THE OPTIMAL NUMBER OF FEMALE FLOWERS AND THE FRUITS-TO-FLOWERS RATIO IN PLANTS UNDER POLLINATION AND RESOURCES LIMITATION

The optimal allocation of resources to pollination efficiency and pollination limitation on an evolutionary time scale can lead to high numbers of female flowers and low fruits-to-flowers ratios. Damage to young fruits by genetic defects, by insects, or by climatic factors is probably an important factor that increases the optimal number of female flowers and decreases the fruits-to-flowers ratio. This includes parental selection for seed or fruit equality. Short-term increases in either pollination probability or resource levels do not increase fruit production by plants that had reached an evolutionary equilibrium under constant conditions. Random variation in pollination level, young-fruit survival, or resource level can cause a significant increase in the number of female flowers and a decrease in the fruits-to-flowers ratio, especially when the cost of flowers or young fruits is low relative to that of mature fruits. The evolutionarily stable sex ratio in most plants selects for a high number of functionally male flowers. The cost of adding female reproductive organs to male flowers to produce bisexual flowers is considerably less than the cost of producing female flowers. This lower cost may provide the selective advantage that explains the prevalence of plants with bisexual flowers. -Authors