We aimed to determine whether humic acid (HA) can alleviate the injury of millet caused by drought and its potential mechanism. Millet seeds (Jingu 21 and Zhangza 10) were soaked in different concentrations of HA (0, 50, 10, 200, and 300 mg L-1) for 12 h. The physiological and photosynthetic characteristics of millet seedlings, including growth parameters, osmotic regulators, antioxidase activity, photosynthesis, chlorophyll fluorescence, and P700 parameters, were determined before and after drought stress. HA significantly promoted the growth of millet seedlings under drought stress. Pretreatment with 100 mg L-1 or 200 mg L-1 HA significantly increased free proline, soluble protein, and activity of the antioxidant enzyme system (superoxide dismutase, peroxidase, and catalase) in both Zhangza 10 and Jingu 21. The accumulation of reactive oxygen species ([Formula: see text] and H2O2) was reduced in HA treatments compared with that of the control (P < .05). Moreover, HA (100 mg L-1) significantly increased net photosynthetic rate, stomatal conductance, effective quantum yield of photosystem II, relative photosynthetic electron transfer rate of photosystem II, and photochemical quenching. HA also reduced intercellular CO2 concentration and non-photochemical quenching. Furthermore, 200 mg L-1 HA significantly increased the maximum P700, effective quantum yield of photosystem I, and relative photosynthetic electron transfer rate of photosystem I in Zhangza 10 and decreased non-photochemical energy dissipation in Jingu 21 and Zhangza 10 under drought stress. HA promoted the growth of millet seedlings under drought stress by promoting the osmotic adjustment ability and antioxidant capacity of seedlings and increased photosynthesis.
Keywords: Drought tolerance; humic acid; millet seedlings; photosynthesis; physiological characteristics.