The tendency for leaves to dehydrate irreversibly (Fig. 6) in response to increases in root hydraulic conductivity is ameliorated by rapid signalling from roots to shoots that results in a slowing of water loss from the foliage. This is achieved by a prompt decrease of stomatal apertures, leaf expansion and, in tomato at least, by marked petiolar epinasty. Epinasty is the outcome of a downward re-orientation of whole leaves and leaflets that involves growth promotion on the upper (i.e., adaxial) surface. Epinasty reduces the amount of energy incident on the foliage and thus will slow the rate of water loss.