Drought perception by plants - Do cells of droughted plants experience water stress?

Because of regulations at the whole-plant level, cells of droughted plants do not necessarily experience dehydration. In fact, recent data suggest that they usually do not in the range of water deficits compatible with agriculture. In this range, leaf water potential and cell turgor are frequently maintained at high values in spite of decreasing soil water status. As a consequence, 'water stress' cannot be defined by plant water status, except when very rapid and severe water deficits cause catastrophic events such as xylem embolism or severe cell dehydration. In water deficits compatible with agricultural situations, plant water status is in many cases tightly controlled by the plant. 'Water stress' should then be defined by water statuses at plant boundaries, i.e. soil and air. Partial maintenance of plant water status under water deficit is allowed by controls of stomatal conductance, root and leaf expansions and leaf senescence. These processes involve both chemical and hydraulic signallings from roots. All these controls tend to reduce transpiration (stomatal closure, reduced leaf growth or leaf senescence) or to increase water uptake (maintenance of root growth or increase in root/shoot ratio). During relatively mild stresses, the role of abscisic acid, frequently considered as a 'stress hormone', in fact appears to be to avoid dehydration at the cellular level. In the cases described here, selecting plants for better resistance to cell dehydration may not be the best strategy for improving 'drought resistance' of crops.