Enhanced salt tolerance in maize plants induced by H2O2 leaf spraying is associated with improved gas exchange rather than with non-enzymatic antioxidant system

Hydrogen peroxide (H2O2) is an essential signaling molecule that mediates plant responses against several biotic and abiotic stresses. H2O2 pretreatment has emerged as a signaling way, inducing salt stress acclimation in plants. Here, we analyzed the effects of H2O2 leaf pretreatment on the non-enzymatic defense system (ascorbate and glutathione), plant growth, relative water content (RWC), relative chlorophyll content, H2O2 content, and gas exchange in maize plants under NaCl stress. The results showed that salinity reduced the leaf area and shoot and root dry mass as compared to control, and the leaf spraying with H2O2 significantly improved the growth of salt stressed plants. Photosynthesis and transpiration, stomatal conductance and intercellular CO2 concentration were strongly decreased by salinity after 7 and 14 days of salt exposure; however, the decrease was lower in plants sprayed with H2O2. The improved gas exchange in H2O2-sprayed stressed plants correlated positively with higher RWC and relative chlorophyll content and lower leaf H2O2 accumulation under NaCl stress conditions. Ascorbate and glutathione did not play any obvious effects as non-enzymatic antioxidants in the ROS scavenging. In conclusion, the salt tolerance induced by H2O2 leaf pretreatment is attributed to a reduction in the H2O2 content and maintenance of RWC and chlorophyll in maize leaves. These characteristics allow maize plants to maintain high rates of photosynthesis under salt stress and improve the growth. KEywORdS: ascorbate, glutathione, hydrogen peroxide, salinity, Zea mays. INTROdUCTION The production of reactive oxygen species (ROS) is a normal event of oxidative metabolism in plants but their generation is further enhanced in response to various biotic and abiotic stresses, such as salinity (Møller et al. 2007). Salinity is a limiting environmental factor, which impairs plant growth and development. It affects approximately 20% of the world’s cultivated area and nearly half of the world’s irrigated lands (Sairam and Tyagi 2004). In excess, ROS can damage DNA, proteins, chlorophyll and membrane functions. The main ROS produced are hydrogen peroxide (H2O2), superoxide ( O2ˉ) and hydroxyl (•OH) radicals (Azevedo Neto et al. 2008). Plants have evolved complex defense mechanisms to avoid an imbalance between generation and scavenging of ROS (Azevedo Neto et al. 2008, Gill and Tuteja 2010). ROS may be scavenged by both enzymatic and non-enzymatic pathways; nonetheless, the failure to control ROS may lead to oxidative stress (Wang et al. 2013). The enzymatic defense system includes superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), guaiacol peroxidase (GPX, EC 1.11.1.7) and ascorbate peroxidase (APX, EC 1.11.1.11) (Azevedo Neto et al. 2008, RESEARCH ARTICLE