Large pollen at high temperature: an adaptation to increased competition on the stigma?

Abstract

Pollen availability is a major constraint of plant reproductive success. Because pollen size trades-off with the quantity of produced grains, the link between climate characteristics and the determination of pollen size is of fundamental importance. To minimize the rate of water loss due to desiccation, a plant should produce larger grains that also have a lower surface-to-volume ratio. We used a comparative analysis to examine the hypothesis predicting increase in pollen size as a response to desiccation intensity. To test the hypothesis, we correlated the data on pollen size with the climate characteristics, temperature and desiccation intensity of the flowering period, for 232 plant species of 11 taxonomic groups. The analysis showed a positive relationship between the pollen size and temperature, but not with the desiccation intensity. We discuss the potential mechanisms by which increased temperature is an indicator of high competition among pollen grains on the stigma, which in turn is expected to promote large pollen. Our work provides insight into the temperature dependence of pollen production in plants and reveals a link between environmental temperature and the intensity of limitation of plant reproductive success by pollen availability. The result is relevant in the context of global climate change. We also discuss why environmental temperature has to be controlled in studies dealing with pollen production, particularly in investigations of size-number trade-off.