The effects of nectar secondary compounds on self-pollen transfer and female plant reproduction

Pollen movement within and among plants affects inbreeding, plant fitness, and the spatial scale of genetic differentiation. Although a diversity of studies have assessed how plant and floral traits influence pollen movement, fewer studies have assessed how nectar components affect pollen transfer. As many as 55% of plant species produce secondary compounds in their nectar (hereafter toxic nectar). The existence of toxic nectar is surprising given that nectar is typically thought to attract pollinators. Here we tested the hypothesis that toxic nectar may benefit plants by discouraging pollinators from foraging on too many flowers per plant, thus reducing levels of geitonogamy (self-pollen transfer). We tested this hypothesis using the self-incompatible plant Gelsemium sempervirens (Loganiaceae). Gelsemium produces the alkaloid, gelsemine, in its nectar, and gelsemine has been shown to be deterrent to foraging bee pollinators. We manipulated concentrations of nectar gelsemine and found that high nectar gelsemine reduced the proportion of flowers probed by pollinators and reduced self-pollen transfer by at least one-third. Using greenhouse hand-pollinations, we found that some self-pollination treatments reduced seed production relative to some outcrossed treatments. To test the effects of nectar gelsemine and self-pollination on reproduction of Gelsemium, we manipulated nectar gelsemine and access to self pollen and measured seed output.  Surprisingly, the benefits of toxic nectar in reducing geitonogamy and impacting seed output were equivocal. Results suggest that any plant-fitness benefits associated with toxic nectar through reduced self-pollination will be strongly dependent on the degree of pollinator and pollen limitation in the field.