Temperature and water stress affect plant-pollinator interactions in Borago officinalis (Boraginaceae)

Climate change alters the abiotic constraints faced by plants, including increasing temperature and water stress. These changes may affect flower development and production of flower rewards, thus altering plant-pollinator interactions. Here, we investigated the consequences of increased temperature and water stress on plant growth, floral biology, flower-reward production, and insect visitation of a widespread bee-visited species, Borago officinalis. Plants were grown for 5 weeks under three temperature regimes (21, 24, and 27°C) and two watering regimes (well-watered and water-stressed). Plant growth was more affected by temperature rise than water stress, and the reproductive growth was affected by both stresses. Vegetative traits were stimulated at 24°C, but impaired at 27°C. Flower development was mainly affected by water stress, which decreased flower number (15 ± 2 flowers/plant in well-watered plants vs. 8 ± 1 flowers/plant under water stress). Flowers had a reduced corolla surface under temperature rise and water stress (3.8 ± 0.5 cm² in well-watered plants at 21°C vs. 2.2 ± 0.1 cm² in water-stressed plants at 27°C). Both constraints reduced flower-reward production. Nectar sugar content decreased from 3.9 ± 0.3 mg/flower in the well-watered plants at 21°C to 1.3 ± 0.4 mg/flower in the water-stressed plants at 27°C. Total pollen quantity was not affected, but pollen viability decreased from 79 ± 4% in the well-watered plants at 21°C to 25 ± 9% in the water-stressed plants at 27°C. Flowers in the well-watered plants at 21°C received at least twice as many bumblebee visits compared with the other treatments. In conclusion, floral modifications induced by abiotic stresses related to climate change affect insect behavior and alter plant-pollinator interactions.