Answer: Stomata open when guard cells become turgid (rise in osmotic concentration) and close when they lose turgor.
Stomatal opening and closing depends on the turgidity of the guard cells. The thick inner wall and radially arranged cellulose microfibrils make the guard cells bow apart when turgid (pore opens) and straighten when flaccid (pore closes).
(a) Starch–sugar interconversion theory (Steward, 1964):
- In light: photosynthesis in guard cells uses CO2, so pH rises (about 7). At high pH the enzyme phosphorylase converts starch into glucose-1-phosphate, which is converted to glucose. Osmotic concentration of guard cells increases, water enters from neighbouring cells by endosmosis, guard cells become turgid and the stoma opens.
- In dark: CO2 accumulates from respiration, forming carbonic acid and lowering pH (about 5). Phosphorylase now converts sugar back to starch, osmotic concentration falls, water leaves the guard cells by exosmosis, they become flaccid and the stoma closes.
(b) Proton transport theory (K+ ion pump; Levitt, 1974):
- Opening (light): starch in guard cells is converted to malic acid, which dissociates into malate and H+. H+ ions are pumped out actively (using ATP) into subsidiary cells, and in exchange K+ ions enter the guard cells; Cl– also enters. K+ with malate/Cl– raises the osmotic concentration, water enters, guard cells become turgid and the stoma opens.
- Closing (dark): the process reverses; K+ (and malate/Cl–) moves out of the guard cells, H+ moves in, osmotic concentration falls, water leaves and the stoma closes. Abscisic acid (ABA) also promotes closure by causing loss of K+.
