Pollen thermotolerance of a widespread plant in response to climate warming: possible local adaptation of populations from different elevations

One of the most vulnerable phases in the plant life cycle is sexual reproduction, which depends on effective pollen transfer, but also on the thermotolerance of pollen grains. Pollen thermotolerance is temperature-dependent and may be reduced by increasing temperature associated with global warming. A growing body of research has focused on the effect of increased temperature on pollen thermotolerance in crops to understand the possible impact of temperature extremes on yield. Yet, little is known about the effects of temperature on pollen thermotolerance of wild plant species. To fill this gap, we selected Lotus corniculatus s.l. (Fabaceae), a species common to many European habitats and conducted laboratory experiments to test its pollen thermotolerance in response to artificial increase in temperature. To test for possible local adaptation of pollen thermal tolerance, we compared data from six lowland (389 - 451 m a.s.l.) and six highland (841 - 1030 m a.s.l.) populations. We observed pollen germination in vitro at 15, 25, 30, and 40{degrees}C. While lowland plants had stable germination rate at a wide range of temperatures between 15 and 30{degrees}C, with reduced germination rate observed only at extremely high temperatures (i.e., 40{degrees}C), the germination rate of highland plants was reduced already when the temperature reached 30{degrees}C, temperature commonly exceeded in the lowland during warm summers. This suggests that lowland populations of L. corniculatus may be locally adapted to higher temperature for pollen germination. On the other hand, pollen tube length decreased with increasing temperature in a similar way in lowland and highland plants. The overall average pollen germination rate significantly differed between lowland and highland populations, with highland populations displaying higher germination rate. On the other hand, the average pollen tube length was slightly smaller in highland populations. In conclusion, we found that pollen thermotolerance of L. corniculatus is reduced at high temperature and that the germination of pollen from plant populations growing at higher elevations is more sensitive to increased temperature, which suggests possible local adaptation of pollen thermotolerance.