LINKED CONTROL OF TRANSPIRATION, LEAF WAT ER-CONTENT AND CHEMICAL MESSAGES FROM THE ROOTS IN SUNFLOWER PLANTS

Dramatic symptoms are frequently associated to water deficit, especial ly reduction in growth rate, leaf wilting or rapid senescence. In an e conomic context in which irrigation is to be reduced, it is necessary to precisely predict the consequences of such water deficit on soil wa ter balance and yield (quantity and quality). In situ evaluation of pl ant water status and its possible consequences are crucial conditions for adequate interpretation of field trials or surveys. Both predictio n and evaluation require a better analysis of the control of water flu x through the plant, which determines plant behaviour on short timesca les. Controls avoiding plant dehydration occur at the whole plant leve l, and not at single-cell level, and involve the transfer of messages from roofs to leaves. We show here that leaf water status, water flux and stomatal conductance can be quantitatively predicted in fluctuatin g conditions if stomatal conductance is controlled by chemical message s from roots. Conversely, leaf water status appears consistently as a consequence of stomatal control and evaporative demand, without any in fluence on stomatal behaviour.