Soil water transfer within the root zone by cotton plants resulting from differences in soil water potential

Abstract

A growth chamber experiment was conducted to test the hypothesis that an unirrigated plant could survive if its root system overlapped with irrigated plants on either side. A box 1.8 m long, 0.2 m deep, and 0.24 m front to back was built, with two vertical barriers running front to back, dividing it into three compartments. Two plants were grown entirely in the center compartment, while two plants on either side were grown astride the barriers, so that each had half its root system in the center and half in an outer compartment. The outer compartments were watered daily, while the center compartment was allowed to dry, creating a gradient in water potential between center and outer compartments. Water content throughout the box was measured by gamma densitometry at the start and conclusion of each 12 hour light period, while stem flow in each plant was monitored with a heat balance method. Leaf water potentials were measured with pressure chambers and isopiestic psychrometers. When the difference in water potential between center and outer compartments neared 1.0 MPa, nightly movement of water from outer compartments into the center compartment was detected on four consecutive nights. Such movement could only have taken place through root systems of the outer row plants. Amounts transferred were insufficient to replace the water lost daily from the center compartment by transpiration. Data analysis suggests that water movement was less than it might have been, were it not for an inability of the xylem water potential of cotton to fully equilibrate with soil water potential during the night: a persistent disequilibrium of 0.4 to 0.6 MPa was noted, even when transpiration was effectively eliminated. Measurements of the stem flow of each plant supported the accuracy of the measured changes in soil water content. Computer simulation showed that incorporation of the observed disequilibrium in calculation of uptake and exudation produced results consistent with the measured data. Field scale simulation produced results similar to the growth chamber experiment: water moved through irrigated plants from wet to dry soil, but not in amounts sufficient to support an unirrigated center row.