Question

Stomata opening in most of the plants is during the day instead of night because

• A. Enzymes convert starch into sugar at night.
• B. Formation of sugar increases osmotic concentration of the cell sap, which results in more absorption of water, which provides turgidity to the guard cells.
• C. The starch converts into sugar by enzymatic activity during the night and the whole process of the day becomes reversible, thus resulting in the closure of stomata.
• D. All of the above.

Answer 1

The correct option is B Formation of sugar increases osmotic concentration of the cell sap, which results in more absorption of water, which provides turgidity to the guard cells.

Most widely accepted theory for opening and closing of guard cells is the malate or potassium ion pump theory, first proposed by Levitt, 1974. According to this theory, during day time, rise in pH of guard cells (due to photosynthesis and consequent decrease of carbon dioxide) causes hydrolysis of starch to form phospho enol pyruvate (PEP). PEP combines with carbon dioxide to form organic acids like oxalic acid. The organic acids get ionised and the proton passes out of guard cells and potassium ion is actively absorbed and accumulated. Potassium ions combine with malate and passes into small vacuoles. Chloride ions are also absorbed from outside to maintain electroneutrality. As a result of accumulation of ions there is a reduction in water potential of guard cells. The reduction in water potential causes absorption of water from adjacent cells, guard cells become turgid and stomata open.