The Efficiency of Humic Acid for Improving Salinity Tolerance in Salt Sensitive Rice ( Oryza sativa ): Growth Responses and Physiological Mechanisms

High-yielding rice cultivars exhibit a great performance in non-saline fields; however, their growth and productivity are greatly reduced in salt-affected lands. Humic acid has a promising stress-mitigating potential and can be effective in improving salt tolerance in salinity sensitive rice cultivars. Herein, seeds of Giza 177 (high-yielding but salt-sensitive rice cultivar) were primed in 40 mg/l humic acid, sown, and maintained. Then growth and physiological responses of the humic acid-primed plants to increased levels of salinity (EC: 0.55, 3.40, 6.77, and 8.00 mS/cm) were evaluated at the reproductive stage. Increasing salinity induced a progressive retardation in plant height, leaf area, fresh and dry weights. Such retardation was associated with Na + buildup in shoot and root, high electrolyte leakage and accumulation of malondialdehyde, total soluble sugars, sucrose, glucose, proline, total soluble proteins, flavonoids, and phenolics. In contrast, salinity reduced K + , K + /Na + ratio, total carbohydrates, and the activity of catalase, peroxidase, and polyphenol oxidase. Humic acid enhanced growth under non-saline and saline conditions. The humic acid-induced improvement in salt tolerance was associated with the reduction of Na + toxicity, increasing K + /Na + ratio, regulating osmolytes concentration, and enhancing the activities of antioxidant enzymes and thus reduce the oxidative stress. These results indicate that humic acid successfully reduced the salinity-induced plant damage, improved metabolism, and maintained active growth of Giza 177 under saline irrigation.