The Influence of Pollination Intensity on Fertilization Success, Progeny Sex Ratio, and Fitness in a Wind-Pollinated, Dioecious Plant

Abstract

Variation in pollination intensity can influence the degree of gametophytic competition with diverse consequences for plant fitness. Such effects should be particularly important in wind-pollinated plants because they commonly possess uniovulate flowers. In several dioecious species, competition between female- and male-determining microgametophytes has been proposed as a cause of female-biased sex ratios. Here, we examine functional relations among pollination intensity, seed set, sex ratios, and sporophyte vigor in Rumex hastatulus, a wind-pollinated, dioecious plant. Using controlled pollinations, we applied a range of pollen grain numbers (1–71) to female flowers. Pollination intensity significantly predicted percent seed set, which was >80% when more than 15 pollen grains were added to stigmas. We estimated that the proportion of pollen grains participating in fertilization was 0.28 and the proportion of fertilized ovules developing into seed was 0.89. Overall, progeny sex ratios were female biased (0.62), with an increased probability of female offspring over the range of 1–15 pollen grains. We detected no effect of pollination intensity on sporophytic vigor. Our study is the first to investigate the relation between pollination intensity and female fertility in a wind-pollinated species and provides support for the contribution of gametophytic competition to sex ratio variation in plants.