Question

In succulent plants, the stomata open at night and closes by day. Which of the following would be the best hypothesis to explain the mechanism of stomata opening at night only?
(a) ${CO}_{2}$ used up, increased pH results in the accumulation of sugar.
(b) ${CO}_{2}$ accumulates, reduce pH stimulates enzymes resulting in accumulation of carbohydrate
(c) Increased in ${CO}_{2}$ concentration, conversion of organic acids into starch, resulting in the increased uptake of potassium ions and water.
(d) High ${CO}_{2}$ concentration causes accumulation of organic aids in guard cells resulting in the increased concentration of cell sap.

Answer 1

Each stoma is surrounded by two guard cells. The kidney-shaped guard cells contain chloroplasts. A respiratory cavity is present beneath each stoma. The mechanism of the closing and opening of the stomata depends upon the presence of sugar and starch within the guard cells.

Complete answer:
In succulent plants, during the night, there is incomplete oxidation of carbohydrates and accumulation of organic acids without the release of ${CO}_{2}$ due to which carbon dioxide accumulates in the guard cells. Amplified acidification of guard cells is seen due to the accumulation of ${CO}_{2}$ and the formation of bicarbonate ions. Accumulation of organic acid is especially within the sort of malic acid within the dark period when an organic acid is stored within the central cell sap vacuole. Malic acid is the main osmoticum for the increase of guard cell turgor pressure and the contributions of potassium ion and sugar as osmotic materials are feeble.

Additional information: During daylight, photosynthesis takes place in guard cells, as they include chloroplast. The soluble sugars formed during this process may contribute to increasing the osmotic potential of guard cells and hence leading to stomatal opening. However, very small amounts of soluble sugars (osmotically active) are extracted from the guard cells, which are insufficient to affect water potential. As a result of photosynthesis ${CO}_{2}$ concentration in guard cells decreases, which results in increased pH from organic acids, chiefly malic acid during this era in guard cells. The manufacturing of malic acid could release protons that would operate in an ATP- driven proton potassium ion exchange pump, moving protons into the adjacent epidermal cells and potassium ion ions into guard cells and thus may contribute to increasing the pressure of the guard cells and resulting in stomatal opening. The reverse process would occur in darkness.
So, the correct option is ‘(b) ${CO}_{2}$ accumulates, reduces pH, stimulates enzymes resulting in accumulation of carbohydrate’.

Note:
- The stomata could also be found altogether in the aerial parts of the plant. They are never found on their roots. - The guard cells are regular living and include chloroplasts. - The sugar is soluble and raises the concentration of the sap of guard cells. - Stomata remains closed during the night when there's no photosynthesis and because of the accumulation of carbon-dioxide, acid is made that causes the pH to be acidic.