Abstract
The mechanism regulating the growth of adult plants in two determinate bean (Phaseolus vulgaris L.) cultivars was investigated. “Redkloud” plants flowered, formed fruits, and ceased shoot growth earlier than “Redkote” plants. Redkloud attained a smaller plant size, compared to Redkote, by imposing dormancy on axillary buds at an earlier age. In both cultivars, cessation of bud growth coincided with maximum combined fruit length per plant. Removal of fruits caused resumption of axillary bud growth within 4 to 5 days. The amount of new growth induced by fruit removal depended on the cultivar and plant age. In fully developed Redkloud plants, where shoot growth had already ceased, total leaf and shoot number per plant nearly doubled within 2 weeks following fruit removal. A much smaller response was observed in the still growing Redkote plants. Fruits, therefore, are assumed to play a major role in the regulation of shoot growth and total plant size through the control of axillary bud dormancy. It seems that smaller plant size, earlier maturity, and earlier senescence of Redkloud, compared to Redkote, were the result of earlier flowering, and accomplished in part through the growth-inhibiting action of fruits.
The endogenous abscisic acid (ABA) concentration of axillary buds was higher in Redkloud than in Redkote. It increased with plant age in both cultivars. Five days after fruit removal the ABA level in bud tissue dropped to approximately 10 to 30% of the control level. When buds were treated with a solution of ABA containing 5 nanomoles of ABA per bud, growth was substantially inhibited. Fifteen days after ABA application the mean length of growing buds on intact and defruited plants was reduced by 40 and 62%, respectively, compared to the untreated controls. A role for ABA in axillary bud growth regulation was not firmly established, but these data suggest correlation between the growth potential of axillary buds and their ABA concentration.
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