Question

In succulent plants the stomata opens at night and closes by day. Which of the following would be best hypothesis to explain the mechanism of stomata opening at night only?

• A. $C{O}_2$ used up, increased pH result in accumulation of sugar.
• B. $C{O}_2$ accumulates, reduce pH stimulates enzymes resulting in accumulation of carbohydrate.
• C. Increase in $C{O}_2$ concentration, conversion of organic acids into starch, resulting in the increased uptake of potassium ions and water.
• D. High $C{O}_2$ concentration causes accumulation of organic aids in guard cells resulting into the increased concentration of cell sap.

Answer 1

The correct option is D High $C{O}_2$ concentration causes accumulation of organic aids in guard cells resulting into the increased concentration of cell sap.

During day light photosynthesis occurs in guard cells, as they contain chloroplast. The soluble sugars formed in this process may contribute in increasing the osmotic potential of guard cells and hence resulting in stomatal opening. However, very small amounts of soluble sugars (osmotically active) have been extracted from the guard cells, which are insufficient to affect water potential.
As a result of photosynthesis $C{O}_2$ concentration in guard cells decreases, which leads to increased pH up of organic acids, chiefly malic acid during this period in guard cells. The formation of malic acid would produce proton, that could operate in an ATP-driven proton $K^{+}$ exchange pump, moving protons into the adjacent epidermal cells and $K^{+}$ ions into guard cells and thus may contribute in increasing the osmotic pressure of the guard cells and leading to stomatal opening.
Reverse process would occur in darkness.