The movement of abscisic acid in the cotton explant was investigated by bioassay and radioisotope techniques. The rate of movement was 20 to 30 millimeters per hour with most of the abscisic acid moving unchanged through the explant into the basal agar. The rate of movement was the same through the abscission zone as through petiole tissue. Patterns of accumulation and metabolic products are discussed.Abscisic acid has been shown to be a very effective abscissionpromoting agent in the cotton explant bioassay (1,2,13,14). It has been presumed that abscisic acid is rapidly transported in the explant to the abscission zone, its site of action (1), but to date no direct proof of this phenomenon has been offered. An alternative possibility is that abscisic acid is immobile and that it induces the production of an abscission-promoting agent or stimulus which is transported. A study of the movement of ABA in the explant was undertaken to answer the questions (a) does ABA move from the site of application to the abscission zone, (b) what is the rate of movement, (c) does it accumulate in any area of the explant? MATERIAIS AND METHODS All experiments were conducted with cotton explants excised from 14-day-old seedlings of Gossypium hirsutwn L. Acala SJ-1 grown in vermiculite subirrigated with tap water. The seedlings were grown in a growth chamber at 32 i 3 C with a 16-hr photoperiod of 5.8 X 104 erg/cm2.sec provided by cool white fluorescent lamps supplemented by incandescent bulbs.Explants from the cotyledonary nodes were cut with hypocotyl segments 9 mm long and epicotyl and petiole stumps 5 mm long. The cotton explant bioassay method of Addicott et al. (1) was employed except where noted. Briefly, the bioassay involved placing 10 explants (each containing two abscission zones) in a covered Petri dish. The bases of the explants were pushed into a 5-mm layer of 1.5% agar. The cut surfaces of the epicotyl and petioles were then treated with 5-,ul droplets of agar (0.75%) delivered by a 500-,l hypodermic syringe. Abscission was tested for by applying a momentary force equivalent to 5 g to the distal end of the petiolar stumps. Each treatment represented two dishes of 10 explants, and the experiments were repeated three times.The movement of ABA was studied with 2-l4C-abscisic acid produced according to the technique reported by Roberts et al. (11). After treatment the explants were cut into various sections as indicated in Figure 3. The pooled tissue sections were soaked for 24 hr in sealed vials in 2 ml of 95% ethanol made to 0.05 N with acetic acid to leach out labeled compounds. Fifteen milliliters of scintillation fluid (100 g of napthalene; 5 g of PPO [2,5-diphenyloxazole]; 1 liter of 1,4-dioxane) were added to the ethanol solutions in each vial containing the tissue sections. The 14C activity was determined by counting for 20 min per sample in a Beckman liquid scintillation spectrometer model 100.Recovery of the '4C-labeled compounds from the tissue and the basal agar was done by extracting with 80% metha...
