Leaf and Root Growth in Relation to Water Status

flowing aqueous solution (Frensch and Hsiao, 1995).There C was assessed to be large relative to m.On the other hand, studies have pointed to conductance as a major factor limiting growth of cells in the stem (hypocotyl) of soybean [Glycine max (L.) Murr.] seedling with roots in water-deficient vermiculite (Nonami and Boyer, 1990).In that case, the low conductance of the radial path between the xylem and the growing cells was attributed to a layer of small cells ≈200 µm thick separating the ring of xylem vessels from the cortical and epidermal cells (Nonami et al., 1997).For simplicity, the remaining part of this paper will discuss growth mostly in terms of the parameters in Eq. [1].Conductance and Ψ gradient will be mentioned, however, where relevant. RESPONSES OF EXPANSIVE GROWTH TO WATER STRESSIn terms of growth physics and associated processes, more is known about roots than about leaves.Some of the more interesting recent results with roots are discussed first, followed by a brief review of results with leaves for comparison.Growth of leaves has long been known to be highly sensitive to inhibition by water stress (Boyer, 1968).Root growth, on the other hand, is more resistant (Westgate and Boyer, 1985).This differential sensitivity is illustrated with some maize data obtained in our laboratory (Fig. 1).The leaf elongated at a maximal rate in the well-watered control, with Ψ of the growth zone higher than -0.8 MPa.Elongation was reduced when growth zone Ψ was reduced by a few hundredths of a MPa, and stopped when Ψ was reduced by 0.3 MPa, to a value of -1.05 MPa (Fig. 1B).For roots in vermiculite, elongation was also reduced by small reductions in medium Ψ (Fig. 1A).Upon further reductions in Ψ, however, root elongation was less affected, and