Petal infrared transmission warms flowers and reduces microbial abundance

Flowers create distinct internal microclimates, yet how these conditions filter microbial abundance remains poorly understood. We tested whether petal light transmission and internal floral temperature influence microbial abundance in two co-occurring species, Lyonia lucida and Lyonia fruticosa (Ericaceae), which have translucent petal 'windows' that modify internal light environments. We measured internal and external floral temperatures, quantified culturable microbial abundance (yeasts, molds, bacteria), and assessed ultraviolet (UV) and infrared (IR) petal transmission using spectrometry. We then experimentally tested the UV resistance of microbial communities across 108 flowers from 18 plants. Additionally, we used manipulative light experiments to test whether IR radiation drove differences in floral warming between these two species. Petal windows transmitted more UV and IR radiation than pigmented tissue. Internal floral temperature predicted microbial abundance, with warmer flowers harboring fewer microbes. A spectral filter experiment confirmed that IR radiation warms L. lucida flowers but not L. fruticosa, while natural variation in UV transmission did not predict microbial abundance or UV resistance. Petal spectral traits shape floral microclimate and microbial abundance primarily through temperature-mediated filtering linked to IR-driven warming rather than passive UV sterilization. These results position flowers as microhabitats where petal traits constrain microbial communities.