WHY DO PLANTS PRODUCE SURPLUS FLOWERS - A RESERVE-OVARY MODEL

A low fruit:flower ratio has been observed in many plant species, and several functional advantages of nonfruiting flowers have been proposed. One hypothesis is that surplus flowers provide an ovary reserve in case of loss due to unpredictable external mortality at the flower level. In this article a quantitative model is proposed in which the optimal flower number is a function of two fitness costs. First, each flower represents a cost that reduces potential seed production. Second, those cases in which fruit production is ovary limited represent a cost. The model predicts that more flowers may be produced than there are resources necessary for them to develop to maturity, even if the only function of the surplus flowers is to serve as reserve ovaries. The number of surplus flowers increases with increasing fruit-cost:flower-cost ratio and increasing ovary mortality. The presented model suggests a method by which to formulate quantitative, testable predictions on optimal fruit:flower ratios and may thus serve as a null hypothesis for alternative functional explanations of the occurrence of surplus flowers. A scanning of available literature suggests that the proposed model may explain very low fruit:flower ratios in a few species with costly fruits, whereas in species with relatively cheaper fruits more modest numbers of surplus flowers are favored by the proposed mechanism.