Stomatal movement are influenced by a number of environment factors like light, temperature, humidity, water availability and C02 concentration. Internal or endogenous factors include growth hormones, organic acid, K+, Cl- and H+ ions.
(a) Light: In the majority of plants the stomata open in light and close in darkness. The intensity required for stomata opening is quite low. However, in succulents or CAM plants (Crassulacean Acid Metabolism), the stomata remain closed during day time. They open only during dark, e.g., Agave, Opuntia, Pineapple.
(b) Temperature : At 30°C - 40°C, stomata can open in complete darkness, while at 0°C they remain closed even in continuous light. Normally, high temperature above 30°C reduces stomatal opening in many species.
(c) Atmospheric humidity : In humid environment, the stomata remain open for longer periods while in dry environment they remain closed for longer periods.
(d) Water availability : Plants undergo water stress if availability of water is less than the rate of transpiration. Water stress (= water deficit = moisture deficit) brings about stomatal closure due to ABA and rise in DPD of epidermal cells..
(e) Mechanical shock: It causes closure of stomata.
(f) COz concentration : Low C02 concentration usually includes opening of stomata while high C02 concentration closes the same. In some plants more breathing over the leaves causes stomatal closure. However, guard cells are sensitive to C02 concentration only from their inner side (i.e., concentration in the leaf interior). Stomata of a plant transferred to dark C02 free environment will remain closed but they will open in light when internal C02 is utilised.
(g) Oxygen : It is essential for opening of stomata.
(h) pH : Rise in pH is known to be required for opening
of stomata while a fall in pH induces closure of stomata.