Steward's scheme for opening and closing of stomata
Opening of Stomata
Stomatal IT movement has been explained by two important theories starch hydrolysis theory and proton transport concept.
Starch Hydrolysis Theory: This theory was proposed by Lloyd and elaborated by Scarth. It is also called Starch Sugar interconversion. It is based on the interconversion of Starch and sugar and effect of PH on enzymes.
Starch + P→ (Glucose-1-phosphate) n
During the day CO2 is used in photosynthesis, concentration is low in guard cells, as a result, the pH rises to 7.0. Enzyme is activated and starch is hydrolysed to glucose phosphate. This causes endosmosis which brings about turgidity and the guard cells open. During night CO2 accumulates in guard cells and is converted to carbonic acid, PH decreases to 5.0 and causes the conversion of sugar to starch. Since it is insoluble water diffuses out causing exosmosis, guard cells are flaccid and the guard cells close. This theory has the following drawbacks
- Photosynthesis in guard cells is too slow to bring about Osmotic changes.
- Malic acid accumulates in place of sugars.
- Enzymes does not catalyse the reverse reaction.
Proton Transport or Active potassium mechanism:
This theory was explained by Levitt. Guard cells in presence of sunlight use CO2 during photosynthesis. With decrease in CO2 concentration, PH rises causing conversion of starch to sugar and finally, Malic acid is accumulated. This dissociates into H+ and Malate ions and by using ATP exchange H+ for K+ to form Potassium malate. The water potential becomes more negative causing endosmosis and the guard cells become turgid and stomata open.
During the dark Photosynthesis decreases, CO2 concentration increases and PH decreases to 5.0. K+ flows and again Malic acid accumulates. Flaccidity occurs as a result of exosmosis making the guard cells close. Closing is also influenced by ABA and Ca+2 ions.
