Lec 3.Ascent of Sap

LEC 3.ASCENT OF SAP

B.Sc BIOTECHNOLOGY

08.05.2012

TRANSLOCATION OF WATER

Introduction

• Plants absorb water through the roots. This absorbed water reached up to leaves through the stem branches. Most of the water is lost by the transpiration from the leaves. Only small amount of water utilized in vital activities of the plant.

• "The upward movement of absorbed water by the under ground roots towards aerial parts of the plant against the gravitation is called Ascent of sap".

• Process of ascent of sap takes place from a small plant to a quite large plant e.g. in Sequoia semipervense. It has been experimentally proved that water moves upwards through the vessels and tracheids of xylem.

Experiment No. 1 - Balsam

• Take aqueous solution of eosine in a beaker and immerse the cut end of (stock) of balsam plant in it. After sometimes strips of red colour are seen in the stem. The transverse section of this stem confirms because only vessels and tracheids are stained with eosine dye. So that it is proved that the ascent of sap take place through the xylem

Experiment No. 2 - Ringing Experiment

• First of all ringing experiment was carried by Hartig, Malpighi and Stephen Hales.

• We take a branch with leaves immersed in beaker which is filled with water. Now remove all the tissues which are present above the xylem [cortex, cambium and phloem]. Now placed this experiment in a suitable environment for 2 - 3 days. Then we observe that leaves did not wilt. This experiment makes it clear that water moves up only through vessels and tracheids of the xylem.

• Ringing [girdling] experiment has been done only in dicotyledons because their vascular bundles lies in a ring. While vascular bundles are scattered in monocotyledons, so that this experiment can not be possible.

• Structures involved in Ascent of Sap: Various experiments like girdling, staining

and plugging, indicate that the xylem tissue is mainly responsible for the

movement of water. As xylem consists of tracheae otherwise called vessels, they

form a system of fine channels running from roots to all other regions of the

plant body and form a beautifully branched supply system, which is almost

similar to that of arteries in animals.

Theories of Ascent of sap

• Many theories have been put forwards by various co-workers to explain the mechanism of ascent of sap.• (1) Vital Force Theory (2) Root

Pressure Theory (3) Physical Force Theory

(1) Vital force theory

• Vital theories presumes that ascent of sap is due to the vital activity of living cells of the plants. The opinion of the various scientist for ascent of sap as follows :

(A) Westermaier's theory

• Westermaier was the first man who proposed his view about the vital force theory. He explained that force for upward conduction of water is provided by the xylem parenchyma cells and vessels and tracheids are simply acting as water reservoirs.

(B) Theory of Godlewski • He proposed that ascent of sap is brought about by

periodic changes in osmotic pressure of xylem parenchyma and medullary rays. A pumping action starts as a result of this changes that causes ascent of sap. So that a sort of stair case movement of water took place in the plant. Thus it is known as "Relay pump theory" or "Clambering theory".

• Osmotic pressure of parenchyma increased, water from the xylem vessel diffused into it. this water moves up because of atmospheric pressure during this period, value of osmotic pressure of parenchyma decreased (lower down) and hence this water recieved by xylem which is lies at higher level.

(C) Pulsation theory of J.C. Bose • J.C. Bose is known as father of Indian plant

physiology. He proposed pulsation theory.• He assumed that the pulsatory activity like

heart beat is found in the inner cortical cells which are situated just above the endodermis. Resulting of this pulsatory activity, water forced or pumped into cavities (Vessel) of xylem and water moves upward.

• He conducted his experiment on a plant - Desmodium gyrans (Indian telegraph plant) which is member of Papillionaceae family.

• J.C. Bose measured this pulsation activity by "Electric probe" (Galvanometer) apparatus Molish

• A scientist named Molish who supported the pulsation theory of J.C. Bose and he introduced a detail description of pulsation theory.

• According to him the pulsatory activity increased to 14 seconds by the application of narcotics drugs on the plant.

Objections

• The view of vital theory discarded by Straburger and Boucheries. They proved by their experiment that the living cells are not essential for ascent of sap. Because when the living cells are killed (destroyed) by picric acid or HgCl2 solution even then ascent of sap continues.

(2) Root Pressure Theory • This theory was proposed by Priestley.• Root pressure : A positive pressure is present in the xylem

sap of roots.• The term root pressure was coined by Stephan Hales.

Priestley called it "hydrostatic pressure" Root pressure is also known as 'exudation pressure'.

• The highest value of root pressure is found in those plants which are growing in well aerated and sufficient amount of soil during the humidity in the atmosphere.

• Root pressure usually develops during the night when absorption is maximum and transpiration is minimum.

• The maximum value of root pressure falls in the range of 2 - 3 atmosphere.

• The liquid which flows from the freshly decapitated stem of the plant is called bleeding. This is also due to root pressure.

• Guttation is also takes place due to root pressure.

• Ascent of sap through the root pressures only possible in small and herbaceous plants. Therefore it is only applicable for small and herbaceous plants.

• The process of ascent of sap can not be explained by root pressure because it has following limitations

• (1) Gymnosperms lack root pressure and the conifers are very tall plants.

• (2) The maximum transpiration is found during the summer and plants require more water but in that time root pressure is found to be very less.

• (3) The rate of ascent of sap is found rapidly even in the absence of root pressure.

• (4) The rising up of water can not be possible at 2 atmosphere in tall plants. Upto 200 feet height plants require 10 atmosphere root pressure to push the water to the top of the plants.

• (5) A negative root pressure is found in some of the desert plants.• (6) Root pressure is found less in summer and more in winter.• (7) Root pressure takes place in the special conditions when the rate of

absorption is higher and transpiration rate is minimum.• Hence, root pressure is not significant in most of the plants. It is

only effective in low transpiring herbaceous plants.• Therefore, theory explain partially the mechanism of ascent of sap.

(3) Physical Force Theory • According to this theory, the ascent of sap

takes place by the vessels and tracheids through some physical forces. The following theories stated under this heading.

• (A) Capillary force theory :• This theory was proposed by Boehm.

According to him the vessels and tracheid which are present in xylem, acts as capillaries and water rises up by surface tension in their capillaries.

• Limitation of this theory as follows :• (a) Xylem vessels are not freely immersed into the water.• (b) The ends of vessels and tracheids are closed while ends of

capillaries are open.• (c) The diameter of the capillaries should be very thin because

capillary with narrow lumen encourages the entry of more water. But in tall plants the diameter of vessels and tracheids is more. At this height capillary force does not operate.

• (d) Tall plants like Gymnosperms having tracheids instead of vessels which have many transverse septum. Therefore, tracheids are homologous as capillary. But still ascent of sap occurs.

• (e) Lifting power of capillary is not much and can not account for the rise of water exceeding 3 meters. It is capable for few centimeters rise only.

• Capillary theory is completely discarded on the basis of above reasons.

(B) Imbibition force theory • This theory was proposed by Von Sachs.• According to him water rises up in the wall of

the xylem cells through the imbibition end through their lumen. But now it has been clear that water rises up only through the lumen not by the walls.

• The wall of vessels and tracheids of xylem are lignified and lignin is impermeable to water.

• Thus this theory also discarded.

(C) Chain theory

• This theory proposed by Jamin.• According to this theory, the molecules of

water and air are arranged alternatively to form a chain.

• Ascent of sap takes place due to spreading of air in the chain.

(D) Transpiration pull - cohesion force theory

• This theory also known as "Water column theory" or "Cohesive force theory"

• This theory was proposed by Dixon and Jolly.• This is the most accepted theory at the

present time and it accounts satisfactory explanation for the rising of water. This theory is based on the following facts :

(A) Transpiration pull• Water is lost continuously from the leaves of the

plants as a result of transpiration. Water vapour move out from the leaves. As a result of loss of water from the mesophyll cell. Diffusion pressure deficit always remain higher in the mesophyll cells. Due to this mesophyll cells absorb water from adjoining internal mesophyll cells and compensate this loss of water. This loss ultimately compensating by xylem which leads water deficit in the xylem. Rapid transpiration develops a pull or tension in xylem which is called transpiration pull. It is about 20 atmospheres. This pull is called "Negative pressure" because it is develops from aerial parts to the under ground part of the plants.

(b) Cohesion Force of Water

• A force of mutual attraction present between the water molecules is called "Cohesive force".

• Water molecules are held together continuously by cohesive force and to form a continuous water column. This cohesive force is up to 45 . 270 atmosphere.

(c) Adhesive Force• Water molecules are also attached with the wall of

vessels and tracheids through a force called adhesive force.• Both forces work together and maintain the continuity in

between water and cell wall.• Cohesive force and adhesive force works continuously in the

cavity (lumen) of xylem. Both the forces are responsible for maintaining unbroken continuity of water column from the roots to the leaves.

• MacDougal called as "Hydrostatic system" of this continuous water column.

• This water column pulled upwards continuously without breaking from the roots to the leaves by transpiration. Vessels and tracheids of xylem workds as pipe (Tubes).

• The process of ascent of sap a continues through the medium of above mechanisms.

• Evidences in support of Dixon's theory :• (1) Normally, the rate of absorption is equal to the rate of

transpiration water does not come out from the cut end of the stem during transpiration. When this cut end is watered, it absorb water inside the stem. This justifies the water tension in the stem.

• (2) McDougal is found that daily contraction in stem of the trees with the help dendrograph apparatus.

• Maximum contraction is found during a maximum transpiration.

• (3) The maximum value of osmotic pressure of the cells of leaves is found during noon. Because the amount of water is minimum in that time

Objection to Dixon's theory

• (1) The presence of air bubbles in the xylem which may break the continuity of water column.

• (2) Ascent of sap continues even after overlapping cuts.

• * Removal of objections :

• Although air bubbles are present in the xylem, but they are unable to move much distance because

• (i) xylem vessels do not continue for long distances, pits are present between them.

• (ii) Tracheid are also present along the vessels as alternative path. Pits are also present in them.

• (iii) Membrane and wall is present on the pits which is permeable. to water, but air bubbles do not pass out through them. Air bubbles may dissolve in water. Root pressure also helps to remove air bubbles.

• The rate of transpiration is higher than that of rate of ascent of sap during the day time and is called absorption lag.

• Russel and Wooley found the ratio 50 : 1 by comparative study of water movement in apoplast and symplast.

• The origin of root pressure is an active process.• Factors affecting Ascent of sap • (1) Amount of water in soil. (2) High temperature• (3) High atmospheric humidity (4) High

atmospheric pressure• (5) Wind velocity (6) Number of stomata in leaves

Mechanism of Ascent of Sap

VITAL THEORY• Suggestions have been made that living cells adjacent to

conducting pipes of xylem are involved in the translocation of sap.These living cells are postulated to show some kind of pulsating action(just lioke heart beat) and pulsation of cells at successive intervals lifts the sap up. Some proponents of vital theory are

• 1669-Ray-Sap Circulates in Plants like blood in Animals• 1884-Godlewski-Cell pump water• 1927-Bose-Sap ascends by pulsation of cortical

movements• 1938-Dixon and Barlee-Leaves Exude water by vital action

of Leaf cells

• However anatomical studies have failed to show any pumping action in the Xylem.Further numerous experiments have demonstrated clearly that living cells are not involved in the translocation of sap in the stem.

ROOT PRESSURE THEORY• Acc to Root pressure theory,the positive

pressure developed in the roots pumps the sap up in the xyulem ducts.This theory seems to be applcable to many species,which generate considerable amount of root pressure.For example,Grapevines have been shown to generate root pressure up to 5-6 bars,which can support a water column upto 150-160 ft

• However, root pressure cannot account for sap translocation in many situations.In many species,root pressure has not been observed at all and in many others,it is low that it cannot account for upward translocation even upto a few ft.

• COHESION THEORY• BY IRISH BOTANIST HENRI DIXON and a

Physicist Jon Joley(1894)• Supported by E.Askenasy(1895)and

Renner(1895).

• Water has a high Cohesive( Mutual attraction) force.This is primarily because of the formation of Hydrogen bonds between water molecules.Water also shows forces of adhesive with the walls of xylem vessels and tracheids.Because of Cohesive and adhesive forces,it exists as a column in the xylem elements.

Organismal Circulation

Unicellular OrganismsAutotrophic Multicellular Organisms

(Heterotrophic Multicellular Organisms)

Cyclosis in Physarum polycephalum, a slime mold

The correct taxonomic affiliation is unclear.It has been treated as Fungus and Protist.Further study is needed to resolve its position.

This organism consists of one very large cytoplasm (plasmodium) with many nuclei and food vacuoles in the cytosol (coenocytic).

Slime molds can weigh up toward kilogram range and moves their blob-like mass around exclusively by cyclosis.

Here you can see, in a thin region of cytoplasm, that it moves along pathways that are river-like in appearance.

Transport is NOT always unidirectional.http://botit.botany.wisc.edu/courses/img/Botany_130/Movies/Slime_mold.mov

What is the ATP source?

http://www.microscopy-uk.org.uk/mag/imgnov00/cycloa3i.avi

Cyclosis: cytoplasmic streaming…intracellular circulation

Chloroplasts and other organelles have surface proteins with myosin-like activity.

Microfilaments of actin are found just under cell membrane.

ATP and Calcium allow myosin to slide along actin filaments, resulting in circulation of organelles within the cell.

What is the source of ATP?

Can you be more specific?

If light intensity were reduced, what would be the prediction on rate of cyclosis based on your hypothesis?

Elodea canadensis

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Figure 36-3 Page 793

Branch

Root

sys

tem

Shoo

t sys

tem

Stem

Apical budAxillary budNode

Leaves

Node

Internode

Lateral roots

Taproot

The shoot organ system is photoautotrophic, taking in CO2 and releasing O2 in daylight.

The root organ system is chemoheterotrophic, taking in O2 and releasing CO2 in the darkness of the soil environment.

Diffusion is sufficient to exchange gases. But solutes need to be circulated in the large plant body as diffusion is too slow!!

O2 in and CO2 out

CO2 in and O2 out

O2 in and CO2 out


Branch

Root

sys

tem

Shoo

t sys

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Stem


Leaves

Node

Internode

Lateral roots

Taproot

The shoot system produces carbohydrates (etc.) by photosynthesis. These solutes are transported to the roots in the phloem tissue:

Translocation

The root system removes water and minerals from the soil environment. These solutes are transported to the shoot in the xylem tissue:

TranspirationWater and Minerals

Transpiration

Carbohydrate etc.

Translocation


Branch

Root

sys

tem

Shoo

t sys

tem

Stem


Leaves

Node

Internode

Lateral roots

Taproot

Because these pathways involve solutes in water passing in the adjacent tissues of a narrow vascular bundle, this is a circulation system!

Transpiration and Translocation

The water is moving up the xylem, and down the phloem, making a full circuit!

Water and Minerals

Transpiration

Carbohydrate etc.

Translocation


Cortex

Cross section of a eudicot stem Cross section of a monocot stem

Epidermis

PithGround tissue

Vascular bundles

Plants occur in two major groups (and some minor ones)

They differ, in part, in their circulation systems:

Dicots initially have one ring of vascular bundles

Monocots rapidly develop multiple, concentric, rings of vascular bundles

Monocot stem anatomy

Young Monocot

Mature Monocot

vascular bundles

As a monocot plant grows in diameter, new bundles are added toward the outside for increased circulation to the larger plant body.

©1996 Norton Presentation Maker, W. W. Norton & Company

Monocot stem anatomy Is this slice from a young or a mature part of the corn stem?

Let’s take a closer look at the vascular tissues


Monocot stem anatomy: vascular bundle

Translocation

Transpiration

Why must xylem do a lot more transport than phloem?


Dicot circulation: stem anatomy

Dicots start with one ring of bundles…

Let’s take a closer look at the vascular tissues


Dicot stem anatomy: vascular bundle

phloem fibers

functional phloem

vascular cambium

xylem

Translocation

Transpiration

Support of Stem

As a dicot grows, how does it add vascular capacity to become a tree?

Cell Divison: More Xylem and Phloem


Dicot stem anatomy: vascular cambium adds secondary tissues

epidermiscortex1º phloem2º phloemcambium2º xylem1º xylempith


Dicot stem anatomy: vascular cambium adds secondary tissues

Each year the vascular cambium make a new layer of secondary xylem and secondary phloem


Dicot stem anatomy: four year-old stem (3 annual growth rings)

xylem = wood

phloem etc. = bark

All of these tissues were added by the vascular cambium!

Figure 36.29 Page 810 See also part (a)

or less competition in forest?

or more competition in forest?

cambium

phloem

Figure 36.0 Page 791

sapw

ood

heartwood

periderm phloem cambium = bark

pith


Two Xylem Conducting Cells: tracheid developmental sequence

Annular Helical PittedWhen flowering plants are young, water needs are limited, tracheids suffice.

The walls are strengthened with secondary thickenings including lignin.

Protoxylem have stretchable annular or helical thickenings.

Metaxylem have reticulate or pitted and fully rigid walls.

Tracheids have end walls and flow between cells is through pits.


plesiomorphic apomorphic

Compare Fig. 36.26 Page 806Two Xylem Conducting Cells: xylem vessel evolution

As flowering plants age and grow, water needs increase, and tracheids need to be supplemented.

Flowering plants evolved xylem cells with larger cell diameter and perforated end walls to increase water flow.

Vessels have perforated end walls or lack end walls, but lateral flow between cells is still through pits.

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Dicot stem anatomy: xylem parenchyma, vessels, and tracheids

The huge vessel transports lots of water longitudinally, and shows lots of pits for lateral transport

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Dicot stem anatomy: xylem parenchyma, vessels, and tracheids

The huge vessel transports lots of water longitudinally, and shows lots of pits for lateral transport

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Secondary xylem: cross sections of three different species

Vessels, Tracheids have different distribution patterns.Some produce big vessels only in spring wood

Others produce vessels year-round.


Dicot stem anatomy: woody stem circulation

O2 in and CO2 out

This sketch is showing the importance of lateral transport.

In both transpiration and translocation materials must move radially to the interior and to the exterior as well as up and down the plant.

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nyDicot stem anatomy: 2-year old stem showing ray and periderm

Rays transport sugar from the phloem toward the interior… …to keep pith and xylem parenchyma fueled.Rays transport water and minerals from the xylem to the exterior… …to keep the periderm, cortex, and phloem parenchyma hydrated.

phloem


Xylem and Phloem: tissues with many cell types but conduction function

main transpiration flow

main translocation flow

radial transport

toward pith

toward cortex

Mendocino Tree (Coastal Redwood) Sequoia sempervirens

Ukiah, California

112 m tall (367.5 feet)!

This tree is more than ten times taller than is “theoretically possible” based solely upon the length of the column of uncavitated water.

How could this be achieved?

http://www.nearctica.com/trees/conifer/tsuga/Ssemp10.jpg

Transpiration in a tall tree has at least 3 critical components:

Evaporation: pulling up water from above

Capillarity: climbing up of water within xylem

Root Pressure: pushing up water from below

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nyTranspiration: root pressure (osmotic “push”)

guttation

Solutes from translocation of sugars accumulate in roots.

Water from the soil moves in by osmosis.

Accumulating water in the root rises in the xylem.

Water escapes from hydathodes.

This is not “dew” condensing!

Transpiration: root pressure (osmotic “push”)

http://img.fotocommunity.com/photos/8489473.jpg

The veins (coarse and fine) show that no cell in a leaf is far from xylem and phloem (i.e.water and food!).

The xylem of the veins leaks at the leaf margin in a modified stoma called the hydathode.

These droplets are xylem sap.

Root pressure accounts for maybe a half-meter of “push” up a tree trunk.

Capillarity: maximum height of unbroken water column

The small diameter of vessels and tracheids and the surface tension of water provide capillary (“climb”).

Cohesion of water, caused by hydrogen bonds, helps avoid cavitation.

A tree taller than 10.4 m would need some adaptations to avoid “cavitation”

atmospheric pressure keeps water in tube

gravity pulls water down

vacuum created

10.4m

glass tube

water


Conifer stem anatomy: pine xylem tracheids with pits, xylem rays

tracheids with pits

ray parenchyma

vascular cambium

In spite of the limitations of tracheids-only xylem, conifers are among the tallest of trees!


Conifer stem anatomy: bordered pits as “check-valve” for flow

These pit features allow conifers to be very tall and still avoid cavitation in their xylem cells.

As pressures change between adjacent cells, the torus movement blocks catastrophic flow that would result in cavitation.

Plow

Phigh

Transpiration: evaporation (“pull”)

76 cm

vacuum

mercury

water

mercuryWater

evaporating from a porous clay cap also

lifts the mercury!

Transpiration can lift the

mercury above its normal cavitation

height!

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Grown in 32PO4 (radioactive phosphorus) 1 hour

“Cold” medium 6 hours “Cold” medium 90 hours

Is phosphate uptake from soil:transpiration or translocation?In xylem or phloem?

Is phosphate mobilization from lower leaf:transpiration or translocation?In xylem or phloem?

note: fading

new growth black

Modified from: ©1996 Norton Presentation Maker, W. W. Norton & Company

Translocation: How solutes move in phloem

plasmodesmataac

tive

tran

spor

t

Leaf

Root

High Pressure

Low Pressure

Modified from: ©1996 Norton Presentation Maker, W. W. Norton & Company

Translocation: How solutes move bidirectionally in phloem

Leaf sugars amino acids

Developing leaves, apical bud, flowers fruits

Lateral buds, stems, roots, root tip

High Pressure

Low Pressure

Low Pressure


Branch

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Leaves

Node

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Lateral roots

Taproot

Root Pressure:Water moves into the root because of solutes from phloem.Pressure pushes the water up the stem.

Water and Minerals

Transpiration

Carbohydrate etc.

Translocation

Transpiration

Capillarity:Water climbs in the xylem cell walls by adhesion.Water molecules follow by cohesion.

Evaporation:Water evaporates from mesophyll into atmosphere.Water molecules are pulled up the xylem by virtue of cohesion.


Branch

Root

sys

tem

Shoo

t sys

tem

Stem


Leaves

Node

Internode

Lateral roots

Taproot

Pressure = Bulk FlowThe pressure gradient forces phloem sap away from leaves to all sinks (bidirectionally).

Water and Minerals

Transpiration

Carbohydrate etc.

Translocation

Translocation

Root (etc.) = SinksSolutes removed from phloem by active transport.Water follows by osmosis, reducing pressure.

Leaf = SourcePhotosynthesis produces solutes.Solutes loaded into phloem by active transport.Water follows by osmosis, increasing pressure.

Thank you!

Next Lecture:TRANSPIRATION

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Lec 3.Ascent of Sap