Short‐Term Growth Responses of Tall Fescue to Changes in Soil Water Potential and to Defoliation1

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Abstract
Plant elongation growth is modulated by soil water availability and other environmental factors. With limited soil water, plant water potential will be low and cellular elongation will be inhibited. In this study, we measured leaf elongation of tall fescue (Festuca arundinacea Schreb.) during and after periods of limited water availability to roots. Elongation, as measured with angular displacement transducers, was more rapid in plants exposed to a −0.1 bar than a −8 bar water potential rooting medium. Growth in the −0.1 bar rooting medium was less susceptible to inhibition by low relative humidity (RH) or illumination. Flushing the rooting medium of the −8 bar treatment with a −0.1 bar solution gave evidence of rapid recovery. Tillers with short‐term, severe drought stress (wilted) displayed rapid elongation immediately after watering. With long‐term severe drought stress (wilted for 26 to 28 hours), no elongation occurred for almost 2 hours after rewatering. We propose that chronic drought stress causes disruptions in the continuity of water columns between roots and leaves that are repaired by positive root pressures. Low soil water availability and low RH limited elongation in darkness, but the effects were much more dramatic in the light when open stomata accelerated water loss. Partial defoliation immediately increased elongation rates of remaining leaves. This response may partially explain the very rapid elongation frequently observed in recently cut grasses.