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

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Abstract
Water transport is an integral part of the process of growth by cell expansion and accounts for most of the increase in cell volume characterizing growth. Under water deficiency, growth is readily inhibited and growth of roots is favoured over that of leaves. The mechanisms underlying this differential response are examined in terms of Lockhart's equations and water transport. For roots, when water potential (Ψ) is suddenly reduced, osmotic adjustment occurs rapidly to allow partial turgor recovery and re‐establishment of Ψ gradient for water uptake, and the loosening ability of the cell wall increases as indicated by a rapid decline in yield‐threshold turgor. These adjustments permit roots to resume growth under low Ψ. In contrast, in leaves under reductions in Ψ of similar magnitude, osmotic adjustment occurs slowly and wall loosening ability either does not increase substantially or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascular system. This isolation protects the root from low Ψ in the mature xylem and facilitates the continued growth into new moist soil volume. Simulations with a leaky cable model that includes a sink term for growth water uptake show that growth zone Ψ is barely affected by soil water removal through transpiration. On the other hand, hydraulic isolation dictates that Ψ of the leaf growth region would be low and subjected to further reduction by 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 simulation also showed that roots behave as reversibly leaky cable in water uptake. Some field data on root water extraction and vertical profiles of Ψ in shoots are viewed as manifestations of these basic phenomena. Also discussed is the trade‐off between high xylem conductance and strong osmotic adjustment.