RESOURCE AND POLLEN LIMITATIONS TO LIFETIME SEED PRODUCTION IN A NATURAL PLANT-POPULATION

Using experimental manipulations, we examined pollen and resource limitations to seed production and their interaction in a natural population of a monocarpic plant, Ipomopsis aggregata. Our design crossed two factors, pollen level (hand-pollinated or control) and resource level (water added, water and fertilizer added, or control). Both hand-pollination and fertilizing during the blooming season increased total seed production, while watering alone had no effect on any component of reproductive success. Hand-pollination boosted number of seeds per flower, with no effect on flower number. In contrast, fertilizing had its primary effect on the number of flowers produced, while also increasing the number Of seeds per mature fruit in hand-pollinated plants. Fertilizing increased nectar volume, but path analysis detected no indirect effect of this increased reward rate on seed production, suggesting instead that fertilizing had a slight, but direct, effect on seeds per flower. These results argue against a strict dichotomy between pollen limitation and resource limitation of female reproductive success in plants.