Infrared imaging supports warming effects of anthocyanin pigments in flowers of diverse plant taxa

Flower color has primarily been studied in the context of pollinator attraction, although effects on thermal energy balance are also important, especially in the context of global climate change. We used infrared imaging to compare petal temperatures of white versus pigmented cultivars of ten angiosperm taxa under controlled environmental conditions. Excised sets of flowers (n= 6 sets per species) exhibiting white, light, and/or dark anthocyanin (red to purple) coloration were mounted perpendicularly to the sun at mid-day, under clear sky, low wind (<1 m s-1) conditions. Sunlight was filtered through either UV-transparent or UV-opaque film, and petal temperatures were measured using an infrared camera after one minute equilibration. In all species, pigmented flowers were significantly warmer than lighter-colored conspecifics. Mean differences averaged +5.3{degrees}C for darker-colored versus white morphs, +2.9{degrees}C for lighter-colored versus white. Most warming was associated with visible wavelengths, but additional warming under UV-inclusion was also observed in some species. In situ observations of intact landscape plants under low-wind, high-light conditions corroborated experimental results, with differences exceeding 10{degrees}C observed in some taxa. Temperature differences >7{degrees}C were also recorded for purple versus white sections of the same flower in multicolored Viola and Petunia cultivars. Follow-up experiments using dark-pink and white varieties of Impatiens x hybrida corroborated well-known effects of sunlight intensity and wind speed on floral temperatures, helping to explain inconsistent reports in the literature. Our results clearly demonstrate that anthocyanin pigments can have significant and dramatic impacts on floral temperatures, which could be an important factor driving evolution of flower color. In the context of climate change, floral pigments could amplify the effects of rising global temperatures, negatively impacting plant reproduction and crop yields, especially on the warmer end of species ranges. Changes in flower color could also potentially induce shifts in pollinator communities, which could have community-scale effects.