Sex Allocation Plasticity in Response to Resource and Pollination Availability in the Annual Plant Brassica rapa (Brassicaceae)

Abstract

Premise of research. Cosexual plants reproduce as females by maturing seeds from fertilized ovules and as males by fertilizing their own ovules or those of other plants. Condition-dependent sex allocation occurs when the environment elicits quantitative differences in the rate at which fitness increases as resources are invested in male versus female function. Here, we examine whether resource availability and the pollination environment generate plastic responses in floral sex allocation and correlated traits.Methodology. We exposed the cosexual mustard species Brassica rapa to variation in resource availability (altered pot size) and in the pollination environment (unpollinated or fully pollinated). We assessed investment in anther and ovary length, which are known to correlate with pollen and ovule number; in flower buds produced at the onset of reproduction; and in buds produced approximately 15 d later. We also measured plant size and flower size, which are often correlated with increased allocation to female and male function, respectively.Pivotal results. In contrast to our predictions, anther-to-ovary length ratios at the onset of reproduction did not vary across plants of different sizes and were unaffected by the pot size treatment, despite an increase in anther lengths in the small pot treatment. Similarly, neither the pot size nor the pollination treatments elicited plasticity in anther-to-ovary length ratios over the course of reproduction. However, independent of plant size and as predicted, plants produced larger flowers over the course of reproduction in the pollination-absent treatment.Conclusions. We discuss the contexts in which resource availability and the pollination environment can influence sex allocation and provide cautionary advice on effective methods for experimentally generating and measuring sex allocation plasticity.