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Kinetics of Tall Fescue Leaf Elongation: Responses to Changes in Illumination and Vapor Pressure¹

When measured at high resolution, elongation of tall fescue (Festuca arundinacea Schreb.) leaves is shown to be rapidly and highly responsive to step changes in illumination and atmospheric water potential. Furthermore, elongation rates are seen to be quite unstable under presumably constant conditions; sharp fluctuations and rhythmic oscillations are often observed when no external changes are imposed. The diurnal pattern of leaf elongation (15/9 hour day/night, 25 C) displayed higher nighttime rates, a sharp burst of growth at the beginning of the dark period, and a temporary depression of growth at the beginning of the light period. Lag times for responses to changes in illumination were 5 to 6 min. A decrease in vapor pressure deficit from 1.7 kPa to 0.3 kPa resulted in a nearly immediate (<1 min lag time) acceleration of leaf elongation rate. The response to increased vapor pressure was seen in the light and dark. Plants growing in the light and at vapor pressure deficits of around 1.7 kPa frequently exhibited growth oscillations. The frequency of the oscillations in elongation rates ranged from 2 to 3 cycles cycles hour^–1, and the amplitudes ranged from 0.05 to >1 mm hour^–1. Concurrent measurements of transpiration and carbon exchange revealed those two processes to be cycling at the same frequency; however, they were out of phase with elongation. Leaf elongation was at or near a cyclic maximum when transpiration and photosynthesis were at a cyclic minimum. It is argued that leaf elongation rate is, a priori an indicator of the flux of water into the cells that drive leaf expansion. Only changes in leaf water status, as modulated by stomata and vapor pressure deficits, could account for the observed rapid, reversible, and rhythmic changes in elongation/water flux. The results suggest that tall fescue leaf elongation is frequently limited by leaf water status, even in well-watered plants and that stomatal movement and leaf extension are modulated within a common range of leaf water potentials.

Key Words: Festuca arundinacea • Auxanometry • Diurnal growth • Leaf water potential • Transpiration • Apparent photosynthesis • Growth oscillations

² Members of the agronomy faculty.

Received for publication October 13, 1981.

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