Effect of climate, anther morphology and pollination syndrome on pollen availability in Penstemon

Traditionally, pollen presentation is thought to be a function of pollinator type and visitation frequency. However, despite the repeated observation that pollen presentation is influenced by flower morphology and abiotic factors, these aspects have been little studied in the wild. Here, we evaluated the effect of climate, anther morphology, and pollination syndrome on anther dehiscence time (the length of time an anther takes to fully dehisce after a flower opens). We recorded anther dehiscence time in twelve species of Penstemon including the four major anther types and the two most common pollination syndromes. We also conducted an experiment to measure the effect of humidity and temperature on anther dehiscence. We found that anther morphology was correlated with anther dehiscence time. Anthers with wide openings take the longest time to dehiscence. These results provide some support for the hypothesis that anther dehiscence time has evolved to decrease pollen wastage. Contrary to the assumption that bird-pollinated species have simultaneous pollen presentation, hummingbird-pollinated species had longer anther dehiscence time than most bee-pollinated species. The experiment revealed that high humidity and low temperature increase anther dehiscence time. Our results suggest that pollen presentation is influenced by anther morphology, pollination syndrome, and the physical environment. Optimal pollen presentation presumably maximizes conspecific pollen transfer and reduces pollen thieving.