Effect of Older Fruits on Abortion and Abscisic Acid Concentration of Younger Fruits in Phaseolus vulgaris L

Abstract

In two cultivars of bean (Phaseolus vulgaris L., Redkloud and Redkote) the older fruits growing at the base of racemes aborted less frequently than the younger ones above them. When older fruits at the base of racemes were removed, the abortion rate of the younger ones was reduced and their abscisic acid (ABA) concentration was lowered. Thirteen days after fruit removal, 36 to 45% of the younger fruits remained viable on treated plants while less than 12% of the younger fruits were viable on control plants. On these intact controls the ABA concentration of young fruits was at least twice that of defruited plants. A similar difference was found when the ABA content was expressed on a per fruit basis, suggesting a direct regulatory influence of older fruits over the ABA content of younger fruits.Premature fruit abscission, often of a large percentage of fruits on a plant, is a common phenomenon that has been reported for a number of plant species including apples (9), beans (15), and cotton (4,8). In beans, abscission appears to be the last step in the process of fruit abortion, which is characterized by cessation of seed development, flattening of pod walls, and loss of green color (15). Events leading to fruit abortion may include a decrease in the concentration of auxins (9) and an increase in the concentration of ethylene and ABA (4,8). However, neither the regulation of these hormonal changes within a fruit nor the coordination of fruit abortion with over-all plant development is well understood.In the companion article, fruits were shown to inhibit the growth of axillary buds of bean plants (15). The present report describes the competitive inhibition of young fruits by adjacent older fruits of bean plants. MATERIALS AND METHODSBean plants (Phaseolus vulgaris L. cv. Redkloud and cv. Redkote) were grown in a controlled environment growth chamber. The conditions of plant growth and methods of ABA analysis were described in the companion paper (15). The effect of older fruits on the abortion rate and ABA content of the younger ones was tested as follows. Fruits within each raceme were classified into categories of "older" (those at the base of the raceme), "younger" (those in the upper part of the raceme), and "aborting" RESULTS Effect of Older Fruits on the Growth and Abortion of YoungerFruits. The relative age of developing bean fruits within a raceme affected their rate of abortion: older fruits at the base of the raceme aborted less frequently than the younger ones above them. When the older fruits were removed from each raceme, the abortion rate of the younger fruits greatly decreased in both cultivars (Table I). On treated plants (older fruits removed), 36 to 45% of the younger fruits remained viable, while on controls the number was below 12%. Only 18 to 33% of the older fruits aborted on control plants. The effect of older fruit removal was highly significant, as shown by analysis of variance, on the number of viable younger fruits (F = 45.5, P = 0.001). Over 82% of the total variation in young...

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