Photosynthesis takes place in guard cells, too, since they contain chloroplasts in contrast to the subsidiary cells. This activity again is related to the rise of the osmotic value and thus also to the opening of the stomata. Controlling cycles regulating the movements of stomata. That much about observations and measurements. Good evidence exists that light exerts its effect mainly by decreasing the intercellular and intracellular concentration of carbon dioxide.
Carbon dioxide is faster consumed than supplied due to the mesophyll s photosynthesis.
This causes an intercellular deficit close to the stomata s opening. The photosynthetic activity within the guard cells themselves leads to a decrease of the intracellular level of carbon dioxide, and causes simultaneously that water is drawn from the subsidiary cells. It turned out that the water-uptake is preceded by an uptake of potassium ions. Protons are given off to the subsidiary cells. The ion flows are quantitatively enough to explain a rise of the turgor that is large enough for the guard cell movements, but the primary stimulus is still not clarified.