Influence of Soil Water Stress on Evaporation, Root Absorption, and Internal Water Status of Cotton

Abstract

Diurnal variations in leaf water potential, diffusion resist.ance, relative water content, stem diameter, leaf temperature, and energy balance components were measured in cotton (Gossypium hirsutum L. var. Lankart 57) during drought stress under field conditions. A plot of leaf water potential against either relative water content or stem diameter during the 24-hour period yielded a closed hysteresis loop. The relation between ceil hydration and evaporation is discussed.Despite low soil water potential in the main root zone, significant plant evaporation rates were maintained. Root absorption rates as a function of soil depth were calculated from water content profiles measured with a neutron probe.The maximal root absorption rate of 3.5 X 10-' day-' occurred at the 75-centimeter depth, welil below the main root zone.Stomatal resistance of individual leaves during the daylight hours remained nearly constant at 2.5 seconds centimetereven though leaf water potentials approached -30 bars. A growth chamber study indicated stomatal closure occurred at potentials near -16 bars. Possible implications of high soil water stress in relation to stomatal funetion and growth are discussed. Based on an energy balance method, the actual to potential plant evapotranspiration ratio was 0.43 for the 24-hour period, indicating partial stomatal closure. A surface resistance, r,, of 4.0 seconds centimeter-' was calculated for the incomplete canopy with the use of the energy balance data. Alternatively, a canopy resistance of 1.3 seconds centimeter-' was attained from a relationship between leaf area and stomatal resistance of individual leaves. If the soil resistance was assumed to be very large and the canopy resistance was weighted for the fractional ground cover of the crop, the calculated surface resistance was 4.3 seconds centimeter-. Under these conditions, the two independent estimates of r, were in essential agreement.Significant enhancement of our understanding of the response of internal plant water status to changes in the evaporative demand, in the water potential in the root environment, and in illumination have been made by theoretical analysis and by experiments under controlled conditions, but parallel studies with plants growing in a natural environment are relatively rare. Because of a prolonged drought in central Texas, the 1969 summer growing season provided an opportunity for field study of the water status of cotton under high soil water stress. Accordingly, diurnal variations in plant and soil water status and evaporation rate during drought stress were measured for purposes of comparison with theoretical predictions and with results from experiments under controlled conditions.Field studies by Weatherley (32) and Slatyer (23) have shown that plant water status is directly influenced by evaporative demand until a critical soil water potential is reached. The transpiration rates presented in these studies were inferred from a technique with the use of excised leaves and cannot be interpreted in absolute...

Variability in adaptive mechanisms to water deficits in annual and perennial crop plants

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<i>AtDREB2A-CA</i> Influences Root Architecture and Increases Drought Tolerance in Transgenic Cotton

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Concurrent Comparisons of Stomatal Behavior, Water Status, and Evaporation of Maize in Soil at High or Low Water Potential

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