Absorption of visible light by floral anthocyanins increases flower temperatures: implications for a warmer world

With rising global temperatures, understanding the mechanisms that influence plant thermal energy balance are critical to forecasting plant and pollinator responses. Flower temperatures impact visitation by insect pollinators, and several temperature-sensitive processes central to plant reproduction. Anthocyanin pigments absorb strongly in visible and ultraviolet wavebands, but clear support for a warming effect of pigments on flowers has remained elusive. We used infrared imaging to measure petal temperatures of horticultural varieties of plants differing in floral anthocyanin content. Excised sets of flowers were mounted perpendicularly to the sun, and exposed to direct ambient sunlight filtered either through Aclar (UV-transparent) or Courtguard (UV-opaque) under low-wind (< 1 m s-1) conditions. Petal temperatures were measured after one minute, and treatment order alternated between replicates. Results showed that pigmented flowers were consistently significantly warmer than white conspecifics (2.9°C warmer on average for lightly-pigmented varieties, 5.3°C warmer for darkly-pigmented). Most species showed no significant difference in petal temperatures under UV inclusion versus exclusion, indicating most warming by anthocyanins can be attributed to absorption of visible light. UV-enhanced warming was observed in some species, which could be due to the presence of side groups that enhance UV-absorption by the anthocyanin molecules. However, these differences were slight (always <1°C), and usually white varieties exhibited differences as well, suggesting other flavonoids too may contribute to floral warming. Additional experiments focusing on dark pink and white varieties of Impatiens x hybrida corroborated previous reports that the most dramatic warming effects occur under high-light, low-wind conditions. In the context of climate change, warmer temperatures could potentially drive the replacement of darkly-colored flowers with more lightly-colored morphs (or species), especially if flower temperatures exceed thermal maxima for fertilization and seed development. This pressure may be especially strong in species with floral morphologies conducive to heat trapping or in low-wind environments (e.g., growing in dense stands or close to the ground). Changes in flower color could also potentially induce shifts in pollinator communities, which could have community-scale effects.