Sugars and beyond: Nectar nutrition and phytochemicals under drought stress

Nectar and pollen provide essential nutrients to pollinators and other insects. Both the quantity and quality of these resources are affected by environmental conditions, including water availability. Drought stress reduces the quantity of floral rewards by causing plants to produce less nectar per flower and fewer flowers per plant. However, less is known about the effects of drought on the quality or chemical composition of floral resources, with most studies justifiably focusing on fluctuations of crucial nutrients, like nectar sugars and pollen proteins and lipids. Conversely, the impact of environmental stress on secondary metabolites or phytochemicals in nectar and pollen has not been explored, despite their importance to pollinator health and foraging behavior. Here, we tested the hypothesis that plants undergoing drought stress produce pollen and nectar with differing phytochemical profiles than well-watered plants. We tested two field-grown sunflower genotypes under three levels of drought stress ranging from severe to none. Using high resolution mass spectrometry, we identified metabolites that are differentially abundant in drought-stressed and unstressed plants, including drought-induced decreases in chemicals that confer health benefits to pollinators. These results suggest that drought could lead to not only lower quantities, but also lower quality resources for pollinators.