MECHANISM OF TRANSPIRATION IN PLANTS [THEORIES]

Types of transpiration, Theories of transpiration mechanism

1. This Photo by Unknown Author is licensed under CC BY-SA TRANPIRATION IN PLANTS

2. TRANSPIRATION The loss of water in the form of vapour from living tissue of aerial parts of the plant is called TRANSPIRATION. The loss of water due to transpiration is quite high Almost 98-99% of water absorbed by a plant is lost in transpiration.

3. MAGNITUDE OF TRANSPIRATION 2 Lts /day in SUNFLOWER 36-45 Lts /day in APPLE 54 Gallons in CORN PLANT in one growing season. 30% of rainfall water is lost by deciduous forest.

4. Depending upon the plant surface transpiration is of 4 types: Stomatal transpiration Cuticular transpiration Bark transpiration Lenticular transpiration

5. 1) STOMATAL TRANSPIRATION Most important type of transpiration. Constitutes 50-97% of total transpiration. It is the loss of water through special apertures which are present in leaves called STOMATA. Few stomata are found on green stem, flower and fruits. Stomata expose the wet interior part of plant to the atmosphere. Water vapour therefore, pass outwardly through stomata by diffusion. Stomatal transpiration continues till the stomata are open.

6. STOMATAL APPARATUS Tiny pores found in the epidermis of leaves and other soft aerial parts. Allow exchange of gases between plant and atmosphere. Number of stomata present per cm square 1,000-60,000. In mesophytic plants ,stomata occur both on adaxial and abaxial surfaces . While in grasses and monocots ,their number is almost equal on both the surfaces.

7. STRUCTURE OF STOMATA Each stoma is surrounded by two small specialized cells called guard cells . Guard cells are connected to adjacent epidermal cells by plasmodesmata . They are rapidly influenced by turgor changes . Microfibrils are oriented in a specific way to allow the opening and closing of stomata . Inner walls (concave side) of guard cells is thick and outer wall (convex side) is thin ,hence they are kidney shaped .

8. 2) CUTICULAR TRANSPIRATION It occurs through the cuticle or epidermal cells of leaves and exposed parts of the plant. In common land plants cuticular transpiration is only 3-10% of the total transpiration. In herbaceous shade loving plants where cuticle is very thin, cuticle transpiration may be upto 50% of the total. This transpiration continues throughout day and night.

9. 3) BARK TRANSPIRATION Occurs through corky covering of the stem. Its measured rate is more than lenticular transpiration due to larger area. Cork is generally impermeable. So, water loss occurs through IMBIBITION . Occurs continuously through day and night.

10. 4) LENTICULAR TRANSPIRATION It is found only in woody stems and some fruits where lenticels occurs. Constitutes major part of water loss by deciduous trees during leafless stage. Occurs continuously day and night and there is no mechanism to stop or reduce it. Total water loss through them is only fraction of total i.e. 0.1%.

11. MECHANISM OF STOMATAL MOVEMENT Three main theories : HYPOTHESIS OF GUARD CELL PHOTOSYNTHESIS CLASSICAL STARCH HYDROLYSIS THEORY MALATE ION PUMP HYPOTHESIS (MODERN THEORY) Stomata functions as turgor operated valves because their opening and closing is governed by turgor changes of the guard cells . INCREASED TURGOR Pore is created DECREASED TURGOR Pore is closed

12. HYPOTHESIS OF GUARD CELL PHOTOSYNTHESIS Guard cell contain chloroplast .During daytime chloroplast perform photosynthesis and thus produces sugar . Sugar INCREASES SOLUTE CONCENTRATION of guard cells as compared adjacent epidermal cells . Thus absorption of water by guard cells from epidermal cells . Turgid guard cells bent outwardly and creates a pore in between.

13. CLASSICAL STARCH HYDROLYSIS THEORY Proposed by SAYRE (1923) Guard cells contains starch when stomata are closed STARCH IS CHANGED TO SUGAR . MECHANISM : In the morning carbon dioxide concentration is low (due to increase in rate of photosynthesis ). Reduced carbon dioxide leads to INCRESE IN PH (due to loss of carbonic acid)

14. Hence glucose increases the OSMOTIC PRESSURE of guard cells , withdraw water from epidermal and subsidiary cells . On absorption of water guard cells swell up and pore is created.

15. THE ABOVE REACTION IS REVERSIBLE i.e. sugar changes back into starch This requires LOWERING OF PH. During the evening time, CARBON DIOXIDE FIXATION stops and RESPIRATION INCREASES THE CONCENTRATION of carbon dioxide in guard cells. Decreased Ph brings about PHOSPHORYLATION OF GLUCOSE.

16. As a result osmotic concentration of guard cells is falls. Hence LOSS of TURGIDITY takes place GUARD CELLS SHRINK PORE CLOSES

17. OBJECTIONS: GLUCOSE NOT FOUND in guard cells during stomatal opening. STARCH SUGAR (chemically slow) while opening and closing of stomata are quite RAPID. Wide changes in guard cells not due to carbon dioxide concentration. BLUE LIGHT is more effective for opening of stomata (not explained by STARCH HYDROLYSIS THEORY). There is no visible changes in the starch content of the guard cells during opening and closing movement.

18. Proposed by LEVITT, 1974. Opening of stomata is accompanied by increase in K+ ion concentration. In open state concentration of K+ ion= 400-800mM In closed state =100mM Stomatal opening is stimulated by sunlight, cytokinin cAMP and other factors MALATE or K+ ION PUMP HYPOTHESIS

19. Blue fraction of LIGHT sensitizes PHOTOTROPIN transfers the signals PROTEIN PHOSPHATASE I (PPI) activates H+ ATPase PROTONS PUMPS OUT FROM THE CYTOSOL (interior of plasma membrane become more negative) Voltage regulated K+ channel opens K+ INTAKE BY GUARD CELLS Cl- ions also intakes through PROTON CHLORIDE ANTIPORTER

20. Some proteins are picked by chloroplast and guard cell CHLOROPLAST and MITOCHONDRIA. Decrease in CO 2 concentration & Increase in pH Causes HYDROLYSIS OF STARCH to form ORGANIC ACIDS. STARCH HEXOSE DIPHOSHPHATE PHOSPHOGLYCERIC ACID PHOSPHOPHENOL PYRUVATE

21. PEP + CO 2 OXALOACETIC ACID (unstable) MALIC ACID H+ MALATE ION H+ pass out of the guard cells & K+ ions enters the cell. Cl ion taken from outside to maintain ionic balance. In onion, Cl alone counterbalances K+. The ions (K+, Cl , malate) pass into the small vacuoles and gets stored & produce an osmotic potential. So, guard cell absorbs water from nearby epidermal cells.

22. Due to elastic nature of guard cell walls, they swell up and increase in volume by 40%-100% . A pore is created in between the two guard cells Sucrose is also involved in developing osmotic potential. Stomatal closer occurs towards evening when light begin to fall. In absence of light, H+ATPase of Plasmalemma stops its activities H+ passes out of the guard cell mitochondria and chloroplast.

23. Decrease in pH of the guard cells. Malate ion present in the guard cells combines with H+ to form MALIC ACID. Undissociated malic acid promotes leakage of ions. Anion channel opens Cl ions comes out of the guard cells Plasma membrane depolarised. Voltage gated K+ channel open. Loss of K ions leads to decrease in osmotic concentration of guard cells as compare to epidermal cells . Pore between the guard cell closes.

24. To form vapours, exposed parts of the plant need a source of heat energy. The dry air of the atmosphere has high DPD (or low water potential) - 13.4 atm at 99% R.H. -140 atm at 90% R.H. -680 atm at 60% R.H. 2055 atm at 20% R.H. Such high DPD or low water potential has overcome various resistances which water molecule have to meet from liquid phase to vapour phase. MECHANISM OF WATER LOSS

25. Intercellular spaces of transpiring organs (Saturated with water vapours) When stomata are open, water vapour are drawn from the stomata to the outside air DUE TO HIGH DPD. This INCREASES the DPD of sub-stomatal air, it draws more water vapour from intercellular spaces. The latter get water vapours from the wet walls of mesophyll cells. Stomatal transpiration continues till stomata are open.

26. FACTORS AFFECTING TRANSPIRATION EXTERNAL OR ENVIRNMENTAL FACTORS RELATIVE HUMIDITY AND VAPOUR DEFICIT ATMOSPHERIC TEMPERATURE SOIL TEMPERATURE LIGHT AIR CURRENTS ATMOSPHERIC PRESSURE SUPPLY OF WATER

27. INTERNAL OR PLANT FACTORS LEAF AREA STOMATA LEAF STRUCTURE LEAF ORIENTATION LEAF SIZE AND SHAPE WATER CONTENT OF LEAVES ROOT/SHOOT RATIO

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