Cracker and Abeles - Abscission : Role of Abscisic Acid

Th2 effect of abscisic acid on cotton (Gossypium hirsutuin L. cv. Acala 4-42) and bean (Phaseolus vulgaris L. cv. Red Kidney) explants was 2-fold. It increased ethylene producticn from the explants, which was found to account for some of its ability to accelerate abscission. Absci ic acd also increased the activity of cellulase. Increased synthesis of cellulaso was not du to an increase in aging of the explants but rather was an effect of abscisic acid on the processes that lead to cellulase synthesis or activity. The ability of a diffusible substance from cotton fruit to accelerate the abscission of cotton exlplants ultimately led to the discovery of what is now known as abscisic acid, a plant hormone capable of regutlating a number of plant processes (9, 16). The role of abscilc acid in abscission however, remains uncer.ain. The substance promotes the abscission of explants, but then so do a large numnber of other compounds, manv of which are not normal constituents of leaves (1). Evidence bo.h for (9) and against (15) a role for thiis hornmone in abscission has been pre ented. However the work of Dale and Milford (15) has been criticizcd becau-e chromotographic behavior of tlheir compound differed from the 8-inhibitor and did not pronlote abscission of young cotton fruit. This paper presents a series of experinments designed to elucidate the role of abscisic acid in the abscission of isolated ab cission zone explants, taking advantage of improved techlniqtues for the measurement of cell separation (14) and the production of cell wall degrading enzymes (4). Materials and Methods Planit Material. The methods used to grow bean (Phaseolus viilgaris L. cv. Red Kidnev) and cotton (Gossypium hirsutumii L. cv. Acala 4-42) plants, and to prepare and store explants, have been described earlier (1,6, 7). Each experimental datum with bean explants represen:s experiments repeated on 3 different occasions witlh 3 sets of 10 explants. Cotton experiments were repeated twice, but since there are 2 separation layers per explant, each experimental datum represents a total of 120 observations. Application of Chemicals. Indole-3-acetic acid (IAA) and abscisic acid were applied by placing a 50 ,ul drop of 1.5% agar containing the IAA or abscisic acid on the distal cut surface of an explant. IAA concentration was 5 X 10-5 M and abscisic acid, except for so-me preliminary concentration curve experiments, was 5 X 10-4 m. The abscisic acid used in these experiments was a gift of the R. J. Reynolds Tobacco Conmpany and consi-ted of 47.3 % d,l-cis,trans abscisic acid (the natural isomer) and ,523 % of the d.l-trans-trans isomer. All concentrations were based on the cis-trats isomer. Samples of dl-cis,tans isomer (SD 16108) were a gift of the Shell Development Company, and a number of preliminary experiments with tthis material indica-ted that there was no essential difference in the proRerties of tIe 2 preparations. Earlier papers from this laboratory have described Lhe trea:mncnt of explants with e.hylene anid CO., in desiccators and gas collection bottles (4), the measurement of ethylene by gas chromatography (8), the injection of actinomycin D (1 1 g//,l water) into the separation layers of explants (6), the determination of celluilase ac.ivity by the lo s of viscosity of sodiuim carboxymethyl cellulose (CMC) (4), and break strength measurements by a recording abscissor ( 14).