Factors affecting stomata opening and closing –
i) There is an endogenous rhythm (a biological clock). Stomata open during the day and close during the night. (Though certain succulents, which are native to hot, dry conditions have a reversed rhythm to enable them to economize on water loss.)
However, stomata continue to open and close on an approximately 24-hour clock (circadian = about a day) even when switched to continuous light. The phase of this opening and closure can be shifted (made to occur at other times of the day) by control of the end of the dark period.
ii) The water balance of a plant affects stomatal aperture. Wilting plants close their
stomata. The plant growth regulator abscisic acid (ABA) seems to act as a mediator
under these conditions. Water stress in the roots can transmit its influence to stomata in
leaves by the signal of ABA.
iii) Low concentrations of CO2 cause stomata to open. If CO2 free air is blown across
stomata in darkness, their stomata open. High CO2 causes stomata to close.
iv) Light causes stomata to open. The minimum light level for opening of stomata in most
plants is 1/1000 to 1/30 of full sunlight, just enough to cause some net photosynthesis.
Blue light (430-460nm) is nearly 10 times as effective as red light (630-680nm). The
wavelengths that are effective in the red part of the spectrum are the same as those that
are effective in photosynthesis that is absorbed by chlorophyll. However, the blue light
effect is quite independent of photosynthesis. Photosynthesis will change intercellular
CO2 concentrations and may have its effect through the mechanism written in point iii)
above.
