Opening and Closing of Stoma

Opening and Closing of Stoma : Opening and closing of the stoma is controlled entirely by the turgor of its two flanking guard cells, and that turgor itself rises and falls over the course of the day because of a corresponding rise and fall in the guard cells' internal osmotic potential. During daytime, guard cells become turgid through endosmosis; because their thin, elastic outer (lateral) walls stretch readily under the resulting turgor pressure while their thick inner walls resist stretching, and because of their kidney or dumbbell shape, the thick inner walls are pulled apart from one another, widening (opening) the stomatal pore. At night, guard cells instead lose water by exosmosis and become flaccid, and this loss of turgor allows the pore to close almost completely.

What actually drives this daily (diurnal) swing in the guard cells' osmotic potential has been explained by two proposed theories:

1. Starch–sugar interconversion theory (Steward, 1964). During the day, the enzyme phosphorylase converts starch stored in the guard cell into sugar. Because sugar is far more osmotically active than the insoluble starch it came from, this conversion raises the guard cell's osmotic potential, drawing water in by endosmosis, so the guard cells become turgid, stretch, and the stoma widens. At night, the reverse reaction takes place — sugar is converted back into starch — lowering osmotic potential, and the loss of internal water pressure that follows brings about closure of the stoma. Starch (stoma closes) ⇌ Sugar (stoma opens): phosphorylase drives the forward reaction by day and the reverse reaction occurs at night. …