Role of Indoleacetic Acid and Abscisic Acid in the Correlative Control by Fruits of Axillary Bud Development and Leaf Senescence

Abstract

When fully filled pods of bean plants were deseeded, the rate of axillary bud growth and the chlorophyll content of leaves were increased. Application of 0.1% indoleacetic acid (IAA) in lanolin on the deseeded pods caused abscission of axillary buds, inhibited growth of the remaining buds, and decreased leaf chlorophyll content. The response of bud development to fruit-applied IAA was concentration dependent between 0.001 and 0.1% IAA (representing from 2 to 200 micrograms IAA per fruit) resulting in greater growth inhibition at higher IAA concentrations. When plants were defruited so that the number of fruits per plant was adjusted to 0, 6, 12, or 18, a dosage effect of fruits on photosynthesis was observed. Removal of all fruits caused a rise in the CO₂-exchange rate (CER). With increasing fruit dosage, plants showed leaf senescence of increasing intensity and a corresponding decline in CER. In contrast to the effect of fruit-applied IAA on leaves and buds, it delayed the senescence of treated fruits. When axillary buds were treated directly with aqueous solutions of IAA, no growth inhibition occurred. Abscisic acid (AbA) applied on deseeded pods, up to a concentration of 0.1% AbA in lanolin, failed to inhibit axillary bud development or to cause leaf senescence. The results support the hypothesis that the correlative control of axillary bud development and leaf senescence by fruits involves the participation of both IAA and AbA. IAA, released by the seeds, may play the role of the correlative signal that moves from the fruit to the target organ, where it stimulates the synthesis or accumulation of AbA. AbA, in turn, may be responsible for the inhibition of axillary bud development and the enhancement of leaf senescence.