SENSORY SYSTEMS IN THE ROOTS OF PLANTS AND THEIR ROLE IN CONTROLLING STOMATAL FUNCTION IN THE LEAVES

Our understanding of the control of water status in plants has progres sed dramatically since the mid-1980s, when it was generally believed t hat the essential mechanisms were located predominantly in the leaves. It was thought that the control mechanisms began to operate only afte r the amount of water lost by transpiration exceeded the supply coming from the roots. Stomatal aperture, the principal means of regulating transpiration, was seen as the major factor that responded to a change in leaf water status. The all-important role of stomata still remains , but few of the other concepts still survive. It is now known that th ere is a complex sequence of cellular processes giving fine control of water loss from the leaves, and it begins at extremities of the root system. Long before leaves experience any measurable shortage of water , the tips of roots are able to ''sense'' that they have come into con tact with drying soil. Only a few of the root tips in part of the soil profile may be affected, but they manufacture a hormone, abscisic aci d, which is transmitted to the leaves to cause stomatal closure. The h ormone causes the stomatal guard cells to lose turgor, using calcium i ons as a second (or third) messenger. Very fine control is achieved no t only of stomatal movements, but also of the subsequent course of the plant's development.