Linked control of transpiration, leaf water content and chemical messages from the roots in sunflower plants

Dramatic symptoms are frequently associated to water deficit, especially reduction in growth rate, leaf wilting or rapid senescence. In an economic context in which irrigation is to be reduced, it is necessary to precisely predict the consequences of such water deficit on soil water balance and yield (quantity and quality). In situ evaluation of plant water status and its possible consequences are crucial conditions for adequate interpretation of field trials or surveys. Both prediction and evaluation require a better analysis of the control of water flux through the plant, which determines plant behaviour on short timescales. Controls avoiding plant dehydration occur at the whole plant level, 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 fluctuating conditions if stomatal conductance is controlled by chemical messages from roots. Conversely, leaf water status appears consistently as a consequence of stomatal control and evaporative demand, without any influence on stomatal behaviour.