Ectotherms are expected to respond strongly to current and projected changes in temperature associated with global climate change. Thermal acclimation ability, and in particular thermoregulatory ability, will strongly influence the impact of these changes. In spite of this, current knowledge about thermoregulation in plants is negligible and even in ectothermic animals, fitness effects are not well understood. Recent studies show that a widespread weed, Plantago lanceolata, can partially thermoregulate reproduction. We tested the hypothesis that this ability is adaptive in thermally fluctuating environments. Understanding the environmental conditions favoring thermoregulation will help us better predict biotic responses to global warming.            Experiments used genotypes resulting from two generations of selection for different degrees of floral reflectance plasticity, which underlies thermoregulatory ability. At cool temperatures only plastic genotypes reduce floral reflectance, which increases the absorption of incoming solar radiation. In one experiment, we induced clones of each genotype to produce “cool” and “warm” phenotypes. We marked spikes (flower clusters) in the flower-bud stage on each clone and transplanted clones into field plots at three times during the reproductive season to allow for natural reproduction. In the second experiment, we transplanted clones of experimental genotypes into field plots to monitor natural flowering phenology.  
Results/Conclusions

In spring when seasonal temperatures were cool, seed production per spike was significantly positively correlated with floral reflectance plasticity, i.e., thermoregulatory ability, for “cool” phenotypes, which differed among experimental genotypes. The correlation disappeared in summer, and reflectance plasticity never affected seed production among “warm” phenotypes. Spike length differences at the beginning of the experiment did not explain these results. Early-season spike production of plastic genotypes resembled that of non-plastic genotypes, although non-plastic genotypes produced spikes for a longer time.            Data help clarify the selection pressures maintaining known geographic variation in thermal acclimation ability in P. lanceolata. Field experimental results indicate that 1) thermoregulation ability is adaptive when reproductive seasons are thermally variable and at times cool and 2) shortening the reproductive season increases the selective advantage of thermoregulation. Results suggest that global warming will reduce the fitness advantage of thermoregulation and alter the genetic structure of higher-latitude/altitude populations.