Differential Physiological Responses of Zea mays and Sorghum bicolor to Drought Stress: Insights into the Ameliorative Role of Humic Acid at the Reproductive and Yield Stages

Abstract

Abstract Drought severely threatens the growth and productivity of cereals, including maize and sorghum. Despite their genetic relatedness, sorghum exhibits better drought tolerance than maize, and the physiological basis behind such differential response is largely unknown. Humic acid (HA) is a stress-relieving biostimulant that can boost drought resilience in plants; however, its mechanism of action remains vague. Herein, the reproductive growth, physiological responses, yield, and grain quality of maize and sorghum under increased field-drought conditions (100%, 80%, and 60% field capacity) and the ameliorative effects of seed priming in HA (100 mg L −1 ) were simultaneously investigated. Drought stress significantly reduced biomass, photosynthetic efficiency, yield traits, and grain quality indices in both species, with sorghum consistently demonstrating smaller reductions. Under severe drought, maize exhibited reductions up to 67.3% in stem dry mass, 68.1% in net photosynthesis, and 63.9% in grain yield, compared to 47.4%, 53.6%, and 54.6% reductions in sorghum, respectively. These differences correlated with lower oxidative damage and higher osmolytes, flavonoids, phenols, and polyphenol oxidase (PPO) in sorghum. HA priming mitigated drought-induced damages and enhanced biomass accumulation, photosynthesis, osmolyte production, non-enzymatic and enzymatic antioxidants, yield traits, and grain quality indices such as grain carbohydrates, protein, oil, and fatty acid composition in both species, with maize being generally more responsive than sorghum. These findings highlight the distinct physiological responses of maize and sorghum to drought and demonstrate that HA priming is a sustainable approach to improve drought tolerance, productivity, and grain quality in these and potentially other crops in drought-prone lands. Graphical Abstract