Salinity stress constrains photosynthesis in Fraxinus ornus more when growing in partial shading than in full sunlight: consequences for the antioxidant defence system

Background and aims An outstanding challenge in plant ecology is understanding the effects of multiple sub-optimal environmental conditions on plant performance. In Mediterranean areas soil salinity builds up during summer because of low soil-water availability coupled with hot temperatures. Although sunlight and soil salinity may strongly interact in determining a plant’s performance, the matter has been poorly explored. Methods Fraxinus ornus grown in outdoors at 45% (shaded) or 100% (sun) sunlight irradiance was supplied with good-quality water with the addition or not of 75 mM NaCl during August. Morpho-anatomical leaf traits, water and ionic relations, leaf gas exchange and PSII performance, the concentrations of individual carotenoids and abscisic acid, the activity of antioxidant enzymes, and the concentrations of ascorbic acid and individual polyphenols were measured. Leaf oxidative stress and damage were investigated through in vivo analysis of stable free radicals and ultra-structural analyses. Results The accumulation of potentially toxic ions in the leaf did not markedly differ in shaded or sun plants treated with root zone salinity. Sun leaves displayed a superior water use efficiency than shaded leaves, irrespective of salinity treatment. Sun leaves indeed had higher both stomatal control and CO2 carboxylation efficiency. Actually, shaded leaves were challenged against a more severe excess of radiant energy than their sun counterparts, in response to salinity. The activity of enzymes aimed at detoxifying hydrogen peroxide either decreased in shaded or increased in sun leaves because of salinity stress. In contrast, the activity of guaiacol peroxidase and the concentration of phenylpropanoids steeply enhanced in shaded or were unaffected in sun leaves because of salt stress. Conclusions Our study allow conclude that salinity may constrain more severely the performance of plants growing at partial shading than at full sun during summer, and unveils a co-operation within the antioxidant defenses network aimed at detoxifying salt-induced ROS generation.