Proton concept: Does it help in the opening and closing of stomata?

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The proton concept of stomatal opening believes in the proton transport mediated mechanism which governs both the stomatal opening as well as closure.

The proton concept may be said to be a synthesis of scarth’s pH theory as well as the active K+ ion transport theory proposed by Walker and Zelitch (1963): The proton transport mechanism is as fol­lows.

Mechanism of sto­matal opening:

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There are two mechanisms for the active transport of potassium ion which are supposed to take place in quick succession. These are –

1. Light induced transport of protons (H+) from the cytoplasm into the chloroplasts (during photosynthesis) creates a negative potential which eventu­ally should stop continued movement of proton. As a result of this K+ ions from the surrounding cells move into the cytoplasm of the guard cells. This will once again increase the positive potential and the movement of proton into the chloroplasts continues. As a result of this the pH of cytoplasm (of guard cells) is raised to 8-9, while that of the chloroplast is reduced to 5. At a higher pH CO, changes to HC03 (in the cytoplasm).

2. Due to photosynthetic activity in the chloroplast, starch is produced and it is converted into phosphoenal pyruvic and (PEP) during glycolysis in the chloroplast. PEP diffuses into cytoplasm (of the guard cells) where in the presence of the enzyme PEP carboxylase it (PEP) reacts with HCO, to form organic acids. These organic acids dissociate in the cytoplasm and release protons (H+).

This in turn will favour the influx of K+ ions into the cyto­plasm of guard cells increasing their osmotic pressure (OP). The increased OP will favour endosmosis resulting in the bulging of the guard cells and oening of the stomata. The influx of K+ ions (into cytoplasm) and eflux of protons energized by H+, K+ ATPase pump. For every molecule of ATP hydrolyzed to ADP (near the plasma membrane) one K+ ion will enter into the cytoplasm in exchange for one H+ ion.

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Mechanism of stomatal closure:

During darkness H+ ions start diffusing out of chloroplast into the cytoplasm.This continues till (COO-), ions are available to combine with H+ to form (CO OH)2 molecules. These organic acid molecules into chloroplast will cause a reverse influx of K+ ions. They move out of the guard cells into the subsidiary cells. This will decrease the’ osmotic pressure of guard cell cytoplasm. As a result, exosmosis takes place and guard cells shrink, bringing about the closure of the stomata.

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