Explosive pollen release, stigma receptivity, and pollen dispersal pattern of Boehmeria caudata Sw. (Urticaceae) in a Brazilian rain forest

Abstract

Male flowers of the wind-pollinated, dioecious shrub Boehmeria caudata Sw. release pollen explosively, possibly stimulated by anther dehydration in response to temporarily dry conditions coupled with hydrostatic pressure in the filament. In the Brazilian Atlantic rain forest, the daily frequency of male flower anthesis peaked between 1000–1200 h (GMT −3) and was positively correlated with rising temperature, wind currents, and direct insolation, but negatively correlated with relative humidity. A generalized linear mixed model predicts the probability of pollen release under different conditions of relative humidity and wind. Receptive stigmas on female flowers were found at any time of day: individual stigmas were receptive (H2O2 test) for at least two days. Beetles and halictid bees were observed feeding on pollen from staminate flowers, but probably did not affect pollination because they did not seem to visit pistillate flowers. Individual plants of this apparently ruderal species grow along road edges and show aggregated (clumped) spatial dispersion. The overall sex ratio we found was 29 females to 43 males, with one monoecious plant in a total transect distance of 6319 m. Pollen traps were placed on the female plants to measure pollen arrival. Pollen capture was higher on the more exposed road-facing side and positively related to male plant density.