Salinity-induced accumulation of organic osmolytes in barley and wheat leaves correlates with increased oxidative stress tolerance: in planta evidence for cross-tolerance

Salinity tolerance in plants is dependent on their abilities to adjust osmotically to reduced soil water potential and to keep intracellular ROS levels under control. Both these processes are believed to rely on de novo synthesis of organic osmolytes (traditionally defined as compatible solutes). However direct in planta evidence for anti-oxidant roles of compatible solutes are scarce. In this work, we induced changes in the level of endogenous organic osmolytes by exposing plants to various levels of NaCl (salinity stress; 50–300 mM range) and then studying sensitivity of leaves to oxidative (UV–B) stress. Increase in the external NaCl concentrations was accompanied by the progressive accumulation in leaf Na⁺. This accumulation was much higher in old leaves compared with young ones. In old leaves, three major inorganic ions (Na⁺, Cl⁻ and K⁺) have made 67.7% and 70.4% of leaf osmotic potential (in wheat and barley, respectively) when exposed to 200 mM NaCl treatment, while in young leaves their contribution was only 43.9% and 46.8%, respectively. Here, organic osmolytes played a substantial role in leaf osmotic adjustment. Increased accumulation of organic osmolytes correlated strongly with activity of PSII in leaves exposed to oxidation inducing UV-B treatment in both species (R² = 0.50 for wheat and 0.71 for barley). We conclude that salinity-induced accumulation of organic osmolytes in barley and wheat leaves correlates with increased oxidative stress tolerance and provides the evidence for a mechanism of cross-tolerance between these two stresses.