Movement of Kinetin and Gibberellic Acid in Leaf Petioles during Water Stress-induced Abscission in Cotton

Abstract

Movement of I14Clkinetin and I'4Clgibberelic acid was examined in cotton (Gossypium hirsutum L.) cotyledonary petiole sections independent of label uptake or exit from the tissue. Sections 20 mimeters in length were taken from well watered, stressed, and poststressed plants. Transport capacity was determined using a pulse-chase technique. Movement of both kinetin and gibberellic acid was found to be nonpolar with a velocity of 1 millimeter per hour or less, suggesting passive diffusion. Neither water stress nor anaerobic conditions during transport of labeled material affected the transport capacity of the petioles.Results suggested strong kinetin binding but weak gibberellic acid binding in the tissue sections. Apparent binding of both growth regulators was unaltered by the experimental conditions. Movement of these two growth regulators within cotton cotyledonary petioles plays a minor role in the stress-induced, foliar abscission process.Foliar abscission resulting from water deficits has recently been shown to be correlated with a decrease in the auxin transport capacity of subtending petioles (3). A clear understanding of the complex interactions of other hormones within the plant during the abscission process is imperative if we are to elucidate mechanisms of abscission induction. ABA levels clearly rise during periods of water stress (18,19), but the changes in extracted ABA from cotton cotyledons do not appear to correlate with induced abscission of cotyledonary leaves (2). Nor does ABA appear to be transported in petiolar segments either prior to, during, or after relief of water stress (2). Although two other major classes of plant hormones, cytokinins and GA3, have been studied in relation to leaf abscission (11, 15, 16) [1,7,12,18-14C] GA3 (1.7 mCi/mmol, Amersham/Searle Corp., Amersham, England) were placed atop each section for 0.5 hr after which they were replaced with agar "chase" blocks containing nonlabeled 10 ,lM kinetin or 10 ,UM GA3, respectively. The relatively high concentration oflabeled GA3 was necessary to obtain sufficient radioactivity in the tissue sections. Half of the sections were allowed to continue transport with chase blocks in place for an additional 0.5 hr and the other half for 2.5 hr. There was no apparent rehydration of stressed petioles throughout the transport experiments. Each 20-mm section was then cut into 10 segments 2 mm in length each of which was pooled with corresponding segments, placed into scintillation vials (total of 10 segments/vial) containing 15 ml of a dioxane-naphthalene-PPO-based scintillation fluid, and shaken for 24 hr at room temperature.