Serial Adjustments in Allocation to Reproduction: Effects of Photosynthetic Genotype

Life history theory predicts that trade‐offs between size and reproductive traits should increase when resources are more limited. This prediction is commonly tested by using plant size as a surrogate for resource status or by comparing trade‐offs across treatments that experimentally manipulate size or environmental resources. In this study, we take a novel and potentially effective approach and compare two well‐characterized genotypes ofAmaranthus hybridusthat differ in photosynthate availability. We consider these genotypes a valuable proxy for inherent genetic variation in resource status because photosynthate is a resource base that will most likely underlie many allocation‐based trade‐offs. We examine a series of hierarchical stages in which adjustments in allocation are predicted to affect seed production, and we test whether these adjustments are more dramatic in the genotype with less available photosynthate. We find that the response to resource limitation was not to decrease allocation to reproduction, alter floral sex ratio, or abort more seed. Rather, the resource limitation increased the trade‐off between seed number and size. By delaying resource‐based adjustments in allocation until the seed maturation stage, this species has a strategy that places priority on maximizing the quantity rather than the quality of offspring. Genetically based differences in photosynthate availability can affect the relationships between size and reproduction and therefore affect fitness.