Exogenous application of salicylic acid mitigates heat-induced oxidative stress in Barley (Hordeum vulgare L.) acid alleviate the damaging effects of heat stress in Chili (Capsicum frutesence L.) by improved antioxidant defense system

Due to climate change, heat stress is becoming one of the most disastrous abiotic stresses and responsible for huge crop losses in different regions of the world. Adverse effects of heat stress can be encountered by the use of phytohormones and salicylic acid (SA) is one of those hormones having capability to protect plants from heat stress. Current research work was planned to explore the defensive role of exogenously applied SA in barley cultivars under high temperature. For this purpose, two weeks old plants of four barley cultivars (Jau-87, B-10007, B-14003 and B-14037) were foliar sprayed with SA (2 mM and 5 mM) for 24 hours and then were exposed to heat stress (42 °C). Our results showed that heat exposure made a prominent decrease in chlorophyll contents, carotenoids, relative water contents, antioxidant enzymes activities (superoxide dismutase, peroxidase and catalase) and anthocyanin activity while increased lipid peroxidation (malondialdehyde), hydrogen peroxide (H2O2) contents and proline accumulation. Foliar application of SA enhanced plants photosynthetic pigments (chlorophyll a, b and total), RWC, proline, and anthocyanin under control and stressed conditions. SA pre-treatment under heat stress reduced H2O2 and MDA levels whereas activities of reactive oxygen species (ROS) scavenging enzymes were elevated by all tested SA concentrations. These results suggest that SA can efficiently remove H2O2 by increasing activity of above mentioned enzymes hence reducing oxidative stress and malondialdehyde (MDA) contents. Proline, an important amino acid that acts as an anti-oxidative system in plants also highly accumulated in SA co-treated heat-stressed plants. Our results also showed that 2mM SA is more effective in protecting barley plants under heat stress as compared to 5mM SA. This concluded that defensive nature of SA regulates different physiological and biochemical processes to induce heat tolerance in barley. 
Key words: Hordeum vulgare L., heat stress, salicylic acid, proline, reactive oxygen species, anti-oxidative system.