SEXUAL STRATEGIES IN PLANTS: I. AN HYPOTHESIS OF SERIAL ADJUSTMENT OFMATERNAL INVESTMENT DURING ONE REPRODUCTIVE SESSION

Abstract

Summary: It is postulated that in one reproductive session the level of maternal expenditure of an angiosperm plant is determined by a temporal series of controls on the number of potential fruit in which an investment is made. The serial adjustment hypothesis has three parts: A. The amount of maternal expenditure is regulated at many developmental units, particularly single flowers and fruit, at three principal sequential stages – the determination of flowers, the development of ovaries and the maturation of fruit. B. At each stage, the initiation or continuation of an investment requires an amount of available resources above a certain threshold. Hence maternal expenditure is continually adjusted to the resources available at each developmental site. C. The pattern of controls that maximizes the maternal fitness of a plant is selected. Many factors affect the relative advantages of regulation at the three stages, therefore species and sexual morphs appear to vary widely in their proportional use of the three stages. Regulation of flower determination has the general advantages of offering bidirectional adjustment of maternal investment, maintaining a constant ratio of maternal to paternal investment, and reducing wasted expenditure. The later stages, ovary and fruit regulation, allow secondary adjustments of maternal investment in unpredictable circumstances and permit adjustment of the relative numbers of polliniferous and seminiferous flowers. The relative advantages of restricting the numbers of developing ovaries, or of maturing fruit, depend principally on the extent to which differences in the capacity of flowers to mature fruit are evident before anthesis.