The Transport of Water from Soil to Shoot in Wheat Seedlings

A technique is described for precisely measuring the drop in water potential, Ay/, between the soil and the leaf xylem of wheat seedlings. The technique was used to explore the relation between transpiration rate and Ay at various times during the monotonic drying of the soil in which the plants were growing. When the soil was wet, the relation was linear, but, as the soil dried, nonlinearities appeared which were, in the main, explicable in terms of simple soil physical models describing the flow of water through the soil to the roots. There was no sign of the major hydraulic resistance at the root: soil interface that other people have recently found. I N T R O D U C T I O N When a soil is wet it has little direct influence on the uptake of water from it by a plant. When it is dry it has a large influence. When it is neither wet nor dry the extent of its influence is a matter of controversy. Some have argued that the conductivity of soil to water is often so small that the transport of water to the plant's roots may limit uptake even when much available water remains in the soil (e.g. Gardner, 1960; Carbon, 1973). Others have argued that the properties of the plant and of its aerial environment dominate the uptake unless the soil is so dry that there is virtually no available water left in it (e.g. Newman, 1969; Lawlor, 1972). Others again have argued that it is neither the soil nor the plant, but the interface between the two that dominates (Herkelrath, Miller, and Gardner, 1977; Faiz and Weatherley, 1978). Tinker (1976) has comprehensively reviewed many of the issues involved. The controversy may have arisen for many different reasons. (1) There is much confusion about the measurement of leaf water potential, y/L (Neumann, Thurtell, and Stevenson, 1974; Bunce, 1978). Since y/L is often used in calculating the gradients of soil water suction near the roots, these calculated gradients may be grossly wrong. (2) Few experimenters have taken the trouble to measure carefully the hydraulic properties of their soils. Hydraulic conductivity decreases by several orders of D ow naded rom http/academ ic.p.com /jxb/article-ct/31/1/333/473180 by gest on 03 D ecem er 2018 334 Passioura—Transport of Water in Wheat magnitude as a soil dries, and does so particularly rapidly in the light soils that many have used. The popular technique of measuring saturated conductivity and using it to calculate unsaturated conductivity (e.g. Hansen, 1974) may be too crude for our purposes. (3) Salts build up at the interface between root and soil when their rate of uptake by the root is less than their convective inflow from the soil (Tinker, 1976). Such a build-up may vary widely between experiments, and, if large, could lead to large decreases in soil water potential close to a root even when none is expected from calculations based on the flow rate of water and the hydraulic properties of the soil. (4) Roots may shrink when water-stressed (Huck, Klepper, and Taylor, 1970), thereby losing much of their hydraulic contact with the soil. A large difference in water potential presumably develops between root and soil (Cowan and Milthorpe, 1968). The shrinkage will depend on many factors, and may depend especially on