Water Relations of Cotton Plants under Nitrogen Deficiency

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Abstract
Suboptimal N nutrition increased the water potential for stomatal closure in water stressed cotton (G. hirsutum L.) leaves. This increased sensitivity to water stress had 2 components, increased accumulation of abscisic acid (ABA) and increased apparent stomatal sensitivity to ABA. Low N increased the threshold water potentials for stomatal closure and ABA accumulation by .apprx. 4 bars and 2 bars, respectively. Low N also greatly increased stomatal response to low concentrations of exogenous ABA applied to excised leaves through the transpiration stream. In low N leaves, kinetin decreased stomatal response to ABA to the level observed with high N leaves. Kinetin by itself had little effect on stomata, nor did it alter stomatal response to ABA in high N leaves. A cytokinin-ABA balance was suggested which is altered by suboptimal N nutrition to favor stomatal closure during stress. Ambient temperature and N nutrition interacted to alter stomatal response to water stress. Stress-induced ABA accumulation and apparent stomatal sensitivity to ABA were independently affected. The effects of each treatment, and their interaction, could be explained as the net result of changes in accumulation and apparent sensitivity. Although the results document environmental control of stomatal response to ABA, either altered partitioning of ABA between active and inactive pools, or altered sensitivity of the guard cells could account for the data.