mechanism of stomatal movement
Stomata function as turgor operated valves because opening and closing of stomata is governed by change in O.P.
or turgidity of guard cells. When guard cells are turgid, stomata open and when guard cells are flaccid, stomata close.
Different theories about the mechanism of stomatal movements have been proposed :
- Hypothesis of guard cell photosynthesis (given by Schwendener, 1881 ).
- Starch-sugar interconversion theory (given by Lloyd, 1908 and by Sayre 1926 and modified by Steward
1964). - Malate or K+ ion pump hypothesis (given by Fujino, 1959 and modified by Levitt, 1974)
Hypothesis of guard cell photosynthesis
According to this, the photosynthetic activity of the guard cells was believed to be responsible for the stomatal movement.
During the day, glucose is photosynthesised and being osmotically active, lowers the water potential of the cell sap, thus
there is an influx of water into the guard cells and the stomata open.
At night, sugar is converted to starch and being osmotically inactive, decreases the osmotic concentration of guard cell.
Thus, water moves out and guard cells become flaccid and stomata close.
Biggest objection to it was that the photosynthetic activity of guard cell chloroplasts was found to be negligible.
Starch-sugar interconversion theory
According to this theory, a change in pH effects the opening and closing of stomata. Starch sugar interconversion
is brought about by a pH dependent enzyme starch phosphorylase. The change in the pH of the guard cell’s 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 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.
Glucose being osmotically active, lowers the water potential of the cell sap, thus there is an influx of water into the
guard cells and the stomata open.
At night, in the absence of light, CO 2 evolved in respiration accumulates in the cell sap and dissolves in water to form
carbonic acid and pH of the cell sap decreases to 5 (acidic).
At this pH, starch synthesis is favoured from glucose-1-phosphate.
Starch being osmotically inactive, decreases the osmotic concentration of guard cell. Thus, water moves out and guard
cells become flaccid and stomata close.
There are many objections to starch-sugar interconversion theory. First of all, it is a slow process and cannot account for
rapid stomatal movements. Secondly, enzyme phosphorylase catalyzes the reaction only in forward direction. Moreover,
sugar has never been found in free state in guard cells of open stomata and starch has never been reported from guard
cells of onion.