Question

Stomata open at day because in day the guard cells have

• A. To help gas exchange
• B. A low pH
• C. A high level of sugar, organic acid ATP and $K^{+}$ ion
• D. Unequally thickened walls

Answer 1

The correct option is C A high level of sugar, organic acid ATP and $K^{+}$ ion

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 CO${}_2$ 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.

Therefore, the correct answer is option C.