Transient dehydration of pollen carried by hot bees impedes fertilization

Abstract

In the intense solar radiation of an alpine climate, small black bees often experience extremely high thoracic temperatures when they are foraging on flowers, but flies forage at lower temperatures. To explore the hypothesis that seed set could be depressed by transient dehydration of pollen at the high temperatures reached by hot bees foraging in sunshine, we compared the effectiveness of single visits by different pollinators to a bowl-shaped flower Potentilla lancinata in alpine meadows, SW China. The ratio of seed set to pollen transferred in individual flowers was monitored over 2 years, indicating that pollen deposited on stigmas by halictid bees produced lower seed set than pollen carried by flies. Scopal pollen applied to stigmas by hand gave good seed set, but in germination tests it burst more frequently than pollen from anthers, implying dehydration. Pollen grains freshly taken from the scopae of solitary bees foraging in sunshine were smaller than those taken from anthers or foraging bees in early-morning overcast conditions, implying dehydration. The effect was reversible: hand pollination showed that scopal pollen was no less effective than fly pollen after removal from the bee. Pollen carried by such bees foraging in intense sunlight in flowers became dehydrated, causing an osmotic mismatch between the pollen and the stigmas. Transient heat-induced dehydration of pollen represents a novel pathway by which climate warming may disrupt plant reproduction, and helps us understand why flies could be more effective pollinators than bees in cool, high-radiation arctic or high-altitude sites.