Exogenously applied growth regulators protect the cotton crop from heat-induced injury by modulating plant defense mechanism

Episodes of extremely high temperature during reproductive stages of cotton crops are common in many parts of the world. Heat stress negatively influences plant growth, physiology and ultimately lint yield. This study attempts to modulate heat-induced damage to cotton crops via application of growth regulators e.g. hydrogen peroxide (H2O2 30ppm), salicylic acid (SA 50ppm), moringa leaf extract (MLE 30 times diluted) and ascorbic acid (ASA 70ppm). Cotton plants were exposed to different thermal regimes by staggering sowing time (field) or exposing to elevated temperatures (38/24 °C and 45/30 °C) for one week during reproductive growth stages (glasshouse). Elevated temperatures significantly induced lipid membrane damage, which was evident from an increased malondialdehyde (MDA) level in cotton leaves. Heat-stressed plants also experienced a significant reduction in leaf chlorophyll contents, net photosynthetic rate and lint yield. Hydrogen peroxide outclassed all the other regulators in increasing leaf SOD, CAT activity, chlorophyll contents, net photosynthetic rate, number of sympodial branches, boll weight and fiber quality components. For example, hydrogen peroxide improved boll weight of heat stressed plants by 32% (supra), 12% (sub) under glasshouse and 18% (supra) under field conditions compared with water treated plants under the same temperatures. Growth regulators, specifically, H2O2 protected physiological processes of cotton from heat-induced injury by capturing reactive oxygen species and modulating antioxidant enzymes. Thus, cotton performance in the future warmer climates may be improved through regulation (endogenous) or application (exogenous) hormones during reproductive phases.