Effects of pollen and nitrogen availability on reproduction in a woodland herb, Lysimachia quadrifolia

Herbivory differentially affects male and female reproductive traits of Cucumis sativus

Herbivory is an important selection pressure in the life history of plants. Most studies use seed or fruit production as an indication of plant fitness, but the impact of herbivory on male reproductive success is usually ignored. It is possible that plants compensate for resources lost to herbivory by shifting the allocation from seed production to pollen production and export, or vice versa. This study examined the impact of herbivory by Helix aspersa on both male and female reproductive traits of a monoecious plant, Cucumis sativus. The effects of herbivory on the relative allocation to male and female flowers were assessed through measurements of the number and size of flowers of both sexes, and the amount of pollinator visitation. We performed two glasshouse experiments; the first looked at the impact of three levels of pre-flowering herbivory, and the second looked at four levels of herbivory after the plants had started to flower. We found that herbivory during the flowering phase led to a significant increase in the number of plants without male flowers. As a consequence there was significantly less pollen export from this population, as estimated by movement of a pollen analog. The size of female flowers was reduced by severe herbivory, but there was no affect on pollen receipt by the female flowers of damaged plants. The decrease in allocation to male function after severe herbivory may be adaptive when male reproductive success is very unpredictable.

Pollen limitation in a northern population of Hepatica acutiloba

Pollen and (or) pollinator limitation may be most likely to occur in spring wildflowers or populations of species near their distribution limits. From 1990 to 1992, we examined a 2-ha population of a perennial spring wildflower, Hepatica acutiloba (Ranunculaceae), at the northern distribution limit of the species. To distinguish between the often confounded effects of pollen and resource limitation, we applied partial and whole-plant treatments. Ramets were open-pollinated, cross-pollinated, open- and cross-pollinated (50% of flowers were open-pollinated, and 50% were hand-pollinated), or self-pollinated. Although there was year to year variation, plants with cross-pollinated flowers produced significantly more viable seeds and heavier seeds than plants with self- or open-pollinated flowers. Comparisons between seed numbers and masses from complete and partial treatments suggested that there was no reallocation of resources among flowers on a plant. From 1990 to 1992, a minimum of 95% of all visits to a subset of ramets of H. acutiloba were from Asclera ruficollis (Coleoptera: Oedemeridae), i.e., virtually the only insect active during flowering. Mark–recapture data for A. ruficollis indicated that even after 6–48 h, 22–40% of the individuals remained within 10 m of the same ramet of H. acutiloba. Observational data indicated that less than 50% of all flowering ramets of H. acutiloba may be visited by A. ruficollis. We conclude that pollen limitation, mediated by reliance on one sedentary pollinator, was the main cause of low seed production in open-pollinated flowers of H. acutiloba. Key words: Hepatica acutiloba, pollen limitation, pollinator limitation, distribution limit, Asclera ruficollis, Oedemeridae.

POLLINATION DYNAMICS IN EPILOBIUM CANUM (ONAGRACEAE): CONSEQUENCES FOR GAMETOPHYTIC SELECTION

Several hand-pollination experiments have shown that pollen tube competition leads to gametophytic selection, and thus affects genotypic and phenotypic characteristics of the next generation. This study is one of the first to quantify natural levels of pollen tube competition. In a population of Epilobium canum, I measured both the amount of pollen deposited on stigmas and the timing of deposition. Approximately 20 tetrads were required for full seed set within fruits. Hummingbirds deposited >20 tetrads at 50-70% of the flowers examined, often in a single load. When pollen arrived in 2 loads, a portion of the ovules within each ovary was probably sired by competing pollen from the second load. Competition may be relatively weak unless at least 30 tetrads compete for ovules. About 20% of the flowers received >30 tetrads in the first load, and 13% acquired > 30 tetrads in 2 loads. The frequency and intensity of pollen tube competition varied among plants. In some styles, 80% of the pollen tubes were excluded from access to ovules, but in others no competition occurred. Further studies of pollination rates and progeny fitness are needed before we can assess the role of pollen tube competition in natural populations. Potential effects of gametophytic selection are discussed.