, thus, reducing the surface area of theleaves and their metabolic activity as well.

1.2. Measurements of photosynthetic rate with photosynthesis system

The basic components of a photosynthesis measurement system are the gasexchange chamber, infrared gas analyzer, flow meters, gas lines, CO2 and watervapor filters, power batteries and a console with keyboard, display and memory.

Leaf chamber architecture, aerodynamics and properties of building materialsprofoundly affect system performance. Precise control of temperature, CO2

concentration, humidity and light has to be achieved. Particularly in close-modesystems, tight sealing of the chamber and use of materials with low adsorption ofwater and CO2 are critical.

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Modern systems measure the CO2 concentration with a non-dispersive infrared gasanalyzer (IRGA). This device includes an infrared source that is shined through agas sampling chamber and then focused on a detector. The energy received at thedetector is the total entering the system minus the energy absorbed by the CO2 inthe sampling chamber. A major problem in IRGA performance is the discriminationbetween CO2 and water vapor, since both gasses absorb energy at similarwavelengths. To solve this problem, the gas sample is dried to a constant watercontent by means of a desiccant before reaching the IRGA. The incorporation ofadvanced computation programs allows the immediate access to data in the fieldand the possibility to detect errors during the measurement.

1.2.1. Using CID-301

Measurement of CO2 uptake, which is instantaneous, non-destructive methodprovides an alternative and direct method for estimating productivity. CO2

assimilation can be measured by different techniques like 14CO2 labelling,conductivity and IR spectroscopic. Infra red gas analysis of CO2 is the mostwidespread contemporary method of determining photosynthesis. It is very sensitivebut costly method. Infra red gas analyser has been used for the measurement of awide range of heteroatomic gas molecules, CO2. H20, NH3, CO, N20, NO andgaseous hydrocarbon. Beside photosynthesis, the instrument also measures thestomatal Conductance/resistance and transpiration rate directly. Three types ofIRGA (infrared gas analyser) are available i.e. (i) closed (ii) semi closed and (iii)open.

Principle

Heteroatomic gas molecules absorb radiation at specific sub-millimeter infra-redwavebands, each gas having a characteristic absorption spectrum. Gas moleculesconsisting of two identical atoms (e.g. N2, 02) do not absorb infrared radiation andthus do not interfere with determination of the concentration of heteroatomicmolecules.

Procedure

i. Switch on the instrument and leave for auto calibration.ii. Select photosynthesis as the parameter to be measured and press enter.iii. Write the area of the chamber to be used according to the crop and press

enter.iv. Adjust the flow rate at 0.50 and press enter.v. Insert the leaf into the chamber and press enter.vi. After few seconds, there will be short beep indicating that the measurement

has been taken.vii. Exclude the leaf and note the data or send it to memory disc for further

analysis on computer.

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1.2.2. Using LI-COR

Procedure

i. Turn on the analyzer pump and fans in the chamber.

ii. Place the CO2 and relative humidity channels in the display.

iii. Clamp the experimental leaf to be measured into chamber.

iv. Wait an appropriate interval until CO2 begins to reduce into the chamber andpress LOG.

v. Enter the leaf area. There are two ways to enter leaf area: 1) during gasexchange: if one can measure area as the leaf site in the chamber, one canset up the console to prompt you for leaf area before the program computesthe values. 2) After readings: one can always edit the data files to entercorrect area and recompute values

vi. To take a new measurement, clear the pad.

Precautionsi. Get a good seal while clamping the leaf.ii. Select the leaf amount and desiccant flow rate that keeps constant humidity.

1.3. Different experiments related to photosynthesis

1.3.1. Carbon dioxide necessity for photosynthesis (Moll’s half leaf method)

Material: A wide mouthed bottle, KOH solution, split cork, a destarched potted plant,starch test apparatus

Principle

Starch is produced by photosynthesis process from CO2 and water using the energyof sunlight. A leaf is kept inside the bottle containing KOH will get light, chlorophylland water but unable to get CO2 as it will be absorbed by KOH. Hence, starchformation will not take place demonstrating that CO2 is essential for photosynthesis.

Procedure

i. Keep the potted plant it in dark till its leaves don’t show positive starch test.

ii. With the help of split cork, insert one half leaf of this destarched plant into thewide mouthed bottle containing potassium hydroxide

iii. To make the apparatus air tight, seal the edges of cork with vaseline.

iv. Put the apparatus in sunlight for few hours.

v. Remove the leaf and test for starch content.

vi. The inner half of the leaf which was devoid of CO2 supply does not turn blue,while the outer half shows positive starch test

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Precautions

i. Destarched the leaf completely before use.ii. The apparatus must be air-tight and receive sufficient sun light.iii. The leaf should not touch KOH solution.

1.3.2. Oxygen (O2) evolution during photosynthesis

Material Required: Beaker, aquatic hydrilla plant, test tube, funnel, water and pondwater

Principle

Photosynthesis is the process by which C O2 of the air is converted into the organicmatter of the green plants with the aid of energy of light. Oxygen is by product ofphotosynthesis which is evolved in the presence of water, light and chlorophyll.

Procedure

i. Place some fresh twigs of aquatic hydrIlla plant under inverted funnel inbeaker filled with water, keeping the cut ends facing towards the tube of afunnel

ii. Place a test tube filled with water upside down over the tube of the funnelpartially dipped in the water of the beaker.

iii. Keep the apparatus in bright sunlight.iv. The air bubbles start emerging out from the cut ends of hydrilla plant which

are collected at the top of the test tube.v. Remove the test tube when sufficient gas is collected.vi. Test it for presence of oxygen by taking glowing splinter to it by introducing

pyrogallol (pyrogaffic acid), which absorbs this gas and the tube gets refilledwith water.

Investigating the need for carbon dioxide in photosynthesis

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Precautions

i. Don’t injure the plant except at one place, where it is cut.

ii. The apparatus should be kept in bright light.

1.3.3. Necessity of light for photosynthesis

Material: Black paper, alcohol, Iodine solution, paper clips, potted plant with widerleaves (e.g. Colacesia, Tropaealum) and water

Principle

The shaded part of leaf could not do photosynthesis due to lack of light, so there isno starch formation indicating that light is essential for the process of photosynthesis.

Procedure

i. Keep the potted plant it in dark till its leaves don’t show positive starch test.

ii. Cover one part of a leaf with black paper and fix it properly by clips.

iii. Keep the whole apparatus in light for few hours.

iv. Pluck the leaf and test for starch with iodine solution.

v. The portion of leaf which was not covered with black paper and receives thelight turns blue with iodine, while the covered portion does not turn blue.

Precautions

i. The leaf must be destarched initially.

ii. The apparatus should be kept in the bright light.

Demonstration of O2 evolution in photosynthesis

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1.3.4. Chlorophyll necessity for photosynthesis

Material: A variegated leaf, apparatus for starch test, potassium hydroxide andwater

Principle

Only green (chlorophyll containing) areas of leaf turned shows positive starch test,while leaf portion devoid of chlorophyll shows negative test, which indicates thatchlorophyll is essential for photosynthesis.

Procedure

i. Take variegated leaves of Coleus or Croton which has received light for fewhours.

ii. Mark out the green parts of the leaf from the non green areas or preparetheir rough outline on plain paper.

iii. Test the leaf for starch content.iv. The leaf portion which is devoid of chlorophyll remain colourless, whereas

the rest of the leaf turns blue black due to the presence of starch.

Precautions

i. The leaves used should be variegated.ii. There should be sufficient sunlight for photosynthesis.

Necessity of Chlorophyll for photosynthesis

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1.3.5. Influence of temperature on photosynthetic rate

Materials: Water, cylindrical jar and Hydrilla plant

Principle

Temperature is one of the major factors which enhance the rate of photosynthesisand then cause to decrease it. Temperature beyond a certain limit can cause deathof a plant.

Procedure

i. Fill the cylindrical jar with water.ii. Keep a glass rod vertically in the jar.iii. Cut a healthy Hydrilla plant at its base.iv. Tie it to the lower portion of the glass rod with thread.v. Keep the jar in a large beaker filled with water at the room temperature.vi. Record the temperature with a thermometer kept in the cylindrical jar.vii. Place the apparatus in sunlight.viii. Determine the rate of photosynthesis by counting the number of oxygen

bubbles evolved per minute at the room temperature and at 30,40,50,60 and70°C by pouring sufficient amounts of hot water in the outer beaker.

Precautions

i. Select healthy plant with sufficient leaves.ii. Apparatus must receive sufficient light for photosynthesis.

.

Effect of temperature on the rate of photosynthesis

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1.3.6. Influence of light intensity on photosynthetic rate

Materials: Three potted plant, shaded and dark place

PrincipleThe rate of oxygen evolution is higher in the plants kept in sunlight and decreaseswith the decreases in intensity of light.

Procedure

i. Take three well watered potted plants.ii. Keep one plant in full sunlight, second plant in shade and third one in

complete darkness for 1-2 days.iii. Determine the rate of photosynthesis in each plant.iv. The photosynthetic rate will be minimum in plant kept in complete darkness,

whereas it will be maximum in plant kept in sunlight, indicating that lightintensity affects the rate of photosynthesis.

Precautions

i. There must be clear difference between light intensity of three pots.ii. Other factors like water, temperature and carbon dioxide must be optimum.

1.3.7. Influence of Light on Photosynthetic Rate – Using an Oxygen Sensor

Materials: Nova5000, 9 g of fresh Elodea, Bright light source (e.g. 150 W halogenlamp), 250 ml glass Erlenmeyer, Rubber cork with a hole that fits the Oxygen sensoror plasticine, A laboratory jack, 1 liter flat water jar (glass or plastic) or tissue culturebottles (2), Oxygen sensor, Optional Temperature sensor (-25 ºC to 110 ºC), Lightsensor (0 - 300 Klx), 0 to 2% Bicarbonate solution.

Principle

Under optimal conditions of carbon dioxide concentrations and temperature,photosynthesis rate depends on light intensity absorbed by the photosynthetic partsof the organism. Light intensity at different distances from a light source is inverselyproportional to the square of the distance.∝ 1Where, ‘I’ is the light intensity and R is the distance from the light source

In this experiment the light intensity is modified by placing the light source at differentdistances from the experimental system. Rate of photosynthesis at various lightintensities is measured by following the concentration of oxygen released to the air inthe process.

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Procedure

i. In this experiment, rate of photosynthesis is measured at variousconcentrations of Bicarbonate solution. Choose four to five concentrations ofBicarbonate in the range of 0% - 2%. Start the experiment with 0.5%Bicarbonate solution.

ii. Follow temperature levels in the water jar throughout the experiment. If watertemperature rises sharply (more than 5ºC in five minutes), stop themeasurements and change the water in the jars.

iii. Mark a line, about 5 cm below the Erlenmeyer edge.iv. Insert the Elodea to the Erlenmeyer: cut the plant to short pieces and arrange

them in parallel to each other to ensure maximal exposure to the light.v. Fill the Erlenmeyer with Bicarbonate solution up to the line marked.vi. Place the Oxygen sensor and tightly close the Erlenmeyer.vii. It is recommended to illuminate the Erlenmeyer containing the Elodea, for five

minutes before the experiment is started. Thereby the solution is saturatedwith oxygen and oxygen release can be measured immediately when theexperiment starts. Otherwise, a lag period of about six minutes is observed.

viii. Click Run on the upper toolbar to begin recording data.ix. Start the experiment with the light source at the maximal distance chosen.

Make sure the light is directed to the Erlenmeyer.x. Switch on the light and follow the oxygen percentage level.xi. Follow photosynthesis rate for 5-8 minutes, until a straight line is received. At

large distances of the light source the rate may be very low.xii. After 5-8 minutes, turn off the light.xiii. Click Stop on the upper toolbar to stop collecting data and save your data by

clicking Save on the upper toolbar.xiv. Move the light source to the second distance and turn the light on again.xv. Repeat stages 10 - 13 at three to four additional distances.

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Chapter 2

RESPIRATIONRespiration is the stepwise oxidation of complex organic molecules and release ofenergy as ATP for various cellular metabolic activities. Respiration involvesexchange of gases between the organism and the external environment. The plantsobtain oxygen from their environment and return carbon dioxide and water vapourinto it. This mere exchange of gases is known as external respiration or breathing incase of animals. It is a physical process.

Glycolysis

In this process, glucose undergoes partial oxidation to form two molecules of pyruvicacid. In plants, this glucose is derived from sucrose, which is the end product ofphotosynthesis, or from storage carbohydrates. Sucrose is converted into glucoseand fructose by the enzyme, invertase, and these two monosaccharides readily enterthe glycolytic pathway. Glucose and fructose are phosphorylated to give rise toglucose-6- phosphate by the activity of the enzyme hexokinase. This phosphorylatedform of glucose then isomerises to produce fructose-6-phosphate. Subsequent stepsof metabolism of glucose and fructose are same. In glycolysis, a chain of tenreactions, under the control of different enzymes, takes place to produce pyruvatefrom glucose.

Various steps of glycolysis

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Utilisation of ATP During Glycolysis:

1. During the conversion of glucose into glucose 6-phosphate

2. During the conversion of fructose 6-phosphate to fructose 1, 6-diphosphate.

There are three major ways in which different cells handle pyruvic acid produced byglycolysis. These are lactic acid fermentation, alcoholic fermentation and aerobicrespiration. Fermentation takes place under anaerobic conditions in manyprokaryotes and unicellular eukaryotes. For the complete oxidation of glucose to CO2

and H2O, however, organisms adopt Krebs’ cycle which is also called as aerobicrespiration. This requires O2 supply.

Fermentation

Fermentation is the process of deriving energy from the oxidation of organiccompounds, such as carbohydrates, using an endogenous electron acceptor, whichis usually an organic compound. This is in contrast to cellular respiration, whereelectrons are donated to an exogenous electron acceptor, such as oxygen, via anelectron transport chain. Fermentation does not necessarily have to be carried out inan anaerobic environment

Aerobic respiration

For aerobic respiration to take place within the mitochondria, the final product ofglycolysis, pyruvate is transported from the cytoplasm into the mitochondria.

The first process takes place in the matrix of the mitochondria while the secondprocess is located on the inner membrane of the mitochondria.

Pyruvate, which is formed by the glycolytic catabolism of carbohydrates in thecytosol, after it enters mitochondrial matrix undergoes oxidative decarboxylation by acomplex set of reactions catalysed by pyruvic dehydrogenase. The reactionscatalysed by pyruvic dehydrogenase require the participation of several coenzymes,including NAD+ and Coenzyme A.

During this process, two molecules of NADH are produced from the metabolism oftwo molecules of pyruvic acid (produced from one glucose molecule duringglycolysis).The acetyl CoA then enters a cyclic pathway, tricarboxylic acid cycle,more commonly called as Krebs’ cycle after the scientist Hans Krebs who firstexplained it.

2.1. Tricarboxylic Acid Cycle

The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid(OAA) and water to yield citric acid. The reaction is catalysed by the enzyme citratesynthase and a molecule of CoA is released. Citrate is then isomerised to isocitrate.

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It is followed by two successive steps of decarboxylation, leading to the formation ofα-ketoglutaric acid and then succinyl-CoA.

In the remaining steps of citric acid cycle, succinyl-CoA is oxidised to OAA allowingthe cycle to continue. During the conversion of succinyl-CoA to succinic acid amolecule of GTP is synthesised. This is a substrate level phosphorylation. In acoupled reaction GTP is converted to GDP with the simultaneous synthesis of ATPfrom ADP. Also there are three points in the cycle where NAD+ is reduced toNADH+H+ and one point where FAD+ is reduced to FADH2.

Electron Transport System (ETS) and Oxidative Phosphorylation

The following steps in the respiratory process are to release and utilize the energystored in NADH+H+ and FADH2. This is accomplished when they are oxidisedthrough the electron transport system and the electrons are passed on to O2

resulting in the formation of H2O. The metabolic pathway through which the electronpasses from one carrier to another, is called the electron transport system (ETS) andit is present in the inner mitochondrial membrane.

Electrons from NADH produced in the mitochondrial matrix during citric acid cycleare oxidised by an NADH dehydrogenase (complex I), and electrons are thentransferred to ubiquinone located within the inner membrane. Ubiquinone alsoreceives reducing equivalents via FADH2 (complex II) that is generated duringoxidation of succinate in the citric acid cycle.The reduced ubiquinone (ubiquinol) isthen oxidised with the transfer of electrons to cytochrome c via cytochrome bc1

complex (complex III).Cytochrome c is a small protein attached to the outer surfaceof the inner membrane and acts as a mobile carrier for transfer of electrons between

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complex III and IV. Complex IV refers to cytochrome c oxidase complex containingcytochromes a and a3, and two copper centres.

Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that ofone molecule of FADH2 produces 2 molecules of ATP. Although the aerobic processof respiration takes place only in the presence of oxygen, the role of oxygen islimited to the terminal stage of the process. Yet, the presence of oxygen is vital,since it drives the whole process by removing hydrogen from the system. Oxygenacts as the final hydrogen acceptor.Unlike photophosphorylation where it is the lightenergy that is utilised for the production of proton gradient required forphosphorylation, in respiration it is the energy of oxidation-reduction utilised for thesame process. It is for this reason that the process is called oxidativephosphorylation.

Respiratory Quotient

The ratio of the volume of CO2 evolved to the volume of O2 consumed in respirationis called the respiratory quotient (RQ) or respiratory ratio.

The respiratory quotient depends upon the type of respiratory substrate used duringrespiration. When carbohydrates are used as substrate and are completely oxidised,the RQ will be 1, because equal amounts of CO2 and O2 are evolved and consumed,respectively. When fats are used in respiration, the RQ is less than 1.

2.2. Photorespiration

Photorespiration or photo-respiration is a process in plant metabolism by whichRuBP (a sugar) has oxygen added to it by the enzyme (rubisco), instead of carbondioxide during normal photosynthesis. This process reduces efficiency ofphotosynthesis in C3 plants.

Simplified biochemistry

Rubisco favors carbon dioxide as a ligand to oxygen, however, photorespirationoccurs when there is a high concentration of oxygen relative to carbon dioxide. Thefirst reaction produces phosphoglycerate and phosphoglycolate (PEP), PGA re-enters the Calvin cycle and is simply converted back to RuBP.

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PPG, however, is more difficult to recycle and has to move from the chloroplast tothe peroxisomes, and then to the mitochondria, undergoing many reactions on theway, before the atoms can return into the Calvin cycle.

Conditions under which photorespiration occurs

Photorespiration can occur when carbon dioxide levels are low, for example, whenthe stomata are closed to prevent water loss during drought. In most plants,photorespiration increases as temperature increases. Photorespiration produces noATP and leads to a net loss of carbon and nitrogen (as ammonia), slowing plantgrowth. Potential photosynthetic output may be reduced by photorespiration by up to25% in C3 plants.

Biochemistry of photorespiration

The oxidative photosynthetic carbon cycle reaction is catalyzed by RuBP oxygenaseactivity:

RuBP + O2 → Phosphoglycolate + 3-phosphoglycerate + 2H+

The phosphoglycolate is salvaged by a series of reactions in the peroxisome,mitochondria, and again in the peroxisome where it is converted into serine and laterglycerate. Glycerate reenters the chloroplast and, subsequently, the Calvin cycle bythe same transporter that exports glycolate. A cost of 1 ATP is associated withconversion to 3-phosphoglycerate (PGA) (Phosphorylation), within the chloroplast,which is then free to reenter the Calvin cycle. One carbon dioxide molecule isproduced for every 2 molecules of O2 that are taken up by RuBisCO.

Photorespiration is a wasteful process because G3P is created at a reduced rate andhigher metabolic cost (2ATP and one NAD(P)H) compared with RuBP carboxylaseactivity. G3P produced in the chloroplast is used to create "nearly all" of the food andstructures in the plant. While photorespiratory carbon cycling results in the formationof G3P eventually, it also produces waste ammonia that must be detoxified at asubstantial cost to the cell in ATP and reducing equivalents.

2.3. Different experiments related to respiration

2.3.1. Measurement of respiration rate using Ganong’s respirometer

Materials: Ganong’s respirometer, germinating seeds (presoaked), Rubber tube,water, wooden stand, a small tube and thread

PrincipleGerminating seeds have a high rate of respiration that can be calculated by usingGanong’s respirometer. A simple respirometer.In plants, carbohydrates, particularlysucrose and starch, are the most important substrates. However some seeds alsocontain stored fats and proteins. designed to measure oxygen uptake or CO2 releaseconsists of a sealed container with the living specimen together with a substance toabsorb the carbon dioxide given off during respiration, such as soda lime pellets or

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cotton wads soaked with potassium hydroxide. The oxygen uptake is detected bydisplacement of manometric fluid in a thin glass U-tube connected to the container.When the organism takes in oxygen it gives off an equal volume of carbon dioxide.As this is absorbed by the soda lime, air is sucked in from the U-tube to keep thepressure constant, displacing the liquid. The rate of change gives a direct andreasonably accurate reading for the organism's rate of respiration.

Procedure

i. Place 25 presoaked seeds of gram or peas in the bulb of Ganong’srespirometer.

ii. Before closing the mouth of the bulb with stopper, 1% KOH solution issuspended in the bulb of small tube.

iii. Fill the water in the tube and note its initial level.iv. Note the changes in the water level at hourly intervals. The water level

increases regularly.v. The rate of respiration can be calculated as increase in water level = (volume

of O2 consumed/time in hr x number of seeds). It can be expressed as rise inwater level/hour/seed.

vi. When respiration takes place in the closed chamber of Ganong’srespirometer bulb, the O2 is continuously depleted from.

Precautions

i. Presoaked seeds should be used.ii. The bulb should be air tight otherwise water level will not rise.iii. Water level rise should not lead the water inside the bulbiv. KOH solution should be accurate

Ganong’s respirometer

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2.3.2. Oxygen is taken up during aerobic respiration.

Material Required: Soda lime, zinc gauze platform, monometer gauze

Procedure:

i. Set up the apparatus as shown. Soda lime absorbs C02 leaving behindoxygen and nitrogen (inert gas) in the tube;

ii. Respiring organisms are placed in the other tube supported by the zinc gauzeplatform.

iii. Liquid levels in the monometer gauze are equalized by means of three waytaps

iv. A separate control is set up containing non-living things.v. A change in the air volume inside the apparatus is shown by the monometer

gauge indicating that oxygen gas is consumed. (Nitrogen being inert does nottake part in respiration).

vi. It clearly shows that oxygen is taken up during aerobic respiration.

Respirometer apparatus

18

2.3.2. Oxygen is taken up during aerobic respiration.

Material Required: Soda lime, zinc gauze platform, monometer gauze

Procedure:

i. Set up the apparatus as shown. Soda lime absorbs C02 leaving behindoxygen and nitrogen (inert gas) in the tube;

ii. Respiring organisms are placed in the other tube supported by the zinc gauzeplatform.

iii. Liquid levels in the monometer gauze are equalized by means of three waytaps

iv. A separate control is set up containing non-living things.v. A change in the air volume inside the apparatus is shown by the monometer

gauge indicating that oxygen gas is consumed. (Nitrogen being inert does nottake part in respiration).

vi. It clearly shows that oxygen is taken up during aerobic respiration.

Respirometer apparatus

18

2.3.2. Oxygen is taken up during aerobic respiration.

Material Required: Soda lime, zinc gauze platform, monometer gauze

Procedure:

i. Set up the apparatus as shown. Soda lime absorbs C02 leaving behindoxygen and nitrogen (inert gas) in the tube;

ii. Respiring organisms are placed in the other tube supported by the zinc gauzeplatform.

iii. Liquid levels in the monometer gauze are equalized by means of three waytaps

iv. A separate control is set up containing non-living things.v. A change in the air volume inside the apparatus is shown by the monometer

gauge indicating that oxygen gas is consumed. (Nitrogen being inert does nottake part in respiration).

vi. It clearly shows that oxygen is taken up during aerobic respiration.

Respirometer apparatus

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Chapter-3

SOIL- PLANT- WATER RELATIONSMovement of water from the soil through the plant to the atmosphere mediatesthrough a widely variable medium (cell wall, cytoplasm, membrane, air space).Fourphysical process are usually involved in the movement of water in living as well asnon living mediums. The main areas of plant-water relationship are water absorption,water conduction and translocation, and water loss or transpiration. These processesare responsible for uptake of plant nutrient, creation of energy gradient andultimately all the metabolic and physiological activities in the plant.

The complete path of water from the soil→ plant → atmosphere forms a continuoussystem, which may be divided into four sequential processes. i) supply of liquid waterto the root surface, ii) entry of water into the root, iii) passage of water in theconducting elements and iv) movement of water vapour through and out of theleaves. The rate of water movement is proportional to the potential gradient andinversely proportional to the resistance to flow. The difference in total water potentialin the soil-plant-atmosphere system could generate a driving force for watermovement from the soil through the plant to atmosphere. On an average, the Ψsoil

varies between - 0.1 to – 20, Ψleaf between - 5.0 to - 50 and Ψatms between - 1000 to -2000 kPa. If this continuum is broken, the driving force would automaticallydisappear.

Water movement in soil-plant atmosphere (SPA) is proportional to the driving forcebetween evaporating surfaces and is inversely proportional to the variousresistances in the pathway. Plants can extract water from the soil only when theirwater potential is lower than that in the soil. This potential difference or waterpotential gradient between the evaporating tissues of the plant and the soil water inthe root zone increases with the evaporative demand and liquid flow resistances inthe pathway.

The rate of water movement (flux) in soil-plant-atmosphere system:

Ψsoil -Ψ root surface Ψ root surface - Ψxylem Ψxylem - Ψleaf Ψleaf - ΨairFlux = --------------------- = ----------------------- = --------------- = ---------------

rsoil rroot rxylem + rleaf rleaf + rair

Resistance to water movement in the soil (rsoil) is governed by its hydraulicconductivity, water content and path length. Water movement in the plant can beconsidered in two phases, i.e. the liquid phase from the root surface to the mesophyIlcell, and the vapour phase from the surface of the leaf mesophyll cell to the stomatalpores. The resistances to flow are relatively low in the liquid phase, being highest inthe roots (at the endodermis), intermediate in the leaves and lowest in the stems.The transformation of water from the liquid to the gaseous phase requires energy,which comes from the solar radiation falling upon the leaf. There are several

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resistances to water movement from within the leaf to the external atmosphere butthe most important are stomatal resistance and the resistance to diffusion throughboundary layer of still air around the leaf.

The total quantity of water required for the physiological functions of the plant isusually less than 1% of all the water absorbed. Most of the water absorbed by theplant is lost through transpiration. If plants fail to replace the water lost bytranspiration, it will result in the loss of turgidity, cessation of growth, and eventuallydeath of the plant.

The plant or leaf water potential at any point in the system can be partitioned into:

Ψℓ = ψ π + ψ p + ψ m + ψ g + ψ i

Except in very dry tissue or in cells with small vacuoles, the matric potential (ψm) isvery small relative to osmotic potential (ψπ) and turgor potential (ψp). Thegravitational potential (ψg) is usually very small for crop plants and is consideredmainly in trees where a vertical change of 10 m affects the ψg by approximately 100kPa. The ψi is the interaction of above explained components of total water potentialis also very small in most of the crop species, hence it can be neglected and than:

Ψℓ = ψ π + ψ p

Assuming no change in cell volume the relationships can be:Ψℓ = ψ π + ψ p

0 = - 20 + 20 bars (turgid)- 10 = - 20 + 10 bars (partially turgid)- 20 = - 20 + 0 bars (flaccid or wilted)

3.1. Measurement of plant water status

Plant water status strongly influences the plant growth and biomass productionthrough its effect on leaf and roots expansion, photosynthesis and other metabolicactivities. Therefore, measurement of plant water status is important forunderstanding plant processes and maximizing yield under different ecosystems.

Material: Leaf samples, water, petri-dishes, humid chamber, blotting paper, punch,forceps and oven

Procedure

Plant water status can be measured by following methods:

3.1.1. Water Content:i. Detach a leaf samples from plants and bring them from the field to laboratory

in a humid chamber.ii. Removing the excess water on the leaf surfaces with blotting paper.iii. Cut the leaf discs (25-30) with the help of a closed punch.iv. Note their fresh weight immediately.

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v. Keep the weighed leaf disc into an oven at 70°C for 24 hours. Record the dryweight of leaf discs.

vi. The water content of the tissue is computed using the following formula:Water content (%) = [(Fresh weight – Dry weight)/Fresh weight] x 100

3.1.2. Relative water contenti. Detach some physiologically active leaves from plants and bring them from

the field to laboratory in a humid chamber.ii. Removing the excess water on the leaf surfaces with blotting paper.iii. Cut the leaf discs (25-30) with the help of a closed punch.iv. Note their fresh weight immediately.v. Float the discs in petri-dishes containing distilled water at room temperature

for attaining maximum turgidity.vi. After 4-5 hours, take out the disc and record their turgid weight after carefully

blotting the excess water.vii. Keep the weighed leaf disc into an oven at 70°C for 24 hours. Record the dry

weight of leaf discs.viii. Calculate the RWC using the formula.

RWC = [(Fresh weight – Dry Weight)/ (Turgid weight – Dry weight)] x 100

3.1.3. Water Saturation Deficiti. Collect the samples and get the values of its RWC following the method

described above.ii. To get water saturation deficit, compute the data in given formula.

WSD (%) = 100-RWCThis technique has been directly linked to water potential.

Precautionsi. Selected leaves must be fresh and partially mature.ii. While cutting the discs, midrib should be removed.iii. Only interveinal area should be taken.iv. Blotting of excess water should be done before recording turgid weights.v. Fresh weight should be taken at the earliest to avoid transpiration losses.

3.2 Processes responsible for water movement in the system

3.2.1 Diffusion

The tendency of the gases, liquid and solid molecules to get evenly distributedthrough out the available space is called diffusion. As a result the molecules movefrom the higher concentration region to the lower concentration region. The diffusionrate of gases is faster than liquids and solutes (solids). The different particles have acertain pressure called as the diffusion pressure which is directly proportional to theconcentration of the diffusing particles. The diffusion takes place always from aregion of higher diffusion pressure to a region of lower diffusion pressure along a

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diffusion pressure gradient. Diffusion of more than one substance at the same timeand place may be at different rates and in different direction, but is dependent ofeach other. A very common e.g. of this is the gaseous exchange in plants. Diffusionor observable movement of particles from one place to another can take place onlywhen there is difference in the concentration of a substance in the different parts of asystem.

Demonstration of the diffusion process

Materials: Beaker, copper sulphate, water

Principle

Kinetic energy in the molecules, ions or even colloidal particles keep them in a stateof perpetual motion. Thus they collide with one another and ultimately get deflectedin the direction of least concentration.

Procedure

i. Fill a beaker about two-third with distilled water.ii. Put a copper sulphate crystal in it and wait for few minutes.iii. After some time, the crystal will disappear and the whole of the water in the

beaker turns to blue color.

3.2.2. Osmosis

The movement of solvent molecules from the region of their higher concentration tothe region of their lower concentration through a semi permeable membrane is calledosmosis. The movement of water or solvent from its higher concentration region toits lower concentration region through a semi permeable membrane without allowingthe diffusion of the solute is called osmosis.

The Importance of Osmosis in Plants

i. In the absorption of water by plantsii. Cell to cell movement of water occurs throughout the plant bodyiii. The rigidity of plant organs is maintained

Diffusion of copper sulphate in Water

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iv. Leaves become turgid and expand due to osmotic pressurev. Growing points of root remain turgid and penetrate the soil particlesvi. The resistance of plants to drought and frostvii. Movement of plants and plant partsviii. Opening and closing of stomata

There are two cells A and B in contact with each other, cell A has a pressurepotential of 4 bars and contain sap with an osmotic potential of -12 bars. Cell B haspressure potential of 2 bars and contain sap with an osmotic potential of -5 bars.

Then,

Ψw of cell A = Ψp + Ψs

=-12+ (+4) = -8 bars

Ψw of cell B = Ψp + Ψs

=-5+ (+2) = -3 bars

Hence water will move from cell B to cell A (ie. Towards lower or more negativewater potential) with a form of (-8(-3)) = -5 bars

A. Process of osmosis in a simple osmometer

Materials: 10% sugar solution, a long stemmed thistle funnel, animal bladder orparchment paper, thread, scissors, stand, beaker, colored water, pencil and rubbersolution

Principle

A semi-permeable membrane having of very small sized pores allows only water topass through the pores but prevent the movement of solute across them. The animalbladder or parchment paper acts as a semi-permeable membrane.

Procedure

i. Cover the thistle funnel mouth with an animal bladder or parchment paper(semi-permeable membrane) with the help of a thread.

ii. With the help of scissors, remove free edges of the bladder as close to thethread as possible.

iii. Seal the free margin of the bladder with the help of rubber solution.iv. Fill the thistle funnel with 10% sugar solution to about 1/3rd neck height and

mark the level as ‘a’.v. Dip the covered end of the thistle funnel in a beaker containing colored

water.vi. After few hours, the sugar solution slowly will become colored and its level in

the vertical neck of the funnel will rise up steadily to point ‘b’.vii. Taste the water of solution in funnel. It will no longer be sweet.

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Precautions

i. The membrane should attach with the thistle funnel properly.ii. The sugar solution must be of sufficiently higher concentrations.

B. Process of osmosis in potato osmoscope

Materials: A large potato tuber, sugar solution, knife, petri dish, water, capillary tubeand marking pencil

Principle

A semi-permeable membrane having very small sized pores allows only water topass through the pores but prevent the movement of solute across them. The tuberwall acts as a semi-permeable membrane.

Procedure

i. Give a flat cut to one end of the potato tuber.ii. On another end of tuber, make a hollow cavity slightly more than half of its

diameter.iii. Remove the skin of the tuber.iv. Place the tuber on its flat cut end in a petri-dish half full of water.v. Fill half of the cavity of the potato tuber with sugar solution.vi. By inserting a pin in the wall of the tuber mark the initial level of the solution.vii. After few hours, the level of sugar solution is found to increase as water

enters in the cavity. It is due to inward diffusion of water (endosmosis).

Demonstration of osmosis

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viii. Repeat the experiment after killing a potato tuber in boiling water. Theprotoplasm is denatured and the cytoplasm does not function as membrane.Thus there is no change in the level of sugar solution in the cavity.

Precautions

i. The cavity of potato must be larger to pour sufficient amount of sugar solution.

ii. The initial level of sugar solution should be marked carefully.

iii. The water level in the Petri dish should be enough to dip a major portion ofpotato tuber.

3.2.3. Plasmolysis

Plasmolysis is the shrinkage of the protoplast of a plant cell from its cell wall due toloss of water under the influence of hypertonic solution. If a plant cell is placed in ahypertonic solution, the plant cell loses water and hence turgor pressure, making theplant cell flaccid. Water loss decreases pressure to the point where the protoplasmof the cell peels away from the cell wall, leaving gaps between the cell wall and themembrane. Plasmolysis can be reversed if the cell is placed in a hypotonic solution.

Plasmolysis only occurs in extreme conditions and rarely happens in nature. It isinduced in the laboratory by immersing cells in strong saline or sugar solutions tocause exosmosis, often using Elodea plants or onion epidermal cells, which havecolored cell sap so that the process is clearly visible.

Potato osmoscope

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Process of plasmolysis and deplasmolysis

Materials: Leaf of Trodescantia, Sugar solution, Pipette, Petridishes, Slides, Coverslips and Water

Principle

Plasmolysis is the shrinkage of the protoplast of a cell from its cell wall under theinfluence of a hypertonic solution whereas the swelling up of a plasmolysedprotoplast under the influence of a hypotonic solution or water is calleddeplasmolysis. The phenomenon of plasmolysis has been exhibited by cells whenthey are kept in hypertonic solutions. The phenomenon of deplasmoylsis has beenexhibited by plasmolysed cells when they are kept in a water or hypotonic solution.

Procedure

i. Peel the lower epidermis of Trodescantia leaf.ii. Divide the leaf into small strips.iii. Place these strips in different concentrations of sugar solution (0.1, 0.2, 0.3,

0.4 M) as well as in fresh water (control).iv. With help of microscope observe the changes in the cells.v. The protoplast of the peelings kept in lower concentration (0.1 M) or in water

remains homogenously distributed. Whereas, the protoplast in the peelingskept in higher concentrations will shrink.

vi. Count the number of cells under the microscope.

Precautions

i. Sugar solution concentration must be correctly prepared.ii. Peelings should be done from lower epidermis of leaf very carefully.iii. Mount the peeling in glycerin carefully and avoid air bubbles and folding of

peelings.

Various stages in plasmolysis

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3.2.4. Imbibition

Imbibition is the phenomenon of adsorption of water by the solid particles of asubstance without forming a solution. It is also defined as the phenomenon by whichthe living or dead plant cell absorbs water by surface attraction Dynamic invasionwith constant flow rate of the displacing fluid. Substance which can imbibe or absorba liquid without forming a solution is known as Imbibants

Importance of Imbibition to the Plants

i. Imbibition is the initial step in seed germination.ii. Imbibition causes swelling of seeds and results in the breaking of testa.iii. Imbibition is dominant in the initial stage of water absorption by roots.iv. The water moves into ovules which are ripening into seeds by imbibition.

One example of imbibition that we can find in nature is the absorption of water byhydrophilic colloids. Matrix potential contributes significantly to water in suchsubstances. Examples of plant material which exhibit imbibition are dry seeds beforegermination. Different types of organic substances have different imbibing capacities.Proteins have a very high imbibing capacity, starch less and cellulose least. That iswhy proteinaceous pea seeds swell more on imbibition than starchy wheat seeds.

Imbibition of water increases the volume of the imbibant which results in imbibitionalpressure. This pressure can be of tremendous magnitude.

3.3. Rooting characteristics and moisture use

The amount of soil moisture that is available to a plant is determined by the moisturecharacteristics of the soil, the depth to which the plant roots extend and theproliferation of the roots. Little can be done to alter soil moisture availability. Greaterpossibilities lie in changing the plant characteristics, enabling it to extend its rootingsystem deeper into the soil, thereby enlarging its reservoir of water. Plants varygenetically in their rooting characteristics. Vegetable crops, such as onions andpotatoes, have a sparse rooting system and are unable to use all the soil waterwithin the root zone. Forage grasses, sorghum, maize and such other crops havevery fibrous, dense roots. Lucerne has a deep root system. Perennial plant hasalready established root depth, and needs only to extend its small roots and roothairs to utilize the entire amount of available soil water.

3.3.1. Moisture extraction pattern within root zone

The moisture extraction pattern is the relative amounts of moisture extracted fromdifferent depths within the root zone. About 40% of the total moisture used isextracted from the 1st, 30% 2nd, 20% 3rd and only 10% from the last quarter of theroot zone. So to have a fair estimate of the soil moisture status, it should bemeasured at different depths within the root zone.

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3.3.2. Root pressure

Root pressure is osmotic pressure within the cells of a root system that causes sapto rise through a plant stem to the leaves. Root pressure occurs in the xylem of somevascular plants under the following conditions:

i. When the soil moisture level is high at night.

ii. When transpiration is low during the day.

When transpiration is high, xylem sap is usually under tension, rather than underpressure, due to transpiration pull. At night in some plants, root pressure causesguttation or exudation of drops of xylem sap from the tips or edges of leaves. Rootpressure is studied by removing the shoot of a plant near the soil level. Xylem sapwill exude from the cut stem for hours or days due to root pressure. If a pressuregauge is attached to the cut stem, the root pressure can be measured.

Root pressure is caused by active transport of mineral nutrient ions into the rootxylem. Without transpiration to carry the ions up the stem, they accumulate in theroot xylem and lower the water potential. Water then diffuses from the soil into theroot xylem due to osmosis. Root pressure is caused by this accumulation of water inthe xylem pushing on the rigid cells. Root pressure provides a force, which pusheswater up the stem, but it is not enough to account for the movement of water toleaves at the top of the tallest trees. The maximum root pressure measured in someplants can raise water only to about 7 meters, and the tallest trees are over 100meters tall.

Measurement of the root pressure in plants by manometer

Materials: A potted plant of Balsam or Bryophyllum, manometer with tube, stand,rubber tube, knife and thread

Moisture-extraction pattern of plants under adequate soil moisture

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Principle

There is rise in mercury level in the manometer is due to the pressure created bywater exudates from the cut end of the stem on account of root pressure generateddue to entry of water by osmosis in root system.

Procedure

i. Fully saturated a potted plant of Balsom or Bryophyllum with water and keep itovernight.

ii. Next morning, cut its stem a few inches above the base with a sharp knife.iii. Attach the cut end of the stem to a manometer fixed to a stand through a

rubber tube and threads.iv. Keep it at a moist and shady place for few hours.v. Measure the initial level of water in the glass tubes.vi. Observe rise in the mercury level in the manomemeter due to the pressure

created by water exuded from the cut end of the stem on account of rootpressure generated due to osmotic entry of water in the roots.

Precautions

i. The plant selected should be succulent.ii. Stem should be cut under water to avoid the entrance of air bubbles in xylem

vessels.iii. Rubber tubing should be fixed carefully.iv. All connection should be made air tight with wax.

Measurement of root pressure

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Chapter-4

TRANSPIRATIONTranspiration is a process similar to evaporation. It is a part of the water cycle, and itis the loss of water vapor from parts of plants (similar to sweating), especially inleaves but also in stems, flowers and roots.

Morphologically a plant consists of roots, stem and leaves. The leaves are bornthroughout the stem in all the plants and are mainly responsible for the loss of water.Similar variation in plant water relations among different species and betweensurfaces also exist. The internal structure of a typical leaf shows that the surface hassmall pores surrounded by two cells. These pores are called stoma and the cellssurrounding them are called guard cells.

The stomata regulate the loss of water as vapours and exchange of carbon dioxidein the leaf and other organs. The leaves maintain their continuity of structure with thestem which has conducting tissues called xylem, the main channels of watertransport, and phloem. The stem maintains its continuity with the root, whicheventually is in contact with the soil. The outermost cell of the root usually getselongated into a long hair which has its continuity with the remaining cells. Otherepidermal cells are also capable of absorbing water but the root hair provides theadvantage of exploring more area because of its enlarged surface. Thus, a largenumber of root hairs draw moisture from their vicinity and supply water to the cortex.

Leaf surfaces are dotted with openings which are collectively called stomata, and inmost plants they are more numerous on the undersides of the foliage. The stoma arebordered by guard cells that open and close the pore. Leaf transpiration occursthrough stomata, and can be thought of as a necessary "cost" associated with theopening of the stomata to allow the diffusion of carbon dioxide gas from the air forphotosynthesis. Transpiration also cools plants and enables mass flow of mineralnutrients and water from roots to shoots.

Portion of a leaf cross-section adjacent to stomata

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Environmental factors affecting rate of transpiration

i. Light: Plants transpire more rapidly in the light than in the dark. This islargely because light stimulates the opening of the stomata (mechanism).Light also speeds up transpiration by warming the leaf.

ii. Temperature: Plants transpire more rapidly at higher temperatures becausewater evaporates more rapidly as the temperature rises. At 30°C, a leaf maytranspire three times as fast as it does at 20°C.

iii. Humidity: The rate of diffusion of any substance increases as the differencein concentration of the substances in the two regions increases. When thesurrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

iv. Wind: When there is no breeze, the air surrounding a leaf becomesincreasingly humid thus reducing the rate of transpiration. When a breeze ispresent, the humid air is carried away and replaced by drier air.

v. Soil water: A plant cannot continue to transpire rapidly if its water loss is notmade up by replacement from the soil. When absorption of water by theroots fails to keep up with the rate of transpiration, loss of turgor occurs, andthe stomata close. This immediately reduces the rate of transpiration (as wellas of photosynthesis). If the loss of turgor extends to the rest of the leaf andstem, the plant wilts.

Guttation

Guttation is the appearance of drops of xylem sap on the tips or edges of leaves ofsome vascular plants, such as grasses. Guttation is not to be confused with dew,which condenses from the atmosphere onto the plant surface. In guttation, water islost in the form of droplets and through hydathodes from the ends of vascular tissuesat the margins of leaves.

Guttation is caused by root pressure (a positive hydrostatic pressure). For example,when earth is extremely well watered and the relative humidity is high, guttation can

Opening and closing of stomata

31

Environmental factors affecting rate of transpiration

i. Light: Plants transpire more rapidly in the light than in the dark. This islargely because light stimulates the opening of the stomata (mechanism).Light also speeds up transpiration by warming the leaf.

ii. Temperature: Plants transpire more rapidly at higher temperatures becausewater evaporates more rapidly as the temperature rises. At 30°C, a leaf maytranspire three times as fast as it does at 20°C.

iii. Humidity: The rate of diffusion of any substance increases as the differencein concentration of the substances in the two regions increases. When thesurrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

iv. Wind: When there is no breeze, the air surrounding a leaf becomesincreasingly humid thus reducing the rate of transpiration. When a breeze ispresent, the humid air is carried away and replaced by drier air.

v. Soil water: A plant cannot continue to transpire rapidly if its water loss is notmade up by replacement from the soil. When absorption of water by theroots fails to keep up with the rate of transpiration, loss of turgor occurs, andthe stomata close. This immediately reduces the rate of transpiration (as wellas of photosynthesis). If the loss of turgor extends to the rest of the leaf andstem, the plant wilts.

Guttation

Guttation is the appearance of drops of xylem sap on the tips or edges of leaves ofsome vascular plants, such as grasses. Guttation is not to be confused with dew,which condenses from the atmosphere onto the plant surface. In guttation, water islost in the form of droplets and through hydathodes from the ends of vascular tissuesat the margins of leaves.

Guttation is caused by root pressure (a positive hydrostatic pressure). For example,when earth is extremely well watered and the relative humidity is high, guttation can

Opening and closing of stomata

31

Environmental factors affecting rate of transpiration

i. Light: Plants transpire more rapidly in the light than in the dark. This islargely because light stimulates the opening of the stomata (mechanism).Light also speeds up transpiration by warming the leaf.

ii. Temperature: Plants transpire more rapidly at higher temperatures becausewater evaporates more rapidly as the temperature rises. At 30°C, a leaf maytranspire three times as fast as it does at 20°C.

iii. Humidity: The rate of diffusion of any substance increases as the differencein concentration of the substances in the two regions increases. When thesurrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

iv. Wind: When there is no breeze, the air surrounding a leaf becomesincreasingly humid thus reducing the rate of transpiration. When a breeze ispresent, the humid air is carried away and replaced by drier air.

v. Soil water: A plant cannot continue to transpire rapidly if its water loss is notmade up by replacement from the soil. When absorption of water by theroots fails to keep up with the rate of transpiration, loss of turgor occurs, andthe stomata close. This immediately reduces the rate of transpiration (as wellas of photosynthesis). If the loss of turgor extends to the rest of the leaf andstem, the plant wilts.

Guttation

Guttation is the appearance of drops of xylem sap on the tips or edges of leaves ofsome vascular plants, such as grasses. Guttation is not to be confused with dew,which condenses from the atmosphere onto the plant surface. In guttation, water islost in the form of droplets and through hydathodes from the ends of vascular tissuesat the margins of leaves.

Guttation is caused by root pressure (a positive hydrostatic pressure). For example,when earth is extremely well watered and the relative humidity is high, guttation can

Opening and closing of stomata

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occur. Guttation produces dew-like drops of water that emerge from the tips of somegrasses and other plants. Modified stomata called hydathodes are the sites of waterexudation with the driving force being root pressure which may help to distributeimportant minerals when transpiration rates are low.

Process of Guttation

At night, transpiration usually does not occur because most plants have theirstomata closed. When there is a high soil moisture level, water will enter plant roots,because the water potential of the roots is lower than in the soil solution. The waterwill accumulate in the plant, creating a slight root pressure. The root pressure forcessome water to exude through special leaf tip or edge structures, hydathodes, formingdrops. Root pressure provides the impetus for this flow, rather than transpiration pull.

Differences between Transpiration and Guttation

Transpiration Guttation

1. Water is lost in the form of watervapours from aerial parts of theplants.

1. Water is lost in the form of liquiddrops from leaf edges only.

2. Water comes out through stomata 2. Water oozes out through hydathodes.

3. The transpired water is pure. 3. The transpired water has mineralsand other organic substances.

4. This process is regulated by theshape of guard cells.

4. This process does not involve thechange in the shape of cells.

5. It takes place only in the presence ofsunlight.

5. It takes place only in the morninghours and at low temperature.

4.1 Distribution of stomata in monocots and dicots Leaves

Stomata are small opening in the leaves that are the site of the gaseous exchange(mainly oxygen and carbon dioxide). The process of transpiration also occursthrough the stomata. They vary widely in size and frequency. Number of stomataalso varies among genotypes of a species and various part of a genotype. Inmonocots, conifers and some dicots, stomata occur in paralIel rows, but in leaveswith netted venation they are scattered. They sometime are sunken below thesurface but occasionally are raised, and usually they open into sub stomatal cavitiesin the mesophyll tissues. When wide-open stomatal pores are usually 3-12 µm wideand 10-30 µm or more in length. The stomata are surrounded by two guard cells,which control their opening and closing. Usually, specialized epidermal cells, calledsubsidiary cells are associated with the guard cells and play a role in guard cellfunctioning.

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Microscopic anatomy of stomata shows that the guard cells are crescent shaped,semi cylinder, and each pair is joined at the ends. One of their most striking featuresof guard cells is the presence of chloroplast, which make them unique amongepidermal cells as other cell are not able to photosynthesize. The cell wall of theguard cell is considerably thicker on the side nearer the stomatal opening;Additionally, fibers of cellulose in the cell wall are arranged in bundles, the radialmicelles, which coverage at the thickened, stomatal site of the cell wall. Thisarrangement of the cell wall fibers causes the concave surfaces of the two guardcells to pull apart when the volume and presence of the cell contents increases, thusopening the stomata. Conversely, when this pair of cells becomes flaccid, it relaxesnot uniformly but in such a way that the stomata close.

The plant response to various stresses begins with the stomatal response. Theresistant varieties are able to partially close the stomata under water limitingconditions. It is possible to measure the stomatal behavior of large number of plantsin the field by using porometer technique.

Distribution of stomata in different plants.

Name of plant Number of stomata per sq. cm.

Upper surface Lower surface

Sunflower (Hellanthus annus) 58 156

Tomato (Lycopersicum esculentum) 12 130

Rajmah (Phaseolus vulgaris) 40 281

Potato (Solanurn tuberosum) 51 161

Maize (Zea mays) 52 68

Oat (Avena sativa) 40 43

Stomatal conductance (m mol/m2/sec): Stomatal conductance of the leaves (top,middle and bottom) can be measured in-situ at with the help of PortablePhotosynthesis System (CID- 310) as per the procedure explained for measuring thephotosynthetic rate in the section 1.2.1, by selecting stomatal conductance instead ofphotosynthesis.

4.2. Different experiments related to transpiration

4.2.1. Demonstration of the transpiration by Bell Jar Method

The water drops come out in the form of water vapors by the aerial parts of the plantduring transpiration and consequently condense on the inner surface of the bell jar.A decrease in the weight of the pot is also due to the loss of water from the aerialportions of plants during transpiration.

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Materials: A well-watered potted plant, Bell jar, a rubber sheet or oilcloth, a glasssheet and Grease or Vaseline

Principle

Bell Jar Method of transpiration is based on the principle that the water vapors canbe seen in the form of water droplets if a transpiring plant is observed thoroughly.

Procedure

i. Select a small well-watered plant.ii. Cover the external soil surface of the pot and its soil thoroughly with oil cloth

or polythene bag.iii. Weigh the whole pot and place it on glass plateiv. Invert a dry bell jar over the pot.v. Keep it for few hours and observe the bell jar.vi. The bell jar becomes misty after sometime and its inner walls contain drops of

water that may flow down the sides of the bell jar.vii. Weigh of the pot again to know the loss in weight due to transpiration.

Precautions

i. The bell jar should be air tight.

ii. The pot should be well watered.

iii. Bell jar should be made up of glass to penetrate light.

iv. The pot surface and soil must be covered carefully to avoid evaporationlosses.

Demonstration of transpiration

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4.2.2. Measurement of transpiration rate using Ganong’s potometer

Ganong’s potometer contains a horizontal graduated capillary tube. One end of thecapillary tube is bent downward, where it has a hole. The other end of the capillaryopens in a wide tube, which bears first a water reservoir and then a wide mouthedbottle for holding a leafy shoot. The water reservoir bears a stopcock at its base toconnect and disconnect it with the rest of the apparatus.

Material Required: Ganong’s potometer, Beaker, melted wax or vaseline, water,stopwatch and twig or branch of a plant

Principle

The amount of water absorbed is almost equal to that of water transpired and thedifference between two is negligible.

Procedure

i. Fill the whole apparatus with water completely through the reservoir.ii. With the help of cork, insert a fresh leafy shoot cut under water into the

mouth of the bottle.iii. Apply melted wax or vaseline to make the apparatus air tight and place it in

sunlight.iv. Introduce an air bubble in the graduated arm with a pipette.v. Dip the free end of tube in beaker containing water. Transpiration will occur

in the plant kept in apparatus and the air bubble will move towards twig.vi. Measure the distance (ℓ) traveled by air bubble in the horizontal tube at

definite time intervals.vii. Calculate the volume of water transpired by the formula:

V=π r2 ℓWhere, V and r denotes volume of water and radius of graduated tube,respectively.

viii. The rate of transpiration is determined in terms of ml/hour/unit area.

Ganong’s potometer

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Precautions

i. The shoot must be cut under water to avoid blocking of vessel by air.ii. Apparatus should be air-tight.iii. The twigs must have sufficient leaves.

4.2.3. Measurement of transpiration by loss of weight method

Material Required: A shoot cut under water, split cork, test tube, water, thread,stand and spring balance

Principle

Weight of the plant is reduced due loss of water in the form of water vapour from theaerial parts of the plant.

Procedure

i. Take a leafy shoot cut under water and defoliate its lower portion for 6-8 cm.ii. Calculate the total surface area of the shoot above the defoliated region.iii. With the help of a split cork, insert the defoliated end of the shoot in a test

tube full of water.iv. Make the joints air-tight.v. Tie the test tube almost vertically to a spring balance hanging from a stand.vi. Read the initial weight of apparatus and allow the shoot to transpire for a

couple of hours.vii. Note the loss of weight at definite intervals and calculate the rate of

transpiration as mg/cm2/hr.

Demonstration of water loss due to the process of transpiration

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Chapter 5

MORPHOGENESISMorphogenesis consists of two primary functions viz. growth and development.Growth means an irreversible increase in cell number, plant size, plant weight, or allof the above. Development/differentiation may be thought of as an increase incomplexity. Internal and external factors influencing cellular differentiation causescell groups to become distinct tissue types and organs through expression of specificgenes. The genes prescribe, according to the function, location, and phenologicalstage of development, the manufacturing of specific enzymes. Growth anddevelopment are frequently associated, but this is not necessarily always the case.For example, increase of dry matter can occur without any further differentiation(deposition of storage material in grain, stem or root) and differentiation can occurwithout a concurrent increase in weight (germination and "growth" of seedlings in thedark). In crop physiology, the term development is usually used as the sequence ofperiodical phenomena of plants (or phenological development).

5.1. Growth

Growth can be defined as an irreversible permanent increase in size of an organ orits parts or even of an individual cell. Generally, growth is accompanied by metabolicprocesses (both anabolic and catabolic), that occur at the expense of energy. Plantgrowth is unique because plants retain the capacity for unlimited growth throughouttheir life. This ability of the plants is due to the presence of meristems at certainlocations in their body. The cells of such meristems have the capacity to divide andself-perpetuate. The product, however, soon loses the capacity to divide and suchcells make up the plant body. This form of growth wherein new cells are alwaysbeing added to the plant body by the activity of the meristem is called the open formof growth.

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5.1.1. Growth Rate

The increased growth per unit time is termed as growth rate. Thus, rate of growthcan be expressed mathematically. An organism, or a part of the organism canproduce more cells in a variety of ways. The growth rate shows an increase that maybe arithmetic or geometrical.

In arithmetic growth, following mitotic cell division, only one daughter cell continuesto divide while the other differentiates and matures. The simplest expression ofarithmetic growth is exemplified by a root elongating at a constant rate.

Mathematically, it is expressed as: = +Where, Lt = length at time ‘t’

L0 = length at time ‘zero’r = growth rate / elongation per unit time.

5.1.2. Phases of growth

All plants pass through various stages of growth. Growth is being expressed bymeans of curve plotted against time. The S-shaped or sigmoid curve is typical ofgrowth pattern of individual organs of a whole plant and of population of plants. Itconsists of five distinct phases:

i. Lag phase: An initial lag period during which internal changes occur that arepreparatory to growth. The increase in size and weight is very slow ornegligible during this period.

ii. Log phase: It is followed by the phase or log period of growth or the grandperiod of growth during straight line during this phase.

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iii. Decreasing growth rate phase: A phase in which growth graduallydiminishes.

iv. Plateau phase: A point at which the organism reaches maturity and growthceases.

v. Senescence phase: Later, senescence and death of organism sets in givingrise to another component of the growth curve.

5.1.3. Important growth stages

i. Maximum Vegetative Growth Stage: During this stage, crops grow at afaster rate. Plants start producing branches or tillers and the crop covers theground, as much as possible, to intercept more radiation. The loss of plantpopulation that occurs during early stages can be overcome in this stage byproducing more number of tillers or branches. The number of ears per unitarea is decided during this stage. This stage is variously called as tilleringstage, active tillering stage, branching stage etc. depending on the crop.

ii. Primordial Differentiation: This stage is called as panicle initiation stage incereals and millets, squaring in cotton, flower bud initiation in sunflower etc.With starting of this stage plants enter reproductive phase. The number offruits or grains is decided during this stage. This stage is more sensitive tomoisture and solar radiation in cereals.

iii. Physiological Maturity: When fruit growth is complete and no longer isphotosynthates translocated to fruits, it is known as physiological maturity.An abscission layer forms between fruit and peduncle. Crops, if necessary,can be harvested at this stage.

iv. Harvest Maturity: At physiological maturity, the moisture percentage ofgrains is about 20 per cent. In addition, all the grains might not be atphysiological maturity. Before, it is a general practice, to allow the crop for 10or more days after physiological maturity to reduce the moisture percentageand also to provide time for the development of late formed grains.

5.2. Development

The developments of a plant/crop refers to the phasic change during the life cyclefrom germination to maturity. It can be considered as a series of discrete periods,each identified by an accompanying process of change in the structure, size orweight of specific organs. The fundamental characteristic of developmentalprocesses is that they are discrete. The seed either germinates or it does not and aleaf primordium is invisible or visible. Hence, they can only be defined in terms oftime, and not in length, volume or weight. The duration of developmental process isusually measured between events that are detected visually.

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5.2.1. Phases of Development

There are many ways to divide and subdivide the life cycles of crop species. First,division of the life cycle into the period before flowering and the period after floweringis relevant for most crops:

A. Based on flowering

i. Pre-flowering phase: The time of planting to flowering. This is also called asvegetative stage.

ii. Post-flowering: The time of flowering to maturity. This is also called asreproductive stage.

B. Based on dry matter accumulation

i. A period of exponential dry matter accumulation during early phases ofdevelopment.

ii. A period of more or less linear dry matter accumulation.

iii. A period of declining rates of dry matter accumulation (and sometimes evena negative rate of dry matter accumulation) during the final phase of the lifecycle when green leaf area declines due to leaf senescence.

Schematic representation of dry matter accumulation during the life cycle

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C. Functional approach

In this approach phenology is treated in terms of the impact of each period on theeconomic product. In the functional approach to phenology, a crop's life cycle can bedivided into three phases:

i. The period of the formation of the factory for the production of raw material(the leaf canopy that intercepts incident solar radiation and the raw materialis reduced CO2). The crop canopy is established during this and a reductionin rate of dry matter accumulation during this period will affect maximum LAI,which may or may not influence absorption of incident solar radiation duringthe period of complete leaf area expansion.

ii. The period of the formation of the manufacturing factory (i.e., theestablishment of seed/grain sink size).

iii. The period during which the economic product is manufactured (i.e., filling ofthe seed/grain).

The formation of the economic product occurs during the final period of the life cycleof the crop. This period is affected potentially most by the relative maturity of thecrop. Because this period is relatively short (approximately 40 days for a maize cropin Ontario), any environmental factor that shortens this period (e.g., a killing frost) willaffect yield relative to the reduction of this period and not relative to the duration ofthe total life cycle.

5.2.2. Rate of Development

Rate of development may be defined as the inverse of the time interval between twostages of development. For instance, if the time interval between two stages ofdevelopment is 10 days, then the mean rate of development would be 0.1/day. Theinterval between the two stages of development is shortest under optimal conditionsfor development and rate of development may be normalized for the maximum rate.There are two conceptional different groups of factors that may influence theduration of the interval.

i. Factors that have a quantitative effect on the duration of the interval betweentwo stages of development. These are environmental factors that influencerates of biochemical processes. The response of rate of development tothese factors is the "temperature like" response.

ii. Factors that act like a trigger at the start of a stage of development, such ashormonal factors that induce flowering, can be either genetically orenvironmentally induced. The response of rate of development to thesefactors is the "photoperiod like" response.

The phyllochron, i.e., the interval between the emergence of two successive leaves,is used during the, "leaf growth" phase to measure rate of development.

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5.3. Factors affecting growth and development

Growth and development of a plant are influenced mainly by solar radiation,temperature, soil moisture, soil aeration and mineral nutrients. In addition, otherfactors also influence crop growth at different stages.

5.3.1. Germination

Germination is influenced by temperature, soil moisture, light, aeration anddormancy of seeds while soil physical conditions and depth of sowing influenceemergence.

i. Temperature: The rate of germination increases linearly with temperatureover the range from the minimum to 2-3°C below optimum temperature. Thedecrease in rate at high temperature and progressive downward shift of thisrate with time is an usual response. The maximum temperature above whichno germination occurs is usually within the range of 35-45°C.

ii. Soil Moisture: Most of the crop seeds germinate well within the moistureregime of field capacity to 50 per cent available soil moisture. The effect ofchange in matric potential of the soil is attributed to the associated increasein mechanical strength of the soil rather than availability of water or tochanges in area of contact with liquid water. For better contact with the seed,the mean diameter of soil aggregates should be less than about one fifth ofthe seed.

iii. Depth of Sowing: After the crop in sown, soil gradually dries out if there isno further addition of moisture to the soil, and if the seeds are sown shallow,they may not germinate. If seeds are sown deep, seeds may not emerge asthe seed reserves may not be sufficient to put forth sufficient growth. Soilcrust is the main hurdle for the emergence of crops like foxtail millet, pearlmillet etc. as the seeds are small. Oxygen content, light and dormancy arealso some of the factors that influence germination and emergence.

5.3.2. Seedling Growth

The early stage of vegetative growth can be called as seedling stage. The seedlinggrowth period starts with autotrophic life of the emerged seedlings and ends withinitiation of tillering or branching. During seedling stage or juvenile stage of the crop,there is no competition for light, nutrients and moisture among them. Leaf and rootare the components that are growing at this stage.

During emergence, the developing growing point is raised to near ground level byelongation of internode above coleoptile. The next 2 to 3 internodes in accropetalsuccession remain arrested and thickened to form crown of the plant. Subsequently,from this crown, both tillers and adventitious roots arise. During seedling growth,seminal roots penetrate downwards and branch at lower layers of the soil. Seminal

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roots support the plant during the juvenile stage. As there are no adventitious rootsduring this stage, the seedlings are uprooted easily with high winds. If the seminalroots cannot succeed in coming in contact with receding soil moisture, the seedlingmay die. Once the crop passes through the seedling stage the possibility ofreduction in number of plants per unit area is far less.

5.3.3. Leaf Growth

In cereals and grasses, the tip of the leaf (distal region) matures earlier while basalregion is still growing. The photosynthates prepared in the distal region are utilized inthe basal portion and part of it may be exported to other plant parts. A similarsequence of events is also seen in the leaves of dicotyledonous plants. The rate ofcell division and expansion are more in earlier formed leaves. Therefore, relativegrowth rates are less at later stages of crop growth.

Solar radiation, temperature, mineral nutrients and water status are important factorsthat decide the size of the leaf. The size that can be reached i.e. potential leaf size isdecided by solar radiation and temperature but its realisation depends on nutrientsupply. Leaf expansion depends on nitrogen supply and high nitrogen applicationleads to larger leaves. The weight and volume of leaves increase with solar radiationbut leaf area is reduced. Leaf expansion is normal if the relative water content (watercontent of leaves compared to water content at saturation) is about 90-100 per cent.If it falls below 70-75 per cent, leaf expansion stops. The relative water contentleaves are more in young leaves compared to old leaves. Cell expansion is moreaffected by moisture stress than cell division.

The leaf of a dicotyledonous plant depends entirely on the supply of carbohydratesfrom older leaves till it unfolds. Its peak period of export of assimilates is when itreaches full size. Net import of nitrogen, phosphorus and potash continues until theleaf reaches its full size. In the earliest stages, most of the nutrients seem to bereceived from older leaves, but as the leaf grows, higher proportion of the nutrients isobtained directly from the roots. The leaf often becomes a net exporter of thesemajor nutrients at full expansion. The subsequent decrease in physiological activityis no doubt associated with these losses and denotes the beginning of senescence.The wheat lamina imports all of its carbohydrates until emergence and even at fullexpansion it imports one fourth of its dry weight from lower leaves. About half of thedry weight of sheath comes from sources other than the attached lamina.

The plant often produces more carbohydrates than are used immediately in growth.This surplus is stored in various tissues and is utilized when the current suppliesbecome inadequate to support the requirements of growth and respiration. At laterstages, senescence sets in and loss of chlorophyll occurs. Subsequently organelleslike plastids, endoplasmic reticulum, mitochondria etc. and all membranes aredisrupted. Gradual loss of water occurs and finally the leaf dies.

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5.3.4. Tillering and Branching

The growth of auxiliary buds into shoots is called branching. The branches that arisefrom basal nodes of the stem or crown are called tillers especially in cereals andgrasses. Initially, tillers appear at a rapid rate and after maximum number of tillers isproduced, some of the late formed tillers die. The rate of production and number oftiller per plant depend on variety, avai1ability of water, mineral nutrients andphotosynthates. At the time of initiation and early growth tillers depend on olderleaves for photosynthates. However, with the emergence of their own leaves theybecome independent. Death of tillers occurs due to insufficient supply ofphotosynthates, nutrients etc. to terminal bud and primordial leave. If the tillersurvives until the shoot has been induced to flower, death does not occur and normalsequence of f1owering and grain filling proceeds. Generally, tiller production stops atpanicle initiation stage and tiller death continues upto emergence of ear on the mainstem. Successive tillers are smaller and yield small amounts of grain. The lateformed tillers are induced to flower early and the inflorescence develops quickly sothat all tillers may come to maturity at about the same time.

5.4. Measurement of crop growth

Size: Growth in plants is defined as an irreversible increase in volume. The largestcomponent of plant growth is cell expansion driven by turgor pressure. During thisprocess, cells increase in volume many fold and become highly vacuolate. However,size is only one criterion that may be used to measure growth.

Fresh and dry weight: Growth also can be measured in terms of change in freshweight—that is, the weight of the living tissue—over a particular period of time.However, the fresh weight of plants growing in soil fluctuates in response to changesin water status, so this criterion may be a poor indicator of actual growth. In thesesituations, measurements of dry weight are often more appropriate.

Cell number: Cell number is a common and convenient parameter by which tomeasure the growth of unicellular organisms, such as the green algaChlamydomonas. In multicellular plants, however, cell number can be a misleadinggrowth measurement because cells can divide without increasing in volume.

Some of the important parameters for measurement of crop growth are as below:

i. Leaf Area Index: Leaf area is important for photosynthesis. Its estimationindicates both assimilating area and growth .For crop production leaf area per unitland area is more important than leaf area of individual plants. Leaf area index is theratio between leaf areas to land area =

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ii. Leaf Area Duration: Yield of dry matter is a function of leaf area, net assimilationrate and duration of leaf area. Leaf area duration of a crop is a measure of its abilityto produce leaf area on unit area of land throughout its life. LAD is calculated by theformula = + − + … … … … … … … + −iii. Absolute Growth Rate: It indicates at what rate the crop is growing i.e. whetherthe crop is growing at a faster rate or slower rate than normal. It is expressed as g ofdry matter produced per day. = −−Where, W1 and W2 are dry weights of plants at time t1 and t2 respectively.

iv. Crop Growth Rate

It is the rate of growth of crop per unit area and expressed as g/m2/day.= −−Where, P is the land area.

v. Relative growth rate (RGR): is a measure used in plant physiology to quantifythe speed of plant growth. It is measured as the mass increase per abovegroundbiomass per day, for example as g/g/day. It is considered to be the most widely usedway of estimating plant growth, but has been criticized as calculations typicallyinvolve the destructive harvest of plants.

RGR is calculated using the following equation.= ( − )−Where, ln = natural logarithm

t1 = time one (in days)t2 = time two (in days)W1 = Dry weight of plant at time one (in grams)W2 = Dry weight of plant at time two (in grams)

vi. Net Assimilation Rate: It indirectly indicates the rate of net photosynthesis. It isexpressed as g of dry matter production per dm2 of leaf area in a day. For calculatingNAR, leaf area of individual plants has to be used but not leaf area index.= − −− −

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Where L1 and W1 are leaf area and dry weight of plants at time t1 and L2 and W2 areleaf area and dry weight of plants at time t2

vi. Rate of crop dry matter accumulation: Rate of crop dry matter accumulation isthe product of total incident solar radiation (Q, MJ/m2/day), the absorptance ofincident solar radiation by the crop canopy (IA', %), and the efficiency of conversionof absorbed solar radiation into plant dry matter (Є, g dry matter/MJ).

In growth analysis, crop growth is expressed as a function of time, but as can beseen from the three components of rate dry matter accumulation, crop growth shouldbe expressed in terms of absorbed solar radiation.

Absorptance of incident solar radiation (IA')

The penetration or transmission of radiation fluxes in a crop canopy of black leaveswith homogeneously arranged leaves of uniform inclination can by approximated bethe Lambert-

Beer's law of absorption: = –Where, It = Transmitted irradiance at the bottom of a canopy with a leaf area (LAI).

Io = Incident irradiance at the top of the canopy (incident solar radiation).k = The extinction coefficient, which is a function of the leaf angle.

Example 1:

The proportion of incident PPFD that is absorbed by a canopy with LAI = 2 and k=0.65 is 73 %, because:

Transmittance = It' = It/Io = e–k x LAI = e–0.65 x 2 = 0.27

Absorptance = IA' = 1 - transmittance = 1 - 0.27 = 0.73

Example 2:

Transmittance (It') is transmitted radiation as a proportion of incident radiation (It/Io)and absorptance (IA') is absorbed radiation as a proportion of incident radiation(IA/Io). Alternatively, it is possible to estimate the LAI that is required to intercept90% of the incident PPFD when k = 0.65 (when absorptance is 90%, thentransmittance is 100% - 90% = 10%):

It' = It /Io = 0.1 = e –k x LAI

→ ln (It /Io) = ln (0.1) = ln( e–0.65 x LAI )→ ln (0.1) = -0.65 x LAI→ LAI = ln (0.1)/ -0.65 = 3.5

Hence, absorptance (IA') in the yield equation can be estimated from LAI and theextinction coefficient of the leaf canopy.

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viii. Radiation use efficiency (RUE): The efficiency of conversion of absorbed solarradiation into plant dry matter (Є) is related to the mean leaf net photosynthetic rateacross the crop canopy. The mean leaf net photosynthetic rate is called netassimilation rate (NAR), which is estimated from rate of dry matter accumulation andmean LAI. Rather than using mean leaf net photosynthesis, which is not verymeaningful when part of the canopy is sunlit and part is shaded, we can use theincrease in crop dry matter accumulation (or crop growth rate, CGR).

Radiation use efficiency (RUE) is estimated empirically from the crop growth rateduring a period of 2 weeks or more and the amount of photosynthetically activeradiation intercepted by the crop canopy (PARi) during the period crop the growthrate is determined:

RUE = CGR / PARi

Where, CGR is in g/m2/day, PARi is in MJ/m2/day, and RUE is in g/MJ The value ofincident PAR is approximately 50% of the value of Q.

The main variables that influence RUE are maximum gross leaf photosynthetic rate,crop respiration rate, stresses that affect leaf photosynthetic rate, and changes inleaf photosynthetic rates associated with phase of development.

Crop RUE Crop RUE

sugar cane 4.0 sorghum 2.6

maize 3.3 barley 2.6

potato 3.3 soybean 2.1

sunflower 3.1 peanut 2.1

wheat 2.9 Grain legumes < 2.0

rice 2.8

5.5. Measurement of plant growth with the help of Arc Auxanometer

Materials: Plotted plant and Arc Auxanometer

Principle

Growth is a permanent change in plants with respect to its size, form, weight andvolume. In case of an arc-auxanometer, there is a wire fixed with the plant apex onone end and a dead-weight on the other. It passes over a pulley which has a pointerattached to it. When the plant's height increases, the pulley rotates and the pointermoves on a circular scale to directly give the magnitude of growth. Sensitiveauxanometers allow measurement of growth as small as a micrometer, allowingmeasurement of growth in response to short-term changes in atmosphericcomposition.

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Procedure

i. An Arc Auxanometer consists of vertical stand with pulley connected to apointer showing movement on the graduated arc scale.

ii. A thread is passed over the pulley with one end tide to the growing point ofthe plant and the other end carrying a weight to keep the thread stretched.

iii. The whole experiment is kept for one or two days and observed.

iv. With the growth in plants, pulley moves downwards resulting in change inpointer showing movement on the ground arc scale.

v. A downward movement of pointer on the graduated arc scale indicates therate of plant growth. Measure the growth on the arc scale.

Precautions

i. Attach pulley with the growing point of the plant very carefully.

ii. The potted plant should be healthy

5.6. Measurement of root depth

Materials: Root auger, hammer, muslin cloth, forceps, hand lens, running water

Principle

Root depth can be measured by taking soil core samples with the help of root augertill the live roots are found.

Procedure

i. Select the location near the crop plant where root depth is to be measured.

Arc Auxanometer

48

Procedure

i. An Arc Auxanometer consists of vertical stand with pulley connected to apointer showing movement on the graduated arc scale.

ii. A thread is passed over the pulley with one end tide to the growing point ofthe plant and the other end carrying a weight to keep the thread stretched.

iii. The whole experiment is kept for one or two days and observed.

iv. With the growth in plants, pulley moves downwards resulting in change inpointer showing movement on the ground arc scale.

v. A downward movement of pointer on the graduated arc scale indicates therate of plant growth. Measure the growth on the arc scale.

Precautions

i. Attach pulley with the growing point of the plant very carefully.

ii. The potted plant should be healthy

5.6. Measurement of root depth

Materials: Root auger, hammer, muslin cloth, forceps, hand lens, running water

Principle

Root depth can be measured by taking soil core samples with the help of root augertill the live roots are found.

Procedure

i. Select the location near the crop plant where root depth is to be measured.

Arc Auxanometer

48

Procedure

i. An Arc Auxanometer consists of vertical stand with pulley connected to apointer showing movement on the graduated arc scale.

ii. A thread is passed over the pulley with one end tide to the growing point ofthe plant and the other end carrying a weight to keep the thread stretched.

iii. The whole experiment is kept for one or two days and observed.

iv. With the growth in plants, pulley moves downwards resulting in change inpointer showing movement on the ground arc scale.

v. A downward movement of pointer on the graduated arc scale indicates therate of plant growth. Measure the growth on the arc scale.

Precautions

i. Attach pulley with the growing point of the plant very carefully.

ii. The potted plant should be healthy

5.6. Measurement of root depth

Materials: Root auger, hammer, muslin cloth, forceps, hand lens, running water

Principle

Root depth can be measured by taking soil core samples with the help of root augertill the live roots are found.

Procedure

i. Select the location near the crop plant where root depth is to be measured.

Arc Auxanometer

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ii. Insert the root auger into the soil with the help of hammer to the depth asper the length of the core of the auger.

iii. Rotate the auger and pull it carefully.

iv. Remove the soil from the auger and keep it in the muslin cloth.

v. Wash the soil sample in the running water until all the soil is washed away.

vi. Observe the live roots on the muslin cloth with the help of hand lens andpluck the roots.

vii. Repeat the process depth wise until the live roots are found in the soilsamples.

viii. The depth upto which the live roots are found will be the rooting depth ofthe crop.

5.7. Measurement of root density

Materials: Root auger, hammer, muslin cloth, forceps, hand lens, running water,oven, electronic balance

Principle

The fresh root samples separated depth-wise are sun dried and then oven dried at65°C till they attain constant weight. The root density is calculated as the dry weightof the roots samples per unit depth.

Procedure

i. Take the soil sample depth-wise (0-15, 15-30, 30-45, 45-60 cm) with thehelp of root auger of known volume near the crop and keep themseparately in sample bags.

ii. Separate the live roots depth wise as procedure in the section 5.6

iii. Keep the roots in envelops, sun dry and oven dry them at 65°C untilconstant weigh is attained.

iv. Weigh the roots in electronic balance.

v. Work out the root density depth-wise with following formula:

Weight of the roots (g)Root density (g/cc) = ------------------------------------------

Volume of the soil (cc)

5.8. Measurement of root growth with Soil Profile Scan System

CI-600 Soil Profile Scan System is designed to scan soil profile and living roots in thesoil. It consists of a rotating linear scan head, a notebook computer and Plexiglasclear tubes.

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Procedure

i. Select the location near the crop plant where root growth is to bemeasured.

ii. Bury or plug the tube into the soil upto the desired depth.

iii. When it is ready to scan, just insert the scan head into the tube.

iv. The computer will control the scan head to automatically rotate the linearCCD element, scanning along the inside wall of the tube. Each scan willcover a 230 mm length of the tube and takes about 10 seconds. Thereadings are directly recorded in the system.

v. Move the scan head to different depths to scan different sections of thesoil profile.

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Chapter-6

EVAPOTRANSPIRATION

Evaporation and transpiration occur simultaneously and there is no easy way ofdistinguishing between the two processes. Apart from the water availability in thetopsoil, the evaporation from a cropped soil is mainly determined by the fraction ofthe solar radiation reaching the soil surface. This fraction decreases over thegrowing period as the crop develops and the crop canopy shades more and more ofthe ground area. When the crop is small, water is predominately lost by soilevaporation, but once the crop is well developed and completely covers the soil,transpiration becomes the main process. At sowing nearly 100% of ET comes fromevaporation, while at full crop cover more than 90% of ET comes from transpiration.

Units

The evapotranspiration rate is normally expressed in millimetres (mm) per unit time.The rate expresses the amount of water lost from a cropped surface in units of waterdepth. The time unit can be an hour, day, decade, month or even an entire growingperiod or year. As one hectare has a surface of 10000 m2 and 1 mm is equal to0.001 m, a loss of 1 mm of water corresponds to a loss of 10 m3 of water perhectare. In other words, 1 mm/day is equivalent to 10 m3/ha/day. Summary of theunits used to express the evapotranspiration rate and the conversion factors is givenbelow:

depth volume per unit area energy per unitarea *

mm/day m3/ha/day lit/sec/ha MJ/m2/day

1 mm day-1 1 10 0.116 2.45

1 m3/ha/day 0.1 1 0.012 0.245

1 lit/sec/ha 8.640 86.40 1 21.17

1 MJ/m2/day 0.408 4.082 0.047 1

* For water with a density of 1000 kg/m3 and at 20°C.

6.1. Reference crop evapotranspiration (ETo)

The evapotranspiration rate from a reference surface, not short of water, is called thereference crop evapotranspiration or reference evapotranspiration and is denoted asETo. The reference surface is a hypothetical grass reference crop with specificcharacteristics. The concept of the reference evapotranspiration was introduced tostudy the evaporative demand of the atmosphere independently of crop type, crop

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development and management practices. As water is abundantly available at thereference evapotranspiring surface, soil factors do not affect ET.

The only factors affecting ETo are climatic parameters. Consequently, ETo is aclimatic parameter and can be computed from weather data. ETo expresses theevaporating power of the atmosphere at a specific location and time of the year anddoes not consider the crop characteristics and soil factors. The FAO Penman-Monteith method is recommended as the sole method for determining ETo. Themethod has been selected because it closely approximates grass ETo at the locationevaluated, is physically based, and explicitly incorporates both physiological andaerodynamic parameters. Moreover, procedures have been developed for estimatingmissing climatic parameters.

Average ETo for different agroclimatic regions (mm/day)

Regions Mean daily temperature (°C)

Cool (~10°C) Moderate (20°C) Warm (> 30°C)

Tropics and subtropics

i. humid and sub-humid 2 - 3 3 - 5 5 - 7

ii. arid and semi-arid 2 - 4 4 - 6 6 - 8

Temperate region

i. humid and sub-humid 1 - 2 2 - 4 4 - 7

ii. arid and semi-arid 1 - 3 4 - 7 6 - 9

Reference crop evapotranspiration

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6.2. Crop evapotranspiration under standard conditions (ETc)

The crop evapotranspiration under standard conditions, denoted as ETc, is theevapotranspiration from disease-free, well-fertilized crops, grown in large fields,under optimum soil water conditions, and achieving full production under the givenclimatic conditions.

Crop evapotranspiration can be calculated from climatic data and by integratingdirectly the crop resistance, albedo and air resistance factors in the Penman-Monteith approach. As there is still a considerable lack of information for differentcrops, the Penman-Monteith method is used for the estimation of the standardreference crop to determine its evapotranspiration rate, i.e., ETo. Experimentallydetermined ratios of ETc/ETo, called crop coefficients (Kc), are used to relate ETc toETo or ETc = Kc ETo.

6.3. Crop evapotranspiration under non-standard conditions (ETc adj)

The crop evapotranspiration under non-standard conditions (ETc adj) is theevapotranspiration from crops grown under management and environmentalconditions that differ from the standard conditions. When cultivating crops in fields,the real crop evapotranspiration may deviate from ETc due to non-optimal conditionssuch as the presence of pests and diseases, soil salinity, low soil fertility, watershortage or waterlogging. This may result in scanty plant growth, low plant densityand may reduce the evapotranspiration rate below ETc.

The crop evapotranspiration under non-standard conditions is calculated by using awater stress coefficient Ks and/or by adjusting Kc for all kinds of other stresses andenvironmental constraints on crop evapotranspiration.

6.4. Measurement of evapotranspiration

6.4.1. Energy balance and microclimatological methods

A. Thornthwaite method

Thornthwaite assumed an experimental relationship between mean monthlytemperature and monthly consumptive use (Cu)= .Where, e = Unadjusted PET in cm per month of 30 days each with 12 hr day time

T = mean monthly temperature, °CI = annual heat index which is some of monthly heat indices {I = (T/5)1.514}

a = an empirical exponent computed by the equation= . − . + . + .

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The unadjusted PET is corrected for actual hrs of the day and days of the month withthe help of a table provided by Thornthwaite (1948). This formula considers onlytemperature and not the other weather parameter.

B. Blaney-Criddle method

Blaney and Criddle (1950) observed that CU by crops was closely related with meanmonthly temperature and day light hours. Their empirical formula is given as:= = =Where, U = Seasonal consumptive use in inches

u = monthly consumptive use in inchesk = crop coefficient determined by actual experimentationf = A function of temperature and day light hr=

Where, t = mean monthly temperature in °F andP = monthly day-light hrs expressed as percentage of yearly day-light hrs.f = p (0.46t+8.13) using t°Cf= 25.4 p x t/100 using t°F

Uniqueness of this formula is that it uses crop coefficient ‘f’ which varies with cropand climatic conditions. Not suitable where variability in weather parameters is high.

C. Penman-Monteith Formula

= . ∆ − + + ( − )∆ + ( + . )Where, ETo = Reference evapotranspiration (mm/day)

Rn = Net radiation at crop surface (MJ/m2/day)G = Soil heat flux density (MJ/m2/day)T = Air temperature at 2m height (°C)u2 = Wind speed at 2m height (m/sec)es = Saturation vapour pressure (kPa)ea = Actual vapour pressure (kPa)es-ca = Saturation vapour pressure deficit (kPa)Δ = Slope vapour pressure curve (kPa/°C)

= Psychrometric constant (kPa/°C)

Net radiation (Rn): The net radiation, Rn, is the difference between incoming andoutgoing radiation of both short and long wavelengths. It is the balance between theenergy absorbed, reflected and emitted by the earth's surface or the differencebetween the incoming net shortwave (Rns) and the net outgoing longwave (Rnl)

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radiation. Rn is normally positive during the daytime and negative during thenighttime. The total daily value for Rn is almost always positive over a period of 24hours, except in extreme conditions at high latitudes.

Soil heat flux (G): In making estimates of evapotranspiration, all terms of the energybalance should be considered. The soil heat flux, G, is the energy that is utilized inheating the soil. G is positive when the soil is warming and negative when the soil iscooling. Although the soil heat flux is small compared to Rn and may often beignored, the amount of energy gained or lost by the soil in this process shouldtheoretically be subtracted or added to Rn when estimating evapotranspiration.

Mean saturation vapour pressure (es): As saturation vapour pressure is related toair temperature, it can be calculated from the air temperature. The relationship isexpressed by:

Where, e°(T) = Saturation vapour pressure at the air temperature T [kPa],

T = Air temperature [°C],

exp[..] 2.7183 (base of natural logarithm) raised to the power [..].

Due to the non-linearity of the above equation, the mean saturation vapour pressurefor a day, week, decade or month should be computed as the mean between thesaturation vapour pressure at the mean daily maximum and minimum airtemperatures for that period:

Using mean air temperature instead of daily minimum and maximum temperaturesresults in lower estimates for the mean saturation vapour pressure. Thecorresponding vapour pressure deficit (a parameter expressing the evaporatingpower of the atmosphere) will also be smaller and the result will be someunderestimation of the reference crop evapotranspiration. Therefore, the meansaturation vapour pressure should be calculated as the mean between the saturationvapour pressure at both the daily maximum and minimum air temperature.

Actual vapour pressure (ea) derived from dew point temperature: As the dewpoint temperature is the temperature to which the air needs to be cooled to make theair saturated, the actual vapour pressure (ea) is the saturation vapour pressure at thedewpoint temperature (Tdew) [°C], or:

55

radiation. Rn is normally positive during the daytime and negative during thenighttime. The total daily value for Rn is almost always positive over a period of 24hours, except in extreme conditions at high latitudes.

Soil heat flux (G): In making estimates of evapotranspiration, all terms of the energybalance should be considered. The soil heat flux, G, is the energy that is utilized inheating the soil. G is positive when the soil is warming and negative when the soil iscooling. Although the soil heat flux is small compared to Rn and may often beignored, the amount of energy gained or lost by the soil in this process shouldtheoretically be subtracted or added to Rn when estimating evapotranspiration.

Mean saturation vapour pressure (es): As saturation vapour pressure is related toair temperature, it can be calculated from the air temperature. The relationship isexpressed by:

Where, e°(T) = Saturation vapour pressure at the air temperature T [kPa],

T = Air temperature [°C],

exp[..] 2.7183 (base of natural logarithm) raised to the power [..].

Due to the non-linearity of the above equation, the mean saturation vapour pressurefor a day, week, decade or month should be computed as the mean between thesaturation vapour pressure at the mean daily maximum and minimum airtemperatures for that period:

Using mean air temperature instead of daily minimum and maximum temperaturesresults in lower estimates for the mean saturation vapour pressure. Thecorresponding vapour pressure deficit (a parameter expressing the evaporatingpower of the atmosphere) will also be smaller and the result will be someunderestimation of the reference crop evapotranspiration. Therefore, the meansaturation vapour pressure should be calculated as the mean between the saturationvapour pressure at both the daily maximum and minimum air temperature.

Actual vapour pressure (ea) derived from dew point temperature: As the dewpoint temperature is the temperature to which the air needs to be cooled to make theair saturated, the actual vapour pressure (ea) is the saturation vapour pressure at thedewpoint temperature (Tdew) [°C], or:

55

radiation. Rn is normally positive during the daytime and negative during thenighttime. The total daily value for Rn is almost always positive over a period of 24hours, except in extreme conditions at high latitudes.

Soil heat flux (G): In making estimates of evapotranspiration, all terms of the energybalance should be considered. The soil heat flux, G, is the energy that is utilized inheating the soil. G is positive when the soil is warming and negative when the soil iscooling. Although the soil heat flux is small compared to Rn and may often beignored, the amount of energy gained or lost by the soil in this process shouldtheoretically be subtracted or added to Rn when estimating evapotranspiration.

Mean saturation vapour pressure (es): As saturation vapour pressure is related toair temperature, it can be calculated from the air temperature. The relationship isexpressed by:

Where, e°(T) = Saturation vapour pressure at the air temperature T [kPa],

T = Air temperature [°C],

exp[..] 2.7183 (base of natural logarithm) raised to the power [..].

Due to the non-linearity of the above equation, the mean saturation vapour pressurefor a day, week, decade or month should be computed as the mean between thesaturation vapour pressure at the mean daily maximum and minimum airtemperatures for that period:

Using mean air temperature instead of daily minimum and maximum temperaturesresults in lower estimates for the mean saturation vapour pressure. Thecorresponding vapour pressure deficit (a parameter expressing the evaporatingpower of the atmosphere) will also be smaller and the result will be someunderestimation of the reference crop evapotranspiration. Therefore, the meansaturation vapour pressure should be calculated as the mean between the saturationvapour pressure at both the daily maximum and minimum air temperature.

Actual vapour pressure (ea) derived from dew point temperature: As the dewpoint temperature is the temperature to which the air needs to be cooled to make theair saturated, the actual vapour pressure (ea) is the saturation vapour pressure at thedewpoint temperature (Tdew) [°C], or:

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Vapour pressure deficit (es - ea): The vapour pressure deficit is the differencebetween the saturation (es) and actual vapour pressure (ea) for a given time period.For time periods such as a week, ten days or a month es is computed using the Tmax

and Tmin averaged over the time period and similarly the ea is computed, usingaverage measurements over the period.

Slope of saturation vapour pressure curve (Δ )

For the calculation of evapotranspiration, the slope of the relationship betweensaturation vapour pressure and temperature, , is required. The slope of the curve ata given temperature is given by.

Where, Δ = Slope of saturation vapour pressure curve at air temperature T[kPa °C-1],

T = Air temperature [°C],

exp[..] 2.7183 (base of natural logarithm) raised to the power [..].

Psychrometric constant ( ): The psychrometric constant, , is given by:

Where, = Psychrometric constant [kPa °C-1],

P = Atmospheric pressure [kPa],

λ = Latent heat of vaporization, 2.45 [MJ kg-1],

cp = Specific heat at constant pressure, 1.013 10-3 [MJ kg-1 °C-1],

= Ratio molecular weight of water vapour/dry air = 0.622.

The specific heat at constant pressure is the amount of energy required to increasethe temperature of a unit mass of air by one degree at constant pressure. Its valuedepends on the composition of the air, i.e., on its humidity. For average atmosphericconditions a value cp = 1.013 10-3 MJ kg-1 °C-1 can be used. As an averageatmospheric pressure is used for each location, the psychrometric constant is keptconstant for each location.

6.4.2. Mass transfer method

This approach considers the vertical movement of small parcels of air (eddies) abovea large homogeneous surface. The eddies transport material (water vapour) andenergy (heat, momentum) from and towards the evaporating surface. By assumingsteady state conditions and that the eddy transfer coefficients for water vapour areproportional to those for heat and momentum, the evapotranspiration rate can be

56

Vapour pressure deficit (es - ea): The vapour pressure deficit is the differencebetween the saturation (es) and actual vapour pressure (ea) for a given time period.For time periods such as a week, ten days or a month es is computed using the Tmax

and Tmin averaged over the time period and similarly the ea is computed, usingaverage measurements over the period.

Slope of saturation vapour pressure curve (Δ )

For the calculation of evapotranspiration, the slope of the relationship betweensaturation vapour pressure and temperature, , is required. The slope of the curve ata given temperature is given by.

Where, Δ = Slope of saturation vapour pressure curve at air temperature T[kPa °C-1],

T = Air temperature [°C],

exp[..] 2.7183 (base of natural logarithm) raised to the power [..].

Psychrometric constant ( ): The psychrometric constant, , is given by:

Where, = Psychrometric constant [kPa °C-1],

P = Atmospheric pressure [kPa],

λ = Latent heat of vaporization, 2.45 [MJ kg-1],

cp = Specific heat at constant pressure, 1.013 10-3 [MJ kg-1 °C-1],

= Ratio molecular weight of water vapour/dry air = 0.622.

The specific heat at constant pressure is the amount of energy required to increasethe temperature of a unit mass of air by one degree at constant pressure. Its valuedepends on the composition of the air, i.e., on its humidity. For average atmosphericconditions a value cp = 1.013 10-3 MJ kg-1 °C-1 can be used. As an averageatmospheric pressure is used for each location, the psychrometric constant is keptconstant for each location.

6.4.2. Mass transfer method

This approach considers the vertical movement of small parcels of air (eddies) abovea large homogeneous surface. The eddies transport material (water vapour) andenergy (heat, momentum) from and towards the evaporating surface. By assumingsteady state conditions and that the eddy transfer coefficients for water vapour areproportional to those for heat and momentum, the evapotranspiration rate can be

56

Vapour pressure deficit (es - ea): The vapour pressure deficit is the differencebetween the saturation (es) and actual vapour pressure (ea) for a given time period.For time periods such as a week, ten days or a month es is computed using the Tmax

and Tmin averaged over the time period and similarly the ea is computed, usingaverage measurements over the period.

Slope of saturation vapour pressure curve (Δ )

For the calculation of evapotranspiration, the slope of the relationship betweensaturation vapour pressure and temperature, , is required. The slope of the curve ata given temperature is given by.

Where, Δ = Slope of saturation vapour pressure curve at air temperature T[kPa °C-1],

T = Air temperature [°C],

exp[..] 2.7183 (base of natural logarithm) raised to the power [..].

Psychrometric constant ( ): The psychrometric constant, , is given by:

Where, = Psychrometric constant [kPa °C-1],

P = Atmospheric pressure [kPa],

λ = Latent heat of vaporization, 2.45 [MJ kg-1],

cp = Specific heat at constant pressure, 1.013 10-3 [MJ kg-1 °C-1],

= Ratio molecular weight of water vapour/dry air = 0.622.

The specific heat at constant pressure is the amount of energy required to increasethe temperature of a unit mass of air by one degree at constant pressure. Its valuedepends on the composition of the air, i.e., on its humidity. For average atmosphericconditions a value cp = 1.013 10-3 MJ kg-1 °C-1 can be used. As an averageatmospheric pressure is used for each location, the psychrometric constant is keptconstant for each location.

6.4.2. Mass transfer method

This approach considers the vertical movement of small parcels of air (eddies) abovea large homogeneous surface. The eddies transport material (water vapour) andenergy (heat, momentum) from and towards the evaporating surface. By assumingsteady state conditions and that the eddy transfer coefficients for water vapour areproportional to those for heat and momentum, the evapotranspiration rate can be

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computed from the vertical gradients of air temperature and water vapour via theBowen ratio.

Other direct measurement methods use gradients of wind speed and water vapour.These methods and other methods such as eddy covariance, require accuratemeasurement of vapour pressure, and air temperature or wind speed at differentlevels above the surface. Therefore, their application is restricted to primarilyresearch situations.

6.4.3. Soil water balance

Evapotranspiration can also be determined by measuring the various components ofthe soil water balance. The method consists of assessing the incoming and outgoingwater flux into the crop root zone over some time period. Irrigation (I) and rainfall (P)add water to the root zone. Part of I and P might be lost by surface runoff (RO) andby deep percolation (DP) that will eventually recharge the water table. Water mightalso be transported upward by capillary rise (CR) from a shallow water table towardsthe root zone or even transferred horizontally by subsurface flow in (SFin) or out of(SFout) the root zone. In many situations, however, except under conditions with largeslopes, SFin and SFout are minor and can be ignored. Soil evaporation and croptranspiration deplete water from the root zone. If all fluxes other thanevapotranspiration (ET) can be assessed, the evapotranspiration can be deducedfrom the change in soil water content (Δ SW) over the time period:

ET = I + P - RO - DP + CR ± Δ SF ± Δ SW

Some fluxes such as subsurface flow, deep percolation and capillary rise from awater table are difficult to assess and short time periods cannot be considered. Thesoil water balance method can usually only give ET estimates over long time periodsof the order of week-long or ten-day periods.

Lysimeters

By isolating the crop root zone from its environment and controlling the processesthat are difficult to measure, the different terms in the soil water balance equationcan be determined with greater accuracy. This is done in lysimeters where the cropgrows in isolated tanks filled with either disturbed or undisturbed soil. In precisionweighing lysimeters, where the water loss is directly measured by the change ofmass, evapotranspiration can be obtained with an accuracy of a few hundredths of amillimetre, and small time periods such as an hour can be considered. In non-weighing lysimeters the evapotranspiration for a given time period is determined bydeducting the drainage water, collected at the bottom of the lysimeters, from the totalwater input. A requirement of lysimeters is that the vegetation both inside andimmediately outside of the lysimeter be perfectly matched (same height and leafarea index). This requirement has historically not been closely adhered to in amajority of lysimeter studies and has resulted in severely erroneous and

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unrepresentative ETc and Kc data. As lysimeters are difficult and expensive toconstruct and as their operation and maintenance require special care, their use islimited to specific research purposes.

6.4.4. ET computed from meteorological data

Owing to the difficulty of obtaining accurate field measurements, ET is commonlycomputed from weather data. A large number of empirical or semi-empiricalequations have been developed for assessing crop or reference cropevapotranspiration from meteorological data. Some of the methods are only validunder specific climatic and agronomic conditions and cannot be applied underconditions different from those under which they were originally developed.

As a result of an Expert Consultation held in May 1990, the FAO Penman-Monteithmethod is now recommended as the standard method for the definition andcomputation of the reference evapotranspiration, ETo. The ET from crop surfacesunder standard conditions is determined by crop coefficients (Kc) that relate ETc toETo. The ET from crop surfaces under non-standard conditions is adjusted by awater stress coefficient (Ks) and/or by modifying the crop coefficient.

6.4.5. ET estimated from pan evaporation

Evaporation from an open water surface provides an index of the integrated effect ofradiation, air temperature, air humidity and wind on evapotranspiration. However,differences in the water and cropped surface produce significant differences in thewater loss from an open water surface and the crop. The pan has proved its practicalvalue and has been used successfully to estimate reference evapotranspiration byobserving the evaporation loss from a water surface and applying empiricalcoefficients to relate pan evaporation to ETo.

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Appendix I: Units and conversion factors for length and area

Length Area1 Meter (m) 100 centimeters

(cm)1 Hectare 10,000 sq. meters, m2

1 Meter 39.37 inches 1 Hectare 1,07,640 sq. feet1 Meter 3.2808 feet 1 Hectare 2.471 acres1 Meter 1.09 yards 1 Hectare 20 kanals1 centimeter (cm) 0.3937 inch 1 Hectare 400 marlas1 Inch (“) 2.54 centimeters 1 Acre 43,560 sq. feet1 Foot (‘) 30.48 centimeters 1 Acre 4840 sq. yards1 Foot 0.3048 meter 1 Acre 4,047 sq meter1 Foot 12 inches 1 Acre 0.4047 hectare1 Yard 91.44 centimeters 1 Acre 8 kanals1 Yard 36 inches 1 Acre 160 marlas1 Yard 3 feet 1 Kanal 20 marlas1 Kilometer (km) 1000 meters 1 Marla 30.25 sq. yards1 Kilometer 0.6214 mile 1 Marla 25.30 sq. meter1 Mile 5280 feet 1 sq. meter 10.764 sq. feet1 Mile 1760 yards 1 sq. cm 0.155 sq. inch1 Mile 1.609 kilometers 1 sq. yard 0.8361 sq. meter1 Nautical mile 1.852 kilometers 1 sq. yard 9 sq. feet1 Nanometer 10-9 meters 1 sq. foot 0.0929 sq. meter1 Micrometer or micron 10-6 meters 1 sq. inch 6.452 sq. centimeter1 Angstrom 10-10 meters 1 sq. kilometer 100 hectares

1 sq. kilometer 0.3861 sq. miles1 sq. kilometer 247.1 acres1 sq. mile 258.99 hectares1 sq. mile 2.59 sq. kilometers

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Appendix II: Units and conversion factors for weight and pressure

Weight Pressure1 Gram (g) 1000 milligram (mg) 1 Bar (b) 106 dynes/cm2

1 Gram 0.0353 ounce 1 Bar 1000 milli bars1 Gram 0.0022 pound 1 Bar 0.987 atmosphere1 Ounce (oz) 28 gram 1 Bar 105 Pascal1 Pound 453.6 gram 1 Milli bar (mb) 103 dynes/cm2

1 Kilogram (kg) 1000 gram 1 Milli bar 100 Pascal (Pa)1 Kilogram 2.205 pounds 1 Atmosphere (atm) 1.013 bars1 Quintal 100 kilograms 1 Kilo Pascal (KPa) 1000 Pascals1 Quintal 220.5 pounds 1 Mega Pascal (MPa) 100 Kilo Pascals1 Metric ton (mt) 1016 kilograms 1 Mega Pascal 106 Pascals1 Metric ton 10 quintals 1 Mega Pascal 10 bars1 Metric ton 1.1 ton (American ton, t)1 Ton (t) 0.91 metric ton

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Appendix III. Conversion factors for water measurement for ET

Volume Flow Rates1 cubic meter 35.314 cubic foot 1 cubic meter/sec

(cumec)35.314 cubic feet/sec(cusec)

1 cubic meter 1.308 cubic yards 1 cubic meter/hr 0.278 litre/sec1 cubic meter 1000 litres 1 cubic meter/hr 4.403 US gallons/min1 Litre 0.0353 cubic foot 1 cubic meter/hr 3.668 Imp. gallons/min1 Litre 0.2642 US gallons 1 litre/sec 0.0353 cubic foot/sec1 Litre 0.2201 Imp. gallons 1 litre/sec 15.852 US gallons/min1 cubic cm 0.061 cubic inch 1 litre/sec 13.206 Imp. gallons/min1 cubic foot 0.0283 cubic meter 1 litre/sec 3.6 cubic meter/hr1 cubic foot 28.32 litres 1 cubic foot/sec 0.0283 cubic meter/sec1 cubic foot 7.40 US gallons 1 cubic foot/sec 28.32 litres/sec1 cubic foot 6.21 Imp. gallons 1 cubic foot/sec 448.8 US gallons/min1 cubic foot 6.39 cubic cm 1 cubic foot/sec 373.8 Imp. gallons/min1 cubic yard 0.7645 cubic meter 1 cubic foot/sec 1 acre inch/hr (approx.)1 US gallon 3.7854 litres 1 cubic foot/sec 2 acre feet/day (approx.)1 US gallon 0.833 Imp. gallons 1 cubic foot/sec 0.06309 litre/sec1 Imp. gallon 1.201 US gallon 1 Imp. gallon/min 0.07573 litre/sec1 Imp. gallon 1.5436 litres1 Acre foot 43560 cubic feet1 Acre foot 1233.5 cubic meter1 Acre inch 3630 cubic feet1 Acre inch 102.8 cubic meter*Imp. = Imperial

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PHOTOSYNTHESIS

1

PHOTOSYNTHESIS

1

PHOTOSYNTHESIS

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MEASUREMENT OF PHOTOSYNTHESIS

2

MEASUREMENT OF PHOTOSYNTHESIS

2

MEASUREMENT OF PHOTOSYNTHESIS

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MEASUREMENT OF PHOTOSYNTHETICALLY ACTIVE RADIATIONS

3

MEASUREMENT OF PHOTOSYNTHETICALLY ACTIVE RADIATIONS

3

MEASUREMENT OF PHOTOSYNTHETICALLY ACTIVE RADIATIONS

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WATER MOVEMENT PROCESSES IN PLANTS

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WATER MOVEMENT PROCESSES IN PLANTS

4

WATER MOVEMENT PROCESSES IN PLANTS

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OPENING AND CLOSING OF STOMATA

5

OPENING AND CLOSING OF STOMATA

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OPENING AND CLOSING OF STOMATA

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METHODS OF ET MEASUREMENT

6

METHODS OF ET MEASUREMENT

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METHODS OF ET MEASUREMENT

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