Primary responses of root and leaf elongation to water deficits in the atmosphere and soil solution

Abstract

Plants experience drought by a limitation of water supply and by enhanced transpiration. Both processes tend to decrease the plant's water potential, but affect growth responses in the root and leaf differently. The evaluation of the underlying mechanisms leads to a discussion of recent studies on biophysical aspects of cell expansion at a cellular, tissue and organ level. Two processes enable roots to compensate rapidly effects of water deficits originating in the medium: (i) adjustment of the minimum pressure in cells required for expansion (yield threshold), and (ii) solute transport within the elongation zone. Limitations of root growth are discussed with respect to hydraulic, mechanical, and solute relations in the root elongation zone. It is argued that the variable nature of both the yield threshold and solute transport challenges the applicability of the Lockhart concept to determine growth-related parameters from steady conditions of turgor and growth. On a whole organ level, the attenuation of xylem pressure along the root is important for the differential response of root and leaf growth. Experimental evidence is presented for the hydraulic separation of the elongation zones, which is closely related to root development and functioning. The data obtained over the past few years have been used to extend mathematical models of growth and water transport in roots.