1. Opening in light is a feature common to the majority of functional stomata, but the current argument is against the traditional view that light is the principal environmental promoter of opening, because stomata can open in the dark in response to CO2 removal and/or temperature increase. In this review, evidence is provided that light is more efficient and effective than other physical factors in both producing and maintaining wide opening. However, light acts on stomata both directly and indirectly, in conjunction with changes in, for example, CO2 balance, water regime and temperature of the leaf tissue.

2. Three general categories of light effects on stomata are recognized: (a) photosynthetic effects driven by metabolic processes, induced or enhanced by light, (b) hydrophotic effects mediating through light-induced changes in epidermal turgor, and (c) photothermal effects arising from light-dependent changes in leaf temperature.

3. Photosynthetic effects involve both CO2 depletion, and starch mobilization, malate synthesis, H+ extrusion, and accumulation of K+ and C1- in guard cells; these processes are triggered by light of different qualities: (a) Both blue and red light are involved in photosynthetic CO2 fixation, utilizing energy from photosynthetic light reaction(s), which provides C precursors for synthesis of stornatal starch. (b) Blue light, but not red, enhances starch mobilization, PEP carboxylase activity and respiration. Accordingly, blue light is postulated to enhance hydrolysis of stornatal starch providing C3 precursors for malate synthesis via PEP-fixation of endogenous CO2; the active extrusion of H+, derived from malate, is coupled with K+ influx to guard cells. Malate and C1- are competitive anions, for K+, and one begins to play a progressively more important role as the other becomes limiting; in intact leaves, however, malate plays a more decisive role. These processes are driven by the energy from blue-light-enhanced respiration. (c) Both photosynthetic fixation and PEP carboxylation act as CO2 sensors, but the exact role of CO2 in the stornatal mechanism has yet to be determined.

4. Hydrophotic and photothermal effects facilitate guard cell expansion by releasing epidermal pressure through enhanced evaporative water loss, and are, therefore, indirect effects of light; photothermal effects may also contribute to metabolic processes outlined in paragraph 3.

5. Stomatal closure in the dark accompanies starch synthesis, malate reduction, efflux of K+ and C1- from guard cells, and accumulation of CO2 in substomatal cavities. Malate may be converted to starch via C2 compounds. Guard cells release K+ and C1- into apoplastic space, from which they are removed by neighbouring cells. The entry of K+ into neighbouring cells is supposed to be coupled with H+ extrusion. These processes are dependent on respiratory energy.

6. The differential abaxial and adaxial stomatal light responses are related to inherent metabolic differences between the two epidermes, but the biochemical basis is not known.