WHY DOES THE SIZE OF REPRODUCTIVE STRUCTURES DECLINE THROUGH TIME IN HYDROPHYLLUM APPENDICULATUM (HYDROPHYLLACEAE)?: DEVELOPMENTAL CONSTRAINTS VS. RESOURCE LIMITATION

A commonly observed pattern in plants is that the size of some reproductive structures declines through a single flowering season. The experimental design of this study allowed me to determine whether architectural constraints or resource limitation is responsible for the seasonal declines in flower size, inflorescence size, and seed weight in the biennial Hydrophyllum appendiculatum (Hydrophyllaceae). Plant resource status was manipulated by varying the number of inflorescences pollinated (two vs. all). Architectural constraints were inferred by comparing the rates of decline in size of reproductive structures in plants that underwent either high or low pollination intensity. The data demonstrated that the relative importance of resource limitation and developmental constraints is trait-specific. Even when not resource stressed, all traits exhibited a seasonal decline, demonstrating that ontogenetic changes occurred in plant architecture. Yet, for two traits, inflorescence size and seed weight, the temporal decline was significantly more severe for heavily pollinated plants. In contrast, the decline in flower size through time was identical in both pollination treatments. Thus, the size of flowers is buffered from the resource status of the plant. These findings illustrate the importance of considering the physical limitations of plant architecture when investigating patterns of plant reproduction.