Proliferation in Nature
Abscissic acid, otherwise known as ABA, is found in many different plants, including ferns and mosses as well as flowering plants. It can be found in every part of the plant, and is synthesised in almost all cells that possess chloroplasts or amyloplasts - organelles similar to chloroplasts except containing no chlorophyll.
ABA as a Stress Hormone
One role of ABA that is well documented is as a stress hormone. When a plant is subjected to difficult environmental conditions, such as very high temperatures, or very reduced water supplies, then it responds by secreting ABA. In a plant under drought conditions, the concentration of ABA in the leaves can rise to forty times that which would normally be present. This high concentration of ABA causes the stomata to close, which reduces loss of water vapour from the leaf.
A Possible Mechanism
It is not known exactly how ABA achieves the closure of stomata, but the fact that the response is very fast indicates that, unlike the effect of gibberellins in seeds, it is not done by regulating the expression of genes. If ABA is applied to a leaf the stomata close within a few minutes. It seems that guard cells have ABA receptors on their plasma membranes, and it is possible that when ABA binds with these it inhibits the proton pump. This would stop the hydrogen ions being pumped out, so potassium ions and water would not enter drawn down the electrical and solute gradients respectively and hence the guard cells would become flaccid and close the stomata.