Give an account on Starch - Sugar Interconversion Theory.

Explanation

i. According to Lloyd (1908), turgidity of guard cell depends on interconversion, of starch and sugar. It was supported by Loftfield (1921) as he found guard cells containing sugar during the daytime when they are open and starch during the night when they are closed.
ii. Sayre (1920) observed that the opening and closing of stomata depends upon chagne in pH of guard cells. According to him stomata open at high pH during day time and become closed at low pH at night. Utilization of `Co_(2)` by photosynthesis during light period causes an increases in pH resulting in the conversion of starch to sugar. Sugar increase in cell favours endosmosis and increases the turgor pressure which leads to opening of stomata. Likewise, accumulation of `CO_(2)` in cells during night decrease the pH level resulting in the conversion of sugar to starch. Starch decreases the turgor pressure of guard cell and stomata close.
iii. The discovery of enzyme phosphorylase in guard cell by Hanes (1940) greatly supports the starch-sugar interconversion theory. The enzyme phosphorylase hydrolyses starch into sugar and high pH followed by endosmosis and the opening of stomata during light. The vice versa takes place during the night.

iv. Steward (1964) proposed a slightly modified scheme of starch-sugar interconversion theory . According to him, Glucose-1-phosphate is osmotically inactive. Removal of phosphate from Glucose-1-phosphate converts which is osmotically active and increases the concentration of guard cell leading to opening of stomata.
Objections to Starch-sugar interconversion theory
i. In monocots, guard cell does not have starch.
ii. There is no evidence to show the presence of sugar at a time when starch disappears and stomata open.
iii. It fails to explain the drastic change on pH from 5 to 7 by change of `CO_(2)`.