Manifold limits on seed production by individual flowering plants

Historically, angiosperm seed production has been considered to be either pollen or resource limited. However, seed production involves more than pollen and seed resources and typically involves multiple ovules in multiple flowers. Therefore, various constraints may act simultaneously.

Proportional components of seed production were assessed for rapid-cycling Brassica rapa plants on which 25, 50, 75, or 100% of flowers were hand outcrossed.

Fruit set increased in a decelerating manner with the percentage of pollinated flowers. For all treatments, only flowers pollinated early on plants produced fruit, although the transition to complete fruiting failure was delayed on poorly pollinated plants. Despite stigmas receiving abundant pollen, only 61% of ovules in pollinated flowers were fertilized, on average. Fertilization success varied within ovaries, with more fertilization of apical ovules in flowers on plants with 75% of pollinated flowers, but more fertilization of basal ovules with 25% pollination. Per-flower fertilization also varied positively with ovule number and the number of basal fruits and negatively with flower position. Almost all fertilized ovules developed into seeds, so variation in seed set depended primarily on the effects on ovule fertilization.

Regardless of pollination treatment, seed production by all plants was jointly limited by both pollen and availability of seed resources and possibly by ovule infertility/receptivity. A simple model demonstrates that such co-limitation can be adaptive if plants produce excess flowers. Interacting effects of the proportion of pollinated flowers and anthesis day on fruit set are consistent with hormonal mediation of resource distribution among fruits.