Q.Stomata of a plant open due to

Stomatal opening and closing is fundamentally controlled by changes in the turgor pressure of the two guard cells bordering each pore, but three separate theories have tried to explain what actually drives that turgor change. Von Mohl's 1856 theory of photosynthesis in guard cells proposed that light-driven photosynthesis inside the guard cells' own chloroplasts raises their sugar content and osmotic pressure, drawing in water and opening the pore, but it was undermined once it became clear that guard-cell chloroplasts are poorly developed and cannot photosynthesise adequately, and that guard cells already hold plenty of stored sugar regardless. The starch-sugar interconversion theory (Lloyd, 1908, with contributions from Loftfield, Sayre and Steward) instead links opening to a pH-driven conversion: during the day, CO2 use in photosynthesis raises guard-cell pH, which - helped by the enzyme phosphorylase, discovered in guard cells by Hanes (1940) - hydrolyses stored starch into osmotically active sugar (later refined by Steward, 1964, to the specific step of converting inactive Glucose-1-phosphate into active glucose), lowering water potential and triggering endosmosis and opening; at night the reverse conversion of sugar back to starch closes the pore. But this theory too has gaps - some monocot guard cells contain no starch at all, and there is no clear evidence that sugar actually appears exactly when starch disappears. The currentl …