Transpirational water loss from flowers is low but unregulated

Flowers play a critical role in reproduction for most of the flowering plants, and maintaining flowers throughout their lifespan can require substantial resources, such as carbon and water. Increases in temperature and aridity due to climate change are shifting the atmospheric conditions experienced by flowers, potentially altering the costs of floral maintenance. However, little is known about floral physiology and the capacity of flowers to regulate water loss. Because many flowers have few, if any, stomata, flowers may not be able to curtail water loss when the atmospheric demand for water vapor is high. Here, we tested whether the surface conductance of of flower petals, tepals, and showy bracts responds dynamically to changes in the vapor pressure gradient driving water loss. We measure the responses of flower surface conductance (gs) to step changes in the vapor pressure gradient on nine species. Across species, gs was low among all species, and there was little, if any, response in gs to step changes in humidity. The lack of response in gs resulted in linear responses of transpirational water loss to variation in vapor pressure deficit. These results suggest that unusually hot, dry conditions could elevate water loss from flowers, leading to premature wilting and senescence, thereby shortening floral longevity.