On Limits to Seed Production

The question is often posed whether seed production islimited by pollen supply or by resources available to the mother plant for maturing seeds (see, e.g., reviews in Bierzychudek 1981; Stephenson 1981; Willson and Burley 1983). Here we offer a simple model showing that natural selection may often act to bring the female's allocation of her reproductive effort to a point where seed production is limited by both pollen supply and provisioning resources. Further, the model indicates that seed production could be expected to respond to a decrease in pollen supply but not to an increase. Suppose a female allocates an amount a of reproductive effort o attracting pollen and producing immature ovules. The number of ovules that can be fertilized increases with a. Assume further that a is taken from a limited total reproductive effort, such that the number of seeds that the mother can provision to maturity decreases with a (fig. 1). For any given value of a, the number of seeds that can be produced is the lesser of these two functions. The greatest number of seeds can be produced at a*, where the functions cross. Below a*, seed production is pollen-limited, and increased allocation to attracting pollen increases eed set. Above a*, seed production is resource-limited, and reduced allocation to attracting pollen increases the resources available for provisioning seeds. The model illustrates the economic principle that internal resources are optimally allocated among competing processes when each resource limits all processes to the same degree (Bloom et al. 1985). Here we consider only two processes, pollen attraction and ovule provisioning, but the principle would still apply if other processes such as pollen production and vegetative reproduction were included. The model is best understood on an evolutionary time scale. If seed production of a species is consistently pollen-limited, natural selection should favor individuals that allocate more resources to pollen attraction. If seed production is consistently provisioning-limited, in ividuals that allocate less to pollen attraction are favored. Of course, the precise position of the two functions will not be the same for all individuals in all years. The allocation to pollen attraction that is selected, a', should give higher seed yields on the average than alternative allocations, but, for a particular individual in a particular year, the functions might intersect o one side or the other of a'. In this sense, the individual could be limited either by pollen acquisition or by resources available for provisioning.