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Essential idea: structure and function are correlated in the xylem of plants.
9.1 Transport in the xylem of plants AHL
One of the key structural features of xylem is the rings of lignin (seen in the lower power scanning EM image). The lignified walls of xylem help them to withstand the very low pressure inside the xylem which drives the transpiration pull.
http://www.nsf.gov/news/mmg/media/images/Sel-lower1_70363.jpg
Understandings, Applications and SkillsStatement Guidance
9.1.U1 Transpiration is the inevitable consequence of gas exchange in the leaf.
9.1.U2 Plants transport water from the roots to the leaves to replace losses from transpiration.
9.1.U3 The cohesive property of water and the structure of the xylem vessels allow transport under tension.
9.1.U4 The adhesive property of water and evaporation generate tension forces in leaf cell walls.
9.1.U5 Active uptake of mineral ions in the roots causes absorption of water by osmosis.
9.1.A1 Adaptations of plants in deserts and in saline soils for water conservation.
9.1.A2 Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.
9.1.S1 Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
9.1.S2 Measurement of transpiration rates using potometers. (Practical 7)9.1.S3 Design of an experiment to test hypotheses about the effect of
temperature or humidity on transpiration rates.
Review 2.2.U2 Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water.
Nature of science:Use theories to explain natural phenomena—the theory that hydrogen bonds form between water molecules explains the properties of water. (2.2)
Cohesion:• This property occurs as a result of the polarity of a water molecule
and its ability to form hydrogen bonds• Although hydrogen bonds are weak the large number of bonds
present (each water molecule bonds to four others in a tetrahedral arrangement) gives cohesive forces great strength
• Water molecules are strongly cohesive (they tend to stick to one another)
http://ib.bioninja.com.au/standard-level/topic-3-chemicals-of-life/31-chemical-elements-and.html
Water droplets form because the cohesive forces are trying to pull the water into the smallest possible volume, a sphere.
Surface tension is caused by the cohesive hydrogen bonding resisting an object trying to penetrate the surface.
n.b. capillary action involves cohesion and adhesion and so dealt with under adhesion.
Review 2.2.U2 Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water.
Nature of science:Use theories to explain natural phenomena—the theory that hydrogen bonds form between water molecules explains the properties of water. (2.2)
Adhesion:• This property occurs as a result of the polarity of a water molecule
and its ability to form hydrogen bonds• Water molecules tend to stick to other molecules that are charged
or polar for similar reasons that they stick to each other • Again similarly individual hydrogen bonds are weak, but large
number of bonds gives adhesive forces great strength
Water droplets stick to surface and seem to defy gravity because of form because the adhesive forces that bond them to the surface of the grass blade.
http://click4biology.info/c4b/9/plant9.2.htm
Capillary action is caused by the combination of adhesive forces causing water to bond to a surface, e.g. the sides of a xylem vessel and the cohesive forces bonding water molecules together. Capillary action is helpful in the movement of water during transpiration and also when you drink using a straw.
http://commons.wikimedia.org/wiki/File:GemeineFichte.jpg
9.1.U2 Plants transport water from the roots to the leaves to replace losses from transpiration. AND 9.1.U3 The cohesive property of water and the structure of the xylem vessels allow transport under tension.
http://www.biology.ualberta.ca/facilities/multimedia/uploads/alberta/transport.swf
9.1.U2 Plants transport water from the roots to the leaves to replace losses from transpiration. AND 9.1.U3 The cohesive property of water and the structure of the xylem vessels allow transport under tension.
This attraction allows water ‘stick’ to the xylem vessel and hence move upwards.
9.1.U4 The adhesive property of water and evaporation generate tension forces in leaf cell walls.
In Summary:• The loss of water from the top of xylem vessels due to evaporation lowers the
pressure inside the vessel and pulls more water into the vessel due to cohesion• Adhesion attracts water molecules to the walls of xylem and vice versa.• Therefore as the water moving upwards (similarly to cohesion) it pulls inward on the
walls of the xylem vessels generating tension - try sucking on a straw when the bottom end is closed.
Edited from: http://www.slideshare.net/gurustip/transport-in-angiospermophytes
9.1.U1 Transpiration is the inevitable consequence of gas exchange in the leaf.
http://passel.unl.edu/pages/animation.php?a=transpiration.swf&b=1094667161
http://www.nsf.gov/news/mmg/media/images/Sel-lower1_70363.jpg
9.1.U3 The cohesive property of water and the structure of the xylem vessels allow transport under tension.
Cell walls are thickened to make them stronger
Wall are impregnated with lignin*. Lignin may be deposited in different ways, such as spirals or rings.
*Lignin is a complex fibrous organic polymer which is strong and rigid. It makes plant stems woody.
Strengthened xylem walls can withstand very low internal pressures without collapsing.
Xylem cells are arranged end to end to form continuous vessels. The reduction of the walls between cells in a vessel makes it easier for water to move between cells
Xylem cells contain no cytoplasm this provides a larger lumen making water transport more efficient. However because the cells are are non-living water transport must be a passive process.
Can you suggest a function of the pits in the cell walls?
9.1.S1 Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
In a cross section (transverse section, TS) of a stem each vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. Xylem vessels can be identified by their large empty lumens and the thickened cell walls.
n.b. Some plant stems, such as monocotyledons, do not possess a cambium and so it is not easy to distinguish between the cortex and pith (both are usually labeled together). In these stems the vascular bundles are not arranged in a ring.
Edited from: http://www.slideshare.net/gurustip/transport-in-angiospermophytes
9.1.S1 Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
Task: draw tissue diagrams of the light micrograph and label the different tissues you can identify.
http://plantphys.info/plant_physiology/images/stemvb.jpg
Species unknown
9.1.S1 Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
Task: draw tissue diagrams of the light micrograph and label the different tissues you can identify.
Zea mays (Corn) stem
http://emp.byui.edu/wellerg/Roots%20and%20Shoots%20Lab/Images/Zea%20Mays%20Stem%20P.jpg
9.1.A1 Adaptations of plants in deserts and in saline soils for water conservation.
9.1.U5 Active uptake of mineral ions in the roots causes absorption of water by osmosis.
Water enters the root hair cells by osmosis
http://www.bbc.co.uk/staticarchive/441a940349a662c2e000ee46215e29024262e92c.gif
http://rajkumarbiology.weebly.com/uploads/9/6/4/2/9642700/431153986.jpg?368
Plants take up water and essential minerals via their roots and thus need a large surface area in order to optimise this uptake.
The root epidermis may have extensions called root hairs which increase surface area for mineral and water absorption
For osmosis to occur there must be a higher concentration of solutes, e.g. mineral ions inside the cell than in the soil water surrounding the roots.A high concentration of solutes means a there is a low concentration of water in the root hair cells compared to the soil water.Therefore water moves down the concentration gradient and enters the root hair cells.
Higher water concentration
Lower water concentration
9.1.U5 Active uptake of mineral ions in the roots causes absorption of water by osmosis.
Active uptake of mineral ions results in a higher concentration of minerals in plants than in the surrounding soil
The use of ATP means that cell must respire (aerobically) to carry out active transport.
9.1.U5 Active uptake of mineral ions in the roots causes absorption of water by osmosis.
Active uptake of mineral ions results in a higher concentration of minerals in plants than in the surrounding soil
You should not need to know the
detailed process by which plant
cells acquire mineral ions.
The use of ATP means that cell must respire (aerobically) to carry out active transport.
9.1.A2 Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.
Water evaporates from the surface of the porous pot
Water is lost from the trough as water moves up the tube to replace water lost from the pot
10m
Place the end of a strip of filter paper in water and the water will gradually move up the paper. What material is paper made from?
Modelling water transport
Place a capillary tube in water and the water moves up the tube - the thinner the tube the higher the water rises.
Setup each model to see it working. For each model discuss both how it models water transport in plants and what its limitations as a model are; when does it cease to be a good representation?
For the porous pot to work there must be an airtight seal between the porous pot and the tube. Additionally allow an air bubble to enter the tube to see water movement more clearly.
Nature of Science: Use models as representations of the real world- mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues. (1.10)
9.1.S2 Measurement of transpiration rates using potometers. (Practical 7) AND 9.1.S3 Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.
Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates.
Potometers vary in design, but all measure transpiration indirectly by looking at the water uptake.
Dependent variable: detail how you will measure transpiration in order that you can calculate a rate.
Independent variable: will it be temperature or humidity? How will this be varied and what range of values will you use?
control variables: What factors could affects the investigation and hence need to be kept constant?
Reliability: how many repetitions do you need? Before answering the question consider how you intend to analyse the data.
http://www.findel-international.com/netalogue/zoom/H24920.jpg
As transpiration occurs a bubble of air moves into the tube and towards the plant (to replace the volume of water transpired).
Essential idea: Structure and function are correlated in the phloem of plants.
9.2 Transport in the phloem of plants AHL
The lower power scanning electron micrograph images show the sieve end plates found on phloem sieve tubes. These perforated walls (in combination with the reduced cytoplasm in sieve cells) gives sieve cells a low resistance to the flow of sap enabling efficient translocation of substances, e.g. sucrose throughout the plant.
http://www.frontiersin.org/files/Articles/26855/fpls-03-00151-r4/image_m/fpls-03-00151-g001.jpg
Understandings, Applications and SkillsStatement Guidance
9.2.U1 Plants transport organic compounds from sources to sinks.
9.2.U2 Incompressibility of water allows transport along hydrostatic pressure gradients.
9.2.U3 Active transport is used to load organic compounds into phloem sieve tubes at the source.
9.2.U4 High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.
9.2.U5 Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.
9.2.A1 Structure–function relationships of phloem sieve tubes.
9.2.S1 Identification of xylem and phloem in microscope images of stem and root.
9.2.S2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.
Review: 9.1.S1 Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
In a cross section (transverse section, TS) of a stem each vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. Xylem vessels can be identified by their large empty lumens and the thickened cell walls.
n.b. Some plant stems, such as monocotyledons, do not possess a cambium and so it is not easy to distinguish between the cortex and pith (both are usually labeled together). In these stems the vascular bundles are not arranged in a ring.
Edited from: http://www.slideshare.net/gurustip/transport-in-angiospermophytes
AKA 9.2.S1 Identification of xylem and phloem in microscope images of stem and root.
Task: draw tissue diagrams of the light micrograph and label the different tissues you can identify.
http://plantphys.info/plant_physiology/images/stemvb.jpg
Species unknown
Review: 9.1.S1 Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
Task: draw tissue diagrams of the light micrograph and label the different tissues you can identify.
Zea mays (Corn) stem
http://emp.byui.edu/wellerg/Roots%20and%20Shoots%20Lab/Images/Zea%20Mays%20Stem%20P.jpg
Review: 9.1.S1 Drawing the structure of primary xylem vessels in sections of stems based on microscope images.
9.2.A1 Structure–function relationships of phloem sieve tubes.
Sieve element cells (form tubes) which transport water and solutes. To be efficient at transport they have reduced quantities of cytoplasm and no nucleus, ribosome or vacuole*.
Companion cells are "life support” for the sieve element cells as they perform certain metabolic functions for sieve elements. Therefore plasmodesmata (microscopic pores in the cell walls) between companion and sieve element cells are larger than in most plant cells to allow for the exchange of metabolites, e.g. ATP.
How is the structure of phloem sieve tubes related to it’s function?
https://www.msu.edu/~walwort8/page2.html#structure
What are sieve element cells and how do they relate to other adjacent tissues?
Diagram of a phloem tissuecross-section
Fibers of sclerenchyma cells and provides structural support for the plant Parenchyma acts as packing
material between other cell types and helps transfer materials to the sieve elements and companion cells
9.2.A1 Structure–function relationships of phloem sieve tubes.
The ends of sieve element cells are connected with other sieve elements together they form a sieve tube.
http://www.uic.edu/classes/bios/bios100/lectf03am/phloem.jpg
How is the structure of phloem sieve tubes related to it’s function?
Sieve plates are found at the interface between two sieve elements cells: they contain large pores in the cell walls speeding the transport of substances between sieve element cells
The rigid cell walls of the sieve tube allow for the building of the high pressures needed to generate hydrostatic pressure (the flow inside the tubes).
How are sieve tubes structured to function?
9.2.U1 Plants transport organic compounds from sources to sinks. AND 9.2.U2 Incompressibility of water allows transport along hydrostatic pressure gradients.
http://highered.mheducation.com/sites/9834092339/student_view0/chapter38/animation_-_phloem_loading.html
http://www.pearsoned.ca/school/science11/biology11/sugartransport.html
Two good animations that help to visualise how plants transport substances such as amino acids and sucrose in phloem vessels.
Watch, take notes and use the rest of the presentation to clarify your understanding and map your notes against objectives.
9.2.U3 Active transport is used to load organic compounds into phloem sieve tubes at the source. AND 9.2.U4 High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect19.htm
• H+ ions are actively transported (using ATP) out of the phloem cell
• High H+ ion concentration gradient builds up outside the cell
• H+ ions flow back into the cell, the energy released is used to co-transport sucrose into the phloem cell.
Sucrose is actively transported into the phloem
High concentrations of solutes cause water uptake by osmosis
• Concentration of sucrose in phloem cells is relatively high
• Consequently the water concentration is relatively low
• Water moves down the concentration gradient from the xylem, through the membrane, into phloem cells, by osmosis
9.2.U2 Incompressibility of water allows transport along hydrostatic pressure gradients. AND 9.2.U5 Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.
• Relatively high concentration of sucrose and water in phloem sieve tubes
• Water is incompressible, i.e. it occupies a fixed volume
• The walls of the sieve tubes are rigid• These two factors cause a build-up of
hydrostatic pressure at the source• Water and the solutes (sucrose and amino
acids) flow down the hydrostatic gradient to the sink where pressure is relatively low
• This is due to the active unloading of sucrose and hence loss of water by osmosis at the sink
Phloem transports water and solutes along hydrostatic pressure* gradients
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect19.htm
*Pressure of water (hydro) before it moves (static)
9.2.S2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.
http://plantsinaction.science.uq.edu.au/sites/plantsinaction.science.uq.edu.au/files/imagecache/figure-medium/plate27AB.TIF-combined.jpg
Aphids possess a stylet: a piercing and sucking mouthpart that is inserted into the plant sieve element to allow the sap to be extracted.
The images shows an aphid feeding and the subsequent flow of sap. The high pressure inside the sieve tube pushes sap into the aphid via the stylet.
Aphids belong to the Hemiptera (true bugs), the only group of insects that have evolved the ability to feed primarily on the plant sap.
Sap is sampled by cutting the aphids stylet after feeding has commensed. The severed stylet remains embedded in the plant and the sap continues to flow.
9.2.S2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.
http://plantsinaction.science.uq.edu.au/sites/plantsinaction.science.uq.edu.au/files/imagecache/figure-medium/plate27AB.TIF-combined.jpg
Using aphids to measure rates of phloem transport.
1. A plant is grown in the lab and one leaf is exposed for a short time to CO
2
containing the radioactive isotope 14C.
2. The 14CO2 will be taken and
incorporated into glucose by the process of photosynthesis. Glucose is converted into sucrose for translocation via the phloem.
3. Aphids are encouraged to feed on the phloem in different locations of the stem at different times.
4. The phloem is then analysed for 14C content and the results can be used to calculate the rate at which substances move through the phloem
Nature of Science: Developments in scientific research follow improvements in apparatus—experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available. (1.8)
http://plantsinaction.science.uq.edu.au/sites/plantsinaction.science.uq.edu.au/files/imagecache/figure-medium/plate27AB.TIF-combined.jpg
Using aphids to measure rates of phloem transport.
1. A plant is grown in the lab and one leaf is exposed for a short time to CO
2
containing the radioactive isotope 14C.
2. The 14CO2 will be taken and
incorporated into glucose by the process of photosynthesis. Glucose is converted into sucrose for translocation via the phloem.
3. Aphids are encouraged to feed on the phloem in different locations of the stem at different times.
4. The phloem is then analysed for 14C content and the results can be used to calculate the rate at which substances move through the phloem
Measurement of phloem rates only became possible after the development of
techniques that radioactively tagged compounds using 14C and then analyse it’s
presence with electronic detectors or photographic film.
https://lh3.googleusercontent.com/-WjkK9THmWQk/VRxnP8OuM4I/AAAAAAAASxk/-6liOy5a2bs/w506-h675/1066991848.jpeg
https://c1.staticflickr.com/3/2347/2573372542_a959ecfd4f_b.jpg
https://bengaltigerc1.files.wordpress.com/2012/08/topiary.jpeg
http://cdn.homedit.com/wp-content/uploads/2013/03/topiary-f1.jpg
Boxwood, Pivet and Yew are plants commonly used for topiary. Topiary involves both encouraging growth in specific directions by use of frameworks and pruning, amongst other techniques. Though human in origin topiary can be considered a change in the environment that causes a change in growth.
9.3 Growth in plants AHL
Essential idea: Plants adapt their growth to environmental conditions.
Understandings, Applications and SkillsStatement Guidance
9.3.U1 Undifferentiated cells in the meristems of plants allow indeterminate growth.
9.3.U2 Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves.
9.3.U3 Plant hormones control growth in the shoot apex. Auxin is the only named hormone that is expected.
9.3.U4 Plant shoots respond to the environment by tropisms.
9.3.U5 Auxin efflux pumps can set up concentration gradients of auxin in plant tissue.
9.3.U6 Auxin influences cell growth rates by changing the pattern of gene expression.
9.3.A1 Micropropagation of plants using tissue from the shoot apex, nutrient agar gels and growth hormones.
9.3.A2 Use of micropropagation for rapid bulking up of new varieties, production of virus-free strains of existing varieties and propagation of orchids and other rare species.
9.3.U1 Undifferentiated cells in the meristems of plants allow indeterminate growth. AND 9.3.U2 Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves.
Growth of a multicellular organism, such as a plant, is produced by either one or a combination of:• Cell enlargement• Increase in cell numbers produced by mitotic cell
division
Edited from: http://www.slideshare.net/gurustip/plant-structure-and-growth-ahl
9.3.U1 Undifferentiated cells in the meristems of plants allow indeterminate growth. AND 9.3.U2 Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves.
9.3.U1 Undifferentiated cells in the meristems of plants allow indeterminate growth. AND 9.3.U2 Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves.
9.3.U3 Plant hormones control growth in the shoot apex.
Auxins are a group of hormones that have a wide range of functions in plants including:• root and shoot growth*• Flowering• fruit development• leaf development• wound response
http://www4.uwsp.edu/biology/courses/botlab/05IA-b-xl%20ColeusTip.jpg
http://plantphys.info/plant_physiology/images/naturalauxins.gif
Hormones are molecules produced by one part of an organism and transported to another to affecting physiological activity, such as growth or metabolism.
*Depending on it’s concentration auxins can either promote or inhibit growth.
9.3.U4 Plant shoots respond to the environment by tropisms.
http://www.kscience.co.uk/animations/auxin.htm
Gravitropism (aka Geotropism) is the response to gravity. This response can be both positive and negative.
Phototropism interactive
animation
Gravitropsimtutorials & animations
Edited from: http://www.slideshare.net/gurustip/plant-structure-and-growth-ahl
http://leavingbio.net/plant%20responses.htm
9.3.U5 Auxin efflux pumps can set up concentration gradients of auxin in plant tissue. 9.3.U6 Auxin influences cell growth rates by changing the pattern of gene expression.
Auxin affects gene expression in shoots:• Cells contain an auxin receptor.• When auxin binds to receptors, transcription of specific genes is promoted.• The expression of these genes causes secretion of hydrogen ions into cell walls.• hydrogen ions loosen connections between cellulose fibres, allowing cell
expansion.
http://www.sciencemag.org/content/312/5775/858/F1.medium.gif
The ways that auxin affect gene expression are varied (e.g. in roots auxin leads to inhibition of growth as opposed to promoting growth in shoots) that and still being researched this is a simple example of one way in which it happens.
9.3.U5 Auxin efflux pumps can set up concentration gradients of auxin in plant tissue. 9.3.U6 Auxin influences cell growth rates by changing the pattern of gene expression.
Different factors can affect transporter proteins and hence the direction in which auxin can move:• The location of transporter proteins can be
changed as the plasma membrane is fluid, e.g. efflux transporters can congregate at the top of cells in roots to move auxin upwards
• Transporter proteins can be activated and/or inhibited by stimuli such as light
http://www.sciencemag.org/content/312/5775/858/F1.medium.gif
Concentration gradients of auxin are necessary to control the direction of plant growth.
Auxin can enter the cell by:• Diffusion• influx transporter proteins
Auxin can move out of the cell by through efflux transporters (called PIN proteins)
This requires that auxin is unevenly transported amongst plant tissues.
9.3.A1 Micropropagation of plants using tissue from the shoot apex, nutrient agar gels and growth hormones.
http://longwoodgardens.org/sites/default/files/wysiwyg/66794.jpghttp://www.bbc.co.uk/staticarchive/a20f41790254c2ae96c013ef544d1f031ad6fc70.jpg
Once roots and shoots are developed, the cloned plant can be transferred to soil.
Micropropagation is used to produce large numbers of identical plants from stock plants*.
*Stock plants possess desired characteristics often derived by selective breeding or genetic modification.
Tissues samples# are sterilized and cut into pieces called explants. The least differentiated material, e.g. shoot apex works the best.
#This process only work because plant tissues are totipotent.
It is important that the media is sterile to prevent the growth of fungi and pathogens.
Hormones stimulate plant growth.
http://longwoodgardens.org/sites/default/files/wysiwyg/66794.jpg
9.3.A2 Use of micropropagation for rapid bulking up of new varieties, production of virus-free strains of existing varieties and propagation of orchids and other rare species.
• Orchids are prized for their beautiful long lasting flowers exhibiting an incredible range of diversity in size, shape and colour.
• It is very difficult to get orchids to breed sexually and to maintain the desired traits• Micropropagation has been so successful that orchids occupy a position as one of the
top ten cut flowers
Micropropagation is used to produce large numbers of identical plants from stock plants.
• Plant virus are transported within the plant through the vascular tissue.• The meristem does not contain vascular tissue• Propagating plants from sterilised vascular tissue produces virus-free
plants
Nature of Science: Developments in scientific research follow improvements in analysis and deduction - improvements in analytical techniques allowing the detection of trace amounts of substances has led to advances in the understanding of plant hormones and their effect on gene expression. (1.8)
http://www.salk.edu/images/pressrelease/2013/619ecker.jpg.png
The interactions between different plant hormones and multiple plant genes are very complex and not fully understood. Our understanding of this area of science is steadily growing due to our ability to detect trace (very small) amounts of molecules.
The commonly held view is that flowering is influenced by abiotic factors such as day length. The importance of biotic factors such as pollination (e.g. by bats), and seed dispersal is being increasingly realised. All factors influence flowering by introducing a selective pressure, which in turn influences the genetics of a population.
9.4 Reproduction in plants AHL
Essential idea: Reproduction in flowering plants is influenced by the biotic and abiotic environment.
http://assets.kew.org/files/assets/KPPCONT_039294.jpg
https://www.nature.nps.gov/biology/invasivespecies/images/houndstongue%20dog.jpg
https://commons.wikimedia.org/wiki/File:Flower_field.jpg
Understandings, Applications and SkillsStatement Guidance
9.4.U1 Flowering involves a change in gene expression in the shoot apex.
9.4.U2 The switch to flowering is a response to the length of light and dark periods in many plants.
9.4.U3
Success in plant reproduction depends on pollination, fertilization and seed dispersal.
Students should understand the differences between pollination, fertilization and seed dispersal but are not required to know the details of each process.
9.4.U4 Most flowering plants use mutualistic relationships with pollinators in sexual reproduction.
9.4.A1Methods used to induce short-day plants to flower out of season.
Flowering in so-called short-day plants such as chrysanthemums, is stimulated by long nights rather than short days.
9.4.S1 Drawing internal structure of seeds.9.4.S2 Drawing of half-views of animal-pollinated flowers.
9.4.S3 Design of experiments to test hypotheses about factors affecting germination.
9.4.U3 Success in plant reproduction depends on pollination, fertilization and seed dispersal.
Reproduction is dependent on the success of each process
9.4.U4 Most flowering plants use mutualistic relationships with pollinators in sexual reproduction.
Pollination is the transfer of pollen from the anther to the stigma.
Cross-pollination (transfer of pollen between different flowers) is preferred as it leads to greater variation in the next generation.
Some flowering plants, e.g. grasses, rely on wind or water for pollination, but most use animals to transfer pollen.
Mutualism is a close association between two organisms where both organisms benefit from the relationship: the animal is rewarded with food in the form of nectar, the plant with successful pollination.
Common types of pollinator include:• birds• bats• Insects, e.g. bees
https://commons.wikimedia.org/wiki/File:Grey-headed_Flying_Fox_%28IMG0526%29.jpg
9.4.U1 Flowering involves a change in gene expression in the shoot apex.
http://ipmb.sinica.edu.tw/IPMB_site1/sites/default/files/field/photo/ft.jpg
Floral initiation is caused by the growth and differentiation of apical cells.
The triggers for the change in gene expression vary between plants, but the most common one is day length (photoperiod)
Cells in the shoot apex change how they divide and differentiate due to changes in their gene expression.
9.4.U2 The switch to flowering is a response to the length of light and dark periods in many plants.
https://smartsite.ucdavis.edu/access/content/user/00002950/bis10v/media/ch19/day_length.swf
9.4.U2 The switch to flowering is a response to the length of light and dark periods in many plants.
9.4.U2 The switch to flowering is a response to the length of light and dark periods in many plants.
http://glencoe.mheducation.com/sites/9834092339/student_view0/chapter41/animation_-_phytochrome_signaling.html
9.4.U2 The switch to flowering is a response to the length of light and dark periods in many plants.
https://smartsite.ucdavis.edu/access/content/user/00002950/bis10v/media/ch19/day_length.swf
9.4.U2 The switch to flowering is a response to the length of light and dark periods in many plants.
9.4.A1 Methods used to induce short-day plants to flower out of season.
Chrysanthemum is short-day plant
For most varieties day length needs to be less than 13 hours for flowers to develop and open
During the summer months (April to September) in temperature countries chrysanthemum will not naturally flower.
This is accomplished by covering the plant with opaque black cloth for 12-15 hours per day. For example place the cover at 5pm and then remove it again at 8am.
Plants need to be covered daily until flower buds begin to show colour.
http://www.ag.auburn.edu/hort/landscape/Potmum.htm
https://commons.wikimedia.org/wiki/File:Chrysanthemum_'Dance'.JPG
9.4.S1 Drawing internal structure of seeds.
9.4.S3 Design of experiments to test hypotheses about factors affecting germination.
https://youtu.be/d26AhcKeEbE
9.4.S3 Design of experiments to test hypotheses about factors affecting germination.
9.4.S3 Design of experiments to test hypotheses about factors affecting germination.
9.4.S3 Design of experiments to test hypotheses about factors affecting germination.
9.4.S3 Design of experiments to test hypotheses about factors affecting germination.
https://youtu.be/pB4ASdELBbQ
Which factors will you investigate (independent variables)? Which need to be kept constant (controlled variables)?
Design an investigation to test how a chosen factor affects germination in a particular species of plant.
Which species of plant/type of seed will you use? It is best to avoid seeds that have long dormancy periods
How will you collect your results, how will you assess whether germination has happened (dependent variable)? How often will you observe your seeds?
Factors affecting germination are numerous, some are specific to particular species such as the Lodgepole pine (Pinus contorta) needs exposure to fire to germinate. A few of the more important factors include:• (soil) temperature• age of the seeds• Hydration / water availability• Level of light, e.g. some seeds need to be buried and therefore require low
light levels
Bibliography / Acknowledgments
Jason de Nys
