Assistant Professor Clemson University Clemson, South Carolina, United States
Abstract: Thermal environments vary across species ranges, establishing the potential for local adaptation of thermal performance optima, tolerance and thermoregulatory mechanisms. Floral temperature is a major determinant of reproductive success in angiosperms, yet whether gametic thermal performance and regulation vary across temperature gradients is unknown.
We characterized long-term flowering season temperatures for the herb, Argentina anserina, at extremes of a 1000m elevational gradient and generated thermal performance curves for pollen performance and seed set in populations at each extreme. We subsequently recorded flower-level ambient and intrafloral temperatures in the field during the 2021 flowering season to compare experimentally determined performance optima and breadths to experienced temperatures.
Performance optima were positioned between the intrafloral means and maxima observed in the field, consistent with local adaptation of thermal optima. However, while high elevation populations experienced cooler thermal environments than low elevation populations, pollen thermal optima were higher in high elevation populations, suggesting that thermal optima cannot be explained by selection via local thermal ranges alone.
Thermal variability did not differ markedly between elevation groups, and we found no evidence of local differentiation of thermal tolerance breadths.
We then tested whether flowers differentially warmed the floral microenvironment between elevation extremes in the field. High elevation flowers warmed significantly more than low, bringing intrafloral temperatures nearer, yet still falling short of their pollen optima. A manipulative experiment demonstrated that stronger warming in high elevation was conferred by the presence of the perianth.
Our results are consistent with the interpretation that (1) local thermal optima can only be partially explained by local thermal ranges, and (2) that local thermal optima contribute to selection on thermal warming mechanisms
