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Endogenous Levels of Abscisic Acid in Water-Stressed Cotton Leaves¹

Studies relating diurnal changes in endogenous levels of certain plant hormones such as abscisic acid (ABA) diurnal patterns of plant water status are generally lacking. Since ABA has been implicated in the response of plants to stress conditions, the effects of plant water deficits on endogenous abscisic acid (ABA) levels in greenhouse and field-grown cotton (Gossypium hirsutum L. ‘Deltapine 16’) were investigated. Greenhouse-grown plants at the 7 to 8 leaf stage and leaves at node 18 of irrigated and nonirrigated field plants were used. Leaves were harvested at intervals throughout a 24-hour period, frozen in liquid N and stored at –30 C. Samples wee then extracted in 80% methanol and assayed for ABA using newly developed, high.pressure, liquid chromatographic techniques. Plant water potential measurements were made at each sampling time. Stomatal diffusion resistance measurements were made on the leaves harvested from the field. A diurnal pattern in the ABA levels was observed in both field and greenhouse plants. Abscisic acid levels were low during the early morning hours when plant water potentials were high. During the day ABA levels increased as plant water potentials decreased, and reached a peak near sunset in both field and greenhouse. Levels of ABA then decreased as plant water potentials increased during the night and early morning hours. ABA levels were always higher in the stressed plants throughout the day in both field and greenhouse with the greater differences occurring in greenhousegrown tissue. The levels of ABA also increased as stomatal conductance increased in the field plants and continued to increase after conductance values began to decline in the afternoon in both stressed and well-watered leaves which may reflect the measurement of ABA levels in whole leaves instead of guard cells alone. The larger differences between the conductance values in the stressed and nonstressed tissue late in the afternoon, however, may be associated with the differences in the ABA levels at that time. Since threshold potentials for initial ABA accumulation as well as diurnal patterns can be established, these assay techniques for ABA, developed specifically for cotton, may be useful in drought tolerance research as a tool for relating these parameters to variances in drought tolerance among genotypes.

Key Words: Gossypium hirsutum L. • Plant water stress • Plant hormone • Liquid chromatography • Stomatal action

² Plant physiologists, ARS-USDA.

Received for publication January 21, 1977.