Reproductive Effort in Plants. 3. Effect of Reproduction on Vegetative Activity

One basic assumption of the various theories proposed to explain differences in resource-allocation patterns among plants is that different structures or activities are alternatives: a gain in one results in a proportional loss in another. We attempted to test this assumption in Agropyron repens by determining whether the resources allocated to reproduction reflect the effect of reproduction on vegetative growth. This was done by comparing the growth of reproductive and vegetative plants grown under conditions differing only in photoperiod. Reproduction in this species was much less costly than the resources allocated to reproduction would indicate. Despite the large proportion of resources allocated to reproduction in most treatments, reproduction had little detrimental effect on total growth; in a few cases, it actually enhanced vegetative growth. There were substantial differences among genotypes and environments in the relationship between the resources allocated to reproduction and its effect on vegetative activity. This suggests that even in comparisons of plants of the same species, resource allocation to reproduction is not necessarily a reliable indicator of the cost of reproduction. Furthermore, since plants can experience different costs per unit of resource allocated to reproduction, it is conceivable that differences among plants in the resources allocated to reproduction may, in part, reflect these differences in cost. A plant with a low cost of reproduction, for example, may be able to allocate a larger proportion of its resources to reproduction. This suggestion represents an alternative to the traditional explanations of life history theory for differences in reproductive effort among plants. In our experiment, the relative differences among different plants in the effect of reproduction on vegetative activity remained the same whether the effect was measured in terms of lost biomass, lost carbon (biomass plus respiration), or lost nitrogen. There were differences, however, between these measures of the cost of reproduction and the cost measured in terms of lost vegetative phosphorus, suggesting that it is important to use the appropriate currency to evaluate the cost of reproduction.