Ecophysiological response of pomegranate plants under long-term combined salt and ozone stress

The progressive salinization of soils irrigated with salty water and salt-water intrusion of groundwater bodies can limit crop production in many areas, especially in the Mediterranean basin. The current background tropospheric ozone (O3) levels are high enough to negatively affect plant physiological and productive performances. In this work, one-year-old saplings of pomegranate (Punica granatum L., cv. ‘Dente di cavallo’) were exposed to two levels of O3 [AOT40 values were 21.51 in ambient air (AA), and 58.74 ppm h, in 2AA] and two levels of salinity, denoted as No Salt (NS) and Salt (S, the electrical conductivity and pH of the irrigation water were 5.5 mS cm-1 and 7.6 with 50 mM of NaCl) for four consecutive months in an O3 FACE open air facility. Under O3 (alone or in combination with salt), plants developed visible stipples of browning tissue localized in the interveinal adaxial leaf surface. At ecophysiological level, salt stress further affected the photosynthetic performance (-17% compared to AA_NS). By contrast, salinity did not induce oxidative damage [as confirmed by unchanged malondialdehyde (MDA) levels]. Under 2AA conditions, O3 alone reduced the stress tolerance, as confirmed by the production of reactive oxygen species (+10 and +225% of anion superoxide and hydrogen peroxide, respectively), the increase of superoxide dismutase activity (+9%) and the concomitant membrane denaturation (+198% of MDA content). According to Bansal calculation, the combination of both stressors had a synergistic effect in terms of oxidative damage and increased activity of catalase.