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Water Uptake by Plants: I. Divided Root Experiments¹

ABSTRACT

The influence of soil water content and soil water potential upon root water uptake was investigated in a controlled laboratory environment. Water extraction rates by roots of winter wheat were measured in situ in a soil column instrumented with a gamma-ray attenuation water-content meter, tensiometers, and psychrometers. In addition, the leaf water potential was measured with an in situ leaf hygrometer. In order to measure the vertical distribution of water uptake, the column was divided into five sections by wax layers which were penetrated by roots, but which prevented movement of water between sections in the soil. A measurement of the average root potential was obtained by allowing a thin "test section" of soil to dry toward equilibrium with the root water potential while the rest of the soil was kept wet. Although 86% of the root zone soil was at a water potential above about –0.1 bar, the roots dried the test section to a potential of –5 hars, indicating a root potential at least this low. Because the average leaf water potential was also –5 bars, the data imply that whereas there was little flow resistance between the roots and the leaves, the resistance to flow between the soil and the root xylem was large. Root extraction rates hegan to decrease rapldly when the soil water content fell below about 0.10 cm³ water/cm³, corresponding to a soil water potential of about –0.1 bar. This result agrees with data in the literature, but is at variance with the soil-water diffusivity-based root uptake model, which predicts little effect of soil dryness upon root uptake at high soil moisture potential.

¹ Published by permission of Director of Research, Wisconsin Agric. Exp. St., Univ. of Wisconsin, Madison, WI 53706. Research supported in part by a grant from the Mobil Foundation.

² Former Research Assistant in Physics, Univ. of Wisconsin, now Physicist, US Geological Survey, Water Res. Div., 345 Middlefield Road, Menlo Park, CA 94025; Professor, Dep. of Physics and Soil Sci., Professor, Dep. of Soil Sci.; both Univ. of Wisconsin, Madison 53706.

Received for publication December 6, 1976. Accepted for publication July 8, 1977.