Question

According to starch-sugar conversion theory of stomatal movement, during stomatal opening

• A. $pH$ is lowered and sugar is converted to starch
• B. $pH$ is increased and sugar is converted to starch
• C. $pH$ is lowered and starch is converted to sugar
• D. $pH$ is increased and starch is converted to sugar

Answer 1

Answer: (D)

$pH$ is increased and starch is converted to sugar

Answer 2

According to Llyod $(1908)$, the conversion of starch into sugar in the day time and vice versa at night causes opening and closing of stomata. Sayre $(1926)$, Scarth $(1932)$ suggested that this conversion is controlled only by $pH$ of the guard cells whereas Steward (1964), regards that this is partly enzymatic and partly controlled by $pH$. The change in the $pH$ of the guard cell's cell sap is due to the presence or absence of $CO_2$ which is dependent on light availability. During day time, in the presence of light, $CO_2$ evolved in respiration is utilized for photosynthesis and hence does not accumulate in the guard cells and $pH$ of the guard cells increases to $7$. Hence the enzyme favours the formation of glucose-$1$-phosphate from starch and inorganic phosphate. Altered sugar concentration increases turgidity of the guard cell due to osmosis and stomata open.