Humic Acid Modulates Growth, Photosynthesis, Hormone and Osmolytes System of Maize under Drought Conditions

With population increasing and global warming, extreme weather events and water resource shortage have become nonnegligible factors restricting maize production. The effects of humic acid (HA) on plant growth and stress tolerance have been extensively studied. However, the underlying physiological and molecular mechanism of how HA influences drought resistance in maize plants remains unknown. In the present study, a pot experiment was conducted with two irrigation levels (W1: drought stress 45-60% of field capacity, W2: well-watered, 75-100% of field capacity), two HA application rates (H0: 0 kg ha(-1), H1: 45 kg ha(-1)), and two maize genotypes (Zhengdan958 and Xianyu335). The results showed that HA applied at 45 kg ha(-1) improved the proportion of macro-aggregates in the soil while increasing the soil water content by 5.0% at W1 compared with H0. The availabilities of P, K, Fe, and Mg in soil were improved, and the activities of rubisco and ATP synthase, the content of IAA, and the concentrations of osmotically active solutes (i.e., soluble sugars, betaine, and proline) in maize were increased with HA application. In addition, the expressions of genes involved in primary metabolic processes, especially photosynthesis, carbon fixation, and hormone and osmotic metabolisms in maize leaves, were regulated. Higher maize yields were achieved in the treatments with HA applied by 19.7% in Zhengdan958 and 14.3% in Xianyu335 than without HA applied under drought stress, respectively. And the water use efficiency of Zhengdan958 and Xianyu335 was 2.16 g kg(-1) and 2.08 g kg(-1) in the treatments with HA applied, respectively. In conclusion, HA application enhanced the drought resistance and growth of maize. The results of this study help to understand the drought resistance mechanism of maize and provide support for crop production in face of water shortage.