Mechanism of Stomatal Movement

Stomatal movement - the opening and closing of the pore between a pair of guard cells - is, at the most basic mechanical level, always caused by changes in guard-cell turgor pressure: rising turgor stretches the guard cells apart and opens the pore, while falling turgor lets them relax back together and close it. What has genuinely been debated, across three successive theories, is what actually triggers that turgor change. Von Mohl's theory of photosynthesis in guard cells (1856) proposed that light-driven photosynthesis inside the guard cells' own chloroplasts raises their sugar content and osmotic pressure, drawing water in and opening the pore; it fell out of favour once it became clear that guard-cell chloroplasts are poorly developed and cannot photosynthesise efficiently, and that guard cells already contain substantial stored sugar regardless of light conditions. The starch-sugar interconversion theory (Lloyd, 1908, with later contributions from Loftfield, Sayre and Steward) instead centres on a pH-driven biochemical switch: daytime photosynthesis lowers CO2 and raises guard-cell pH, which activates the enzyme phosphorylase (discovered in guard cells by Hanes, 1940) to hydrolyse stored starch into osmotically active sugar - refined by Steward (1964) into the specific step of converting osmotically inactive Glucose-1-phosphate into active glucose - lowering water potential and triggering endosmotic opening, with the reverse conversion at night causing closure; but this theory also has real gaps, since some monocot guard cells contain no starch at all, and clear evidence that sugar rises exactly as starch falls has been hard to establish. The theory of active K+ transport (Levit, 1974; Raschke, 1975) is the one now generally accepted: in light, guard-cell starch converts to malic acid, which dissociates to release protons that are actively pumped out to neighbouring subsidiary cells in exchange for K+ ions moving into the guard cell (balanced by accom …