[Physiological and biochemical mechanism of spermidine improving drought resistance in maize seedlings under drought stress]

To explore the role of exogenous spermidine (Spd) in enhancing the resistance of maize to drought stress, 15% polyethylene glycol (PEG-6000) was used to simulate drought stress and with 'Xianyu 335' (drought-insensitive) and 'Fenghe 1' (drought sensitive) as the experiment materials, the effects of Spd (0.1 mmol·L^-1) on the growth, photosynthetic characteristics, chlorophyll content, osmotic adjustment substance, membrane lipid peroxidation and root activity of maize seedlings were investigated. The results showed that the application of Spd significantly promoted the growth of maize seedlings under drought stress, increased chlorophyll content, net photosynthetic rate (P_n), stomatal conductance (g_s), transpiration rate (T_r) and water use efficiency (WUE), and decreased the enhancement of intercellular carbon dioxide concentration (C_i) in 'Fenghe 1'. Moreover, the stomatal and non-stomatal limitations of photosynthetic ability caused by drought stress in 'Fenghe 1' were effectively reduced by exogenous Spd. The application of Spd increased the content of proline and soluble sugar, decreased theO₂^-· generation rate, contents of H₂O₂ and MDA and cell membrane permeability, enhanced the root activity. The changes of drought-sensitive 'Fenghe 1' were greater than drought-tolerant 'Xianyu 335'. These results indicated that exogenous Spd had positive effects on the seedlings to capture and converse solar energy, thus promoting photosynthesis and the growth of maize seedlings. It would also enhance the adaptability of seedlings to drought stress by reducing the production of reactive oxygen species (ROS), increasing the accumulation of osmotic adjustment substances to stabilize the cell membrane system and improving the root vigor. The positive effects of Spd was more obvious for drought-sensitive variety 'Fenghe 1'.