Floral shape and color impact heat accumulation and thermal stability of the floral microenvironment in a subalpine meadow.

Premise: The floral thermal microenvironment impacts plant reproduction through its effects on gametophyte performance and plant-pollinator interactions. Color and shape are axes of floral variation that may mediate floral temperature because they affect the absorption and reflection of solar radiation, but their interactive effects are unclear. Additionally, near infrared (NIR) reflectance is important for organismal temperature, but the contribution of petal NIR reflectance to floral temperature has not been assessed.

Methods: In a subalpine meadow during peak solar radiation, we continuously tracked temperature of the floral microenvironment and air temperature outside of flowers for species that differed in floral shape and dominant floral color (yellow vs. anthocyanic [pink, blue, red]). We related shape, color, surface area, and UV-NIR petal reflectance to floral temperature metrics.

Results: Flowers with exposed reproductive structures ("open") warmed more than those with concealed reproductive structures ("closed"), and this effect was strongest for yellow flowers. Closed flowers were more thermally stable than open flowers regardless of color. Open flowers with larger surface area also warmed more. NIR reflectance (700-900 nm) was strongly correlated with visible reflectance (500-700 nm), so its effect on floral temperature could not be isolated. However, closed flowers with stronger reflectance from 500 to 900 nm were cooler and more thermally stable.

Conclusions: Our results show that floral color, reflectance, and shape interact to influence the temperature of the floral microenvironment of our focal taxa. They provide a predictive framework for how floral traits may change across space and time in response to temperature variation.