EFFECTS OF GROWTH-REGULATORS ON REPRODUCTIVE ABSCISSION IN FABA BEAN (VICIA-FABA CV TROY)

Growth regulators were applied to faba bean (field bean) plants in order to study hormonal control of premature reproductive abscission. Combined data from pairs of racemes sited in a consistently yielding region of the main stem indicated effects of growth regulator treatments. Decapitation increased pod numbers, but the inhibitory influence of the upper shoot could not be replaced completely by indol-3yl-acetic acid (IAA) applied to the cut stem stump. Blockage of auxin transport through the stem with the auxin-transport inhibitors 2,3,5-triiodobenzoic acid (TIBA) or methyl-2-chloro-9-hydroxyfluorene-9-carboxylate (CF, chlorflurenol) reduced pod numbers above the application point. The latter effects might have been indirect and due to elevated ethylene levels, since pod retention in this shoot region was decreased when sprayed with the ethylene-releasing compound 1-aminocyclopropane carboxylic acid (ACC) and increased when sprayed with ethylene-biosynthesis inhibitors 2-aminoethoxyvinyl glycine (AVG) or 1-amino-2-cyclopropylcyclopropane carboxylic acid (cyclopropyl-ACC). Pod numbers were increased when the reproductive structures themselves were sprayed with IAA, gibberellic acid (GA3) or 6-benzylaminopurine (BAP), although a corresponding increase in total seed weight occurred only with the latter growth regulator. Application of BAP or abscisic acid (ABA) as root drenches showed that levels of reproductive abscission were raised or lowered with BAP treatment, depending on its concentration, and lowered with ABA treatment. It is suggested that growth regulator treatments leading to decreased ethylene production in the shoot and/or increased cytokinin and abscisic levels in the xylem sap are those most likely to reduce abscission.