Different Theories of Stomatal Regulation of Transpiration
Mechanism of Stomatal Movement:-
> Stomata function as turgor-operated valves because their opening and closing movement is governed by turgor changes of the guard cells.
> Whenever, the guard cells swell up due to increased turgor, a pore is created between them. With the loss of turgor the stomatal pores are closed.
> Stomata generally open during the day and close during the night with a few exceptions.
>The important factors which govern the stomatal opening are light, high pH or reduced CO2 and
availability of water. The opposite factors govern stomatal closure, viz., darkness, low pH or high CO2 and dehydration.
> There are three main theories about the mechanism of stomatal movements:
a. Hypothesis of Guard Cell Photosynthesis:-
- Guard cells contain chloroplasts.
- During day the chloroplasts perform photosynthesis and produce sugar.
- Sugar increases osmotic concentration of guard cells. It causes absorption of water from nearby epidermal cells.
- The turgid guard cells bend outwardly and create a pore in between.
- However, photosynthetic activity of guard cell chloroplasts seems to be negligible.
b. Classical Starch Hydrolysis Theory:-
- The main features of the theory were spelled out by Sayre (1923).
- It was modified by Steward (1964).
- The guard cells contain starch.
- At low carbon dioxide concentration (in the morning achieved through photosynthesis by mesophyll and guard cells), pH of guard cells rises. It stimulates enzyme phosphorylase. Phosphorylase converts starch into glucose 1- phosphate. The latter is changed to glucose 6-phosphate which undergoes hydrolysis to produce glucose and phosphoric acid. Glucose increases osmotic concentration of guard cells. On account of it, the guard cells absorb water from neighbouring cells, swell up and create a pore in between them.
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- Evening closure of stomata is brought about by increased carbon dioxide content (due to stoppage of photosynthesis) of leaf. It decreases pH of guard cells and brings about phosphorylation of glucose. In the presence of phosphorylase, glucose 1-phosphate is changed into starch. As a result, osmotic concentration of guard cells falls. They lose water to adjacent epidermal cells. With the loss of turgidity, the guard cells shrink and close the pore in between them.
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Objections:-
(i) Glucose is not found in guard cells at the time of stomatal opening.
(ii) Starch ↔ Sugar changes are chemically slow while opening and closing of stomata are quite
rapid.
(iii) Wide changes in pH of guard cells cannot be explained on the basis of carbon dioxide
concentration.
(iv) Onion and some of its relatives do not possess starch or related polysaccharide that can be
hydrolysed to the level of glucose.
(v) Blue light has been found to be more effective than other wavelengths for opening of
stomata. The same cannot be explained by starch hydrolysis theory.
(vi) Hydrolysis of starch theory cannot account for high rise in osmotic pressure found in guard
cells.
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c. Malate or K+ ion Pump Hypothesis (Modern Theory):- The main features of the theory were put forward by Levitt (1974).
i. During Stomatal Opening:-
- According to this theory, pH of the guard cell can rise due to active H+ uptake by guard cell chloroplasts or mitochondria, CO2 assimilation by mesophyll and guard cells.
- A rise in pH causes hydrolysis of starch to form organic acids, especially phosphoenol pyruvate. Starch → Hexose Phosphate → Phosphoenol Pyruvate.
- Phosphoenol pyruvate can also be formed by pyruvic acid of respiratory pathway. With
the help of PEP carboxylase (PEP case), it combines with available CO2 to produce oxalic acid
which gets changed into malic acid.
- Malic acid dissociates into H+ and malate. H+ ions pass out of the guard cells actively. In exchange, K+ ions pass inwardly. Same CI– ions may also enter guard cells along with K+ ions.
- Guard cells maintain their electroneutrality by balancing K+ with malate and Cl–.
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- In the combined state they pass into the small vacuoles and increase the osmotic concentration of the guard cells. As a result guard cells absorb water from the nearby epidermal cells through endosmosis, swell up and create a pore in between them.
ii. During Stomatal Closing:-
- The H+ ions diffuse out of the guard cell chloroplasts. It decreases pH of the guard cell cytoplasm.
- Any malate present in the cytoplasm combines with H+ to form malic acid.
- Excess of malic acid inhibits its own biosynthesis. High CO2 concentration also has a similar effect.
- Un-dissociated malic acid promotes leakage of ions. As a result K+ ions dissociate from malate and pass out of the guard cells.
- Formation of abcisic acid (as during drought or midday) also promotes reversal of H+ = ↔ K+ pump and increases availability of H+ inside the guard cell cytoplasm.
- Loss of K+ ions decreases osmotic concentration of guard cells as compared to adjacent epidermal cells. This causes exosmosis and hence turgidity of the guard cells decreases. It closes the pore
between the guard cells. Simultaneously the organic acids are metabolised to produce starch.