THE UNITS OF CURRENCY FOR PLANT WATER STATUS

The appropriate measure for plant water status is of fundamental importance in plant water relations. One possible approach is to consider a hypothetical sensor for water status at the cell level. It is argued that such a sensor must respond to an intrinsic variable describing the quantity of water present in the cell. Uncertainty about the variable being sensed has affected our interpretion of plant responses to drought. In general, a more rigorous analysis of the effects of tissue hydraulic parameters and further understanding of the pertinent variable for water status are needed to assess such questions. For example, the significance of changes in the elasticity of cell walls in response to drought depends on the measure for water status. A simple model for water transport suggests that a decrease in the elastic modulus may act to maintain turgor for succulent plant species, but it is not clear how increases in the elastic modulus enhance water uptake for non-succulent species.