The osmotic pressure of the stomata is far larger in the guard cells than in the subsidiary cells. This ratio shifts in favour of the subsidiary cells when the stomata are closed. The stomata of most plant species are closed in darkness. Light stimulates opening. The action spectrum is similar to that of photosynthesis.
Blue light is especially effective.
The stomata of CAM-plants , like Crassulaceans, are opened during the night. They depend on the accumulation of carbon dioxide during the night. Opened stomata would cause intolerable transpiration losses in the areas that CAM-plants live in. Photosynthesis starts with the first light of the day, because enough carbon dioxide has been accumulated. Photosynthesis takes place in guard cells, too, since they contain chloroplasts in contrast to the subsidiary cells.