Photosynthetic acclimation of rhododendrons to light intensity in relation to leaf water-related traits

Leaves under high light may suffer from risks caused by excessive light energy and dehydration. However, it remains unclear how leaf water-related traits affect the photosynthetic acclimation of plants to light intensity. We investigated the photosynthesis and leaf water-related traits of two hybrids (Rhododendron ‘XXL’ and ‘Furnivall’s daughter’) and one native species (Rhododendron delavayi) under different growth irradiances to determine the importance of leaf water-related traits on the photosynthetic acclimation of rhododendrons to irradiance. Compared to plants under low light, the values for light saturation point (LSP), stomatal density (SD), leaf dry mass per unit area, and vein density (Dv) in the three rhododendrons were higher under high light, but the values for chlorophyll content and leaf size were lower. There were significant differences in light-saturated photosynthetic rate (Pmax), stomatal conductance (gs), transpiration rate (Tr), and non-photochemical quenching (NPQ) in R. delavayi and Rhododendron ‘XXL’ between plants grown under high and low irradiance levels, but not in ‘Furnivall’s daughter’. The positive correlations of Dv and SD with Pmax, Tr, and LSP indicate that the increase in water transport capacity can improve water supply to meet transpirational demand and regulate photosynthetic performance under high irradiance. Meanwhile, the decrease in chlorophyll content and the increase in NPQ prevent the photodamage of photosystem II by decreasing light absorption and enhancing heat dissipation, respectively, when the plants are exposed to high light. These results suggest that adjusting leaf water-related traits and reducing the damage on photosystem II caused by excess light play important roles in the photosynthetic acclimation of rhododendrons to growth irradiance.