Species‐specific responses to combined water stress and increasing temperatures in two bee‐pollinated congeners ( Echium , Boraginaceae)

Water stress and increasing temperatures are two main constraints faced by plants in the context of climate change. These constraints affect plant physiology and morphology, including phenology, floral traits, and nectar rewards, thus altering plant–pollinator interactions. We compared the abiotic stress responses of two bee‐pollinated Boraginaceae species, Echium plantagineum , an annual, and Echium vulgare , a biennial. Plants were grown for 5 weeks during their flowering period under two watering regimes (well‐watered and water‐stressed) and three temperature regimes (21, 24, 27°C). We measured physiological traits linked to photosynthesis (chlorophyll content, stomatal conductance, and water use efficiency), and vegetative (leaf number and growth rate) and floral (e.g., flower number, phenology, floral morphology, and nectar production) traits. The physiological and morphological traits of both species were affected by the water and temperature stresses, although the effects were greater for the annual species. Both stresses negatively affected floral traits, accelerating flower phenology, decreasing flower size, and, for the annual species, decreasing nectar rewards. In both species, the number of flowers was reduced by 22%–45% under water stress, limiting the total amount of floral rewards. Under water stress and increasing temperatures, which mimic the effects of climate change, floral traits and resources of bee‐pollinated species are affected and can lead to disruptions of pollination and reproductive success.