Leaves and roots of a citrus tree are able to rapidly detect changes in their surroundings . Both the leaves and roots quickly know when the temperature is high or its low, they know when they are thirsty, and even when there is a desiccating wind blowing against them,. Most everyone knows when these things happen the tree closes the stomates in the leaves to reduce the amount of water lost by the tree through transpiration, thereby saving the tree from desiccation, loss of leaves and fruit, shoot or root die back, and possible death. But do you know how the tree does this? Leaves and roots can detect these dangers and quickly come to the rescue by synthesizing a chemical called abscisic acid (ABA) and then sends the acid up the transpiration stream via the xylem to the guard cells of the leaf which upon arrival tells the stomates to close. ABA also carries out other functions - it helps to regulate the tree's dormancy in combination with cytokinins and gibberelins, and though not its major function, ABA also plays a role in leaf, flower, and fruit abscission (hence the name "abscisic acid". - Millet
