Responses to Saturation Deficit in a Stand of Groundnut (Arachis hypogaea L.) 1. Water Use

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Abstract
Stands of groundnut were grown in computer-controlled glasshouses on water stored in an undisturbed soil profile. The maximum saturation vapour pressure deficit (D) of the air was either 1.0, 2.0, 2.5, or 3.0 kPa, and the mean air temperature was 27 °C. Transpiration (E), determined from the soil water balance, was strongly dependent on D, because D influenced both the fraction of incident solar radiation intercepted by foliage (f) and the transpiration rate per unit f (E/f). When D exceeded 2 kPa, canopy expansion was restricted and f reduced during early growth, but differences in f diminished as the canopies closed. E/f increased with D, implying that any restriction of transpiration through stomatal closure at large D was outweighted by a steeper humidity gradient from leaf to air. In all treatments E/f decreased as the soil profile dried. Saturation deficit per se had little influence on the proportional reduction in E/f with time, even though soil water deficit was considerably greater at large D. This lack of response occurred because plants compensated for the greater evaporative demand by extracting larger amounts of water from deep in the profile.