Sensitivity of growth of roots versus leaves to water stress: biophysical analysis and relation to water transport

Water transport is an integral part of the process of growth by cell expans ion and accounts for most of the increase in cell volume characterizing gro wth. Under water deficiency, growth is readily inhibited and growth of root s is favoured over that of leaves. The mechanisms underlying this different ial response are examined in terms of Lockhart's equations and water transp ort. For roots, when water potential (Psi) is suddenly reduced, osmotic adj ustment occurs rapidly to allow partial turgor recovery and re-establishmen t of Psi gradient for water uptake, and the loosening ability of the cell w all increases as indicated by a rapid decline in yield-threshold turgor. Th ese adjustments permit roots to resume growth under low Psi. In contrast, i n leaves under reductions in Psi of similar magnitude, osmotic adjustment o ccurs slowly and wail loosening ability either does not increase substantia lly or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascula r system, This isolation protects the root from low Psi in the mature xylem and facilitates the continued growth into new moist soil volume. Simulatio ns with a leaky cable model that includes a sink term for growth water upta ke show that growth zone Psi is barely affected by soil water removal throu gh transpiration. On the other hand, hydraulic isolation dictates that Psi of the leaf growth region would be low and subjected to further reduction b y high evaporative demand. Thus, a combination of transport and changes in growth parameters is proposed as the mechanism co-ordinating the growth of the two organs under conditions of soil moisture depletion. The model simul ation also showed that roots behave as reversibly leaky cable in water upta ke. Some field data on root water extraction and Vertical profiles of Psi i n shoots are viewed as manifestations of these basic phenomena. Also discus sed is the trade-off between high xylem conductance and strong osmotic adju stment.