Abscission of Citrus Leaf Explants

Abstract

The role of a-naphthaleneacetic acid (NAA) in the control of abscission in Citrus (Citrus sinensis L. Osbeck) leafexplants and its conjugation were studied in non-aged and 24-hour-aged explants. Dipping non-aged explants in 1.5 micromolar NAA for 15 minutes immediately after excision did not delay abscission whereas 150 micromolar NAA effectively delayed it. As incubation time was prolonged up to 24 hours after excision, the delaying effect of both concentrations gradually increased. In general, both concentrations did not delay abscission when applied to 24-hour-aged explants held for an additional period of up to 24 hours. The uptake and conjugation of '4C-NAA to glucose and aspartic acid were similar in petiole, abscission zone, and leaf blade of non-aged and aged tissues, for all NAA concentrations. No correlation was established between the kinetics of abscission and the rate of conjugation in the abscission zone.In Citrus leaf and fruit explants, and in many other plant species, auxin delays abscission when applied during the first few hr after excision (2, 20), but loses its delaying effect-and can even accelerate the separation process-when applied to aged explants (1). This retarding effect could be connected to the delayinf effect of auxin on the activity of hydrolytic enzymes in the AZ (17,22). The mechanism which causes auxin to lose its delaying ability, however, is still obscure. In bean explants this phenomenon is said (8) to be independent of auxin translocation, but is probably mediated by immobility of auxin through the conjugation mechanism.The ability of plant tissues to conjugate IAA and NAA to glucose and aspartic acid is well established (3, 9, 10). We have found (11) an in situ increase in the level of auxin in AZs sections of isolated Citrus leaf during the first hours of abscission, and a marked decrease in endogenous auxin at later stages. This decrease is correlated with the increased failure ofauxin to delay abscission, and we wondered whether it is also correlated with an increase in the conjugation mechanism in the AZ layers. We investigated this question by using NAA-a synthetic auxin which is resistant to peroxidation (5). The data presented in the following are based on kinetic studies of NAA conjugation at various stages of abscission of Citrus leaf explants, which are a sensitive and reliable system for physiological study of abscission, and conjugate NAA effectively. Their performances in abscission studies have been well characterized in this laboratory (1 1, 12, 18, 19).