Thermal regulation of flowers: color-driven differences in radiation absorption, cooling, and heat tolerance

Angiosperm flowers vary widely in color, influencing not only pollinator attraction but also the thermal environment of reproductive organs, with potential consequences for plant fitness under climate change. We examined how color polymorphism shapes the floral energy balance, cooling strategies, and heat tolerance in Vinca minor, which has three color morphs. By combining full-spectrum optical measurements, controlled temperature-response experiments under controlled and natural radiation, and modeling, we quantified petal energy balance components. Darker flowers absorbed more shortwave radiation but maintained similar temperatures to lighter flowers by increasing latent heat loss through higher transpiration. Sensible heat exchange dominated cooling across morphs, yet latent heat loss was critical for offsetting the extra thermal load in dark morphs. Dark and light purple flowers exhibited slightly higher (+4°C) heat tolerance (i.e. temperature where 50% of cell membranes show stress-induced injury) than white flowers, but did not fully compensate for their elevated tissue temperature (+6°C under natural radiation), resulting in narrower thermal safety margins. Our study serves as an integrative case study linking flower color to full-spectrum radiation absorption, water use, and heat tolerance, revealing pathways through which flowers balance thermal regulation.