Opening and closing of stomata takes place due to turgor changes in guard cells. Solutes are taken in by the guard cells from the neighbouring epidernal and mesophyll cells so both osmotic potential and water potential of the guard cells is lowered.
These create a water potential gradient between the guard cells and the neighbouring cells and make the water move into the guard cells. The guard cells become turgid and swell in size, resulting in stomatal opening.
i. With a decline in guard cells solutes, water moves out of the guard cells, making them flaccid to stomata close.
ii. Levitt (1974) proposed active potassium theory. He observed that proton (H*) uptake by guard cells, chloroplasts takes place with the help of ATP. This leads to increase in value of pH in guard cells. Rise in pH converts starch into organic acid (like malic acid).
Malic acid further dissociates to form H+ and malate anion.
iii. The uptake of K+ ion in balanced by one of the following: uptake of CI", transport of H ion from organic acid or by negative changes of organic acid when they lose H+ ions. Thus all these factors lead to the opening of stomata.
iv. The stomata closure is due to excretion of K+ from guard cells surrounding epidermal and subsidiary cells.
Factors affecting stomatal movement
Stomata open in the presence of light and close in darkness. Light intensity required to open the stomata is very low, as compared to the intensity required for photosynthesis. In CAM (Crassulacean Acid Metabolism) plants, stomata open during dark and remain closed during the day. Even moonlight is sufficient to keep the stomata open in some plant species.
Rise in temperature induces stomatal opening while fall in temperature causes closure. At 38°-40°C, stomata open even in darkness. In some plant species, stomata remain closed even under continuous light at 0°C.
3. Carbon dioxide:
Low CO2 concentration induces stomatal opening and vice versa. In closed stomata, external CO2 concentration has no effect.
Essential for stomatal opening.
5. Water availability:
Water stressed (less water available to plant and high transpiration rate) plants induces stomatal closure, due to formation of ABA and lowering of water potential in epidermal cells.
Influx of K+ causes opening of stomata while efflux of K+ from guard cells causes closure of stomata.
7. Mechanical shock:
Causes stomatal closure.
Abscisic acid brings about closure of stomata. Cytokinins are required for keeping the stomata open.
The loss of water in the form of liquid is called guttation. It generally occurs from tips and margins of leaves during night or early morning when there is high atmospheric humidity as during wet seasons.
i. Hydathodes or water pores are special pores on the leaves in the region of guttation which are permanently open pores as their guard cells are immobile.
ii. Gutted water possess minute quantities of both inorganic and organic substances.