Pollination services and pollen limitation in Triodanis perfoliata, a cleistogamous annual plant
Pollen limitation is a phenomenon wherein a scarcity of conspecific pollen limits plant reproduction. Although several studies have explored pollen limitation in wild plant populations, relatively little is known about how pollen limitation affects reproductive allocation. Studies have shown that increased pollen limitation can increase selfing rates and reproductive allocation toward selfing in plants with mixed mating systems, likely because selfing can provide reproductive assurance. However, the effect of pollen limitation on mating system is still not empirically well described.
This study determines if pollen limitation occurs in the cleistogamous annual, Triodanis perfoliata, a plant that reproduces using both open, chasmogamous (CH), facultative outcrossing flowers and closed, self-pollinating cleistogamous (CL) flowers. Four treatments were applied in a natural population in Falkland, NC: (1) No manipulation (ambient pollination) (n=30), (2) Chasmogamous hand-supplemented with outcross pollen (n=28), (3) Bagged chasmogamous hand-supplemented with self pollen (n=28), and (4) Bagged plants to exclude outcross pollen but allow selfing (n=40). This study determines how pollen availability influences resource allocation toward selfing CL flowers in terms of the relative flower number and aggregate seed production.
Comparison of seed production by ambient-pollinated plants to seed production by outcross-supplemented plants suggests pollen limitation of outcross pollen. Preliminary results show that fruits from the outcross supplementation treatment had the highest average CH seed count per fruit. Auto-fertility was only partially effective; bagged plants produced 66% as many seeds as bagged, self-supplemented plants. Mean CL seed count for outcross-supplemented and ambient-pollinated plants are similar (an average of 38.4 and 41.98 CL seeds per fruit, respectively).
Treatments differed in their reproductive allocation to CH flowers, suggesting reproductive allocation changes in response to pollination services. Outcross supplemented plants had the greatest reproductive allocation to chasmogamous reproduction (80% of total reproduction). Open-pollinated plants and bagged, self-supplemented plants had intermediate levels of %CH (72% and 75%, respectively). Bagged plants had the lowest %CH, with just 61%CH.
Results are consistent with theory predicting that optimal pollinator conditions favor reproduction by CH flowers whereas limiting pollinator conditions favor allocation to CL flowers. Findings fit with several models suggesting that stochastic environmental factors may stabilize evolution of cleistogamy by oscillating between CH-favorable and CL-favorable conditions.