Western Carolina University and Clemson University, United States
Abstract: Geographically widespread organisms must cope with divergent thermal environments across their ranges. Local adaptation in thermal tolerance and/or thermoregulatory behaviors are ways in which organisms can persist under extreme thermal environments. Flowering plants are largely sessile and most are ectothermic. They have evolved mechanisms that accumulate or dissipate heat from flowers which house temperature-sensitive gametes (pollen and ovules) and attract temperature-sensitive pollinators. However, whether floral thermoregulatory mechanisms are locally adapted to the thermal environment across populations spanning thermal gradients has not been examined. We measured a suite of floral traits, and floral thermoregulatory ability of a widespread herb, Argentina anserina, in populations spanning >1000m elevation in the San Juan Mountains in Southwest Colorado. With our dataset we evaluated whether putative thermoregulatory traits differed between cooler high elevation populations and warmer low elevation populations. We also identified traits linked with the capacity for solar heat capture. We then conducted a growth chamber experiment in which we exposed low and high elevation genotypes to low- and high-elevation thermal environments and measured temperature-induced floral trait plasticity. In the field, high elevation plants held petals at a higher angle (more ‘cupped’) than low, and responded more strongly to changes in ambient temperature than low by altering petal angle. In high elevation populations, higher petal angle was associated with increased heat accumulation during peak sun. We modeled the reflection of light to show that more cupped petals focus ambient solar radiation within the flower, while flatter petals reflect radiation outside of the flower. All other traits measured (petal size, water content, and floral height) were unassociated with floral warming. In the growth chamber experiment, both high and low elevation genotypes exhibited a strong plastic responses to temperature, holding petals at a lower angle in the warm treatment. High elevation populations however maintained a higher petal angle on average, indicating a genetic component to the petal angle difference between elevation clusters. In combination with previous work showing that gametic thermal optima are higher in high elevation than low, results suggest that petal movements that alter floral morphology are likely locally adapted to local thermal environments and gametic thermal performance optima.
