Experimental Thinning Reduces Inter-ovary Competition Among and Within Inflorescences.

Abstract

Background and aims: Resource availability often limits female reproductive success in plants, especially when multiple flowers within inflorescences simultaneously compete for limited resources. Here, I explored whether relaxing inter-ovary competition-through experimental thinning of inflorescences and/or inflorescence buds -influences resource allocation and enhances reproductive output in Lupinus polyphyllus, a legume species with vertical inflorescences that typically show a decline in fruit and seed production from basal to distal flowers.

Methods: I implemented a gradient of reproductive thinning: (1) no removal (control), (2) removal of half of the current open inflorescences, and (3) removal of both half of the open inflorescences and all inflorescence buds (i.e., future racemes). For three randomly selected remaining inflorescences per plant, I recorded fruit number, and within three sections along the inflorescence (basal, middle, and distal). For each fruit in each of these three inflorescences, I also counted the number of viable seeds, aborted seeds, and unfertilized ovules.

Key results: Results strongly supported the inter-ovary competition hypothesis. Inflorescence removal increased fruit set and viable seed production, reduced seed abortion, especially when both inflorescences and buds were removed. These effects occurred consistently across all positions within inflorescences, suggesting enhanced resource allocation even to distal flowers, which are often resource-limited. Ovule production per flower was unaffected, indicating no over-compensation prior to fertilization. Fertilized ovule numbers declined from basal to distal flowers, consistent with non-uniform pollen receipt, likely influenced by bumblebee foraging behavior. However, fertilization ratios remained high (80-90%) across positions and treatments.

Conclusions: These findings demonstrate that inflorescence thinning can effectively relax resource-based constraints within and among inflorescences, enhancing female success without altering pollination dynamics. This contributes to our understanding of how architectural plasticity and developmental constraints shape reproductive trade-offs in flowering plants.