Abstract:

This research studied the effect of spraying exogenous melatonin on drought resistance of maize seedlings， and explored the mechanism of melatonin alleviation of drought stress in maize variety Zhengdan 985. A hydroponic experiment was conducted， using polyethylene glycol （PEG-6000） to simulate drought stress. The effects of melatonin at different concentrations （0， 10， 50， 100 μmol·L^-1； MT₀， MT₁₀， MT₅₀， MT₁₀₀， respectively） on the growth， root configuration and diameter characteristics， photosynthetic characteristics， antioxidant capacity and osmotic content of maize seedlings under drought stress were studied. Multivariate analysis of the maize seedling drought resistance under the various treatments was undertaken using the subordinate function value method. Compared with the treatment without melatonin， spraying 50 μmol·L^-1 melatonin had the best effect， and significantly increased plant height， aboveground fresh and dry weight， underground fresh and dry weight， stem tolerance index and root tolerance index of the seedlings. The total root length （RL）， root surface area （SA）， root volume （RV） and root tip number （RT） of maize seedlings were also increased significantly by melatonin application. The RL of the finest diameter category （0-0.5 mm） and the SA and RV of the second diameter category （0.5-1.0 mm）， were also significantly increased. Across the treatments， the proportions of total RL and SA （82.40% and 47.32%， respectively） in the （0-0.5 mm） diameter range were highest in the 50 μmol·L^-1 melatonin treatment. In this treatment， compared to the control， shoot malondialdehyde （MDA） levels were decreased by 20.17% （P<0.05）， shoot soluble protein content was maximized， the soluble protein content of underground parts was increased by 49.14%， and leaf photosynthetic rate （P_n） and stomatal conductance （G_s） were increased by 85.83% and 32.81%， respectively. The activities of superoxide dismutase （SOD）， peroxidase （POD）， catalase （CAT） and ascorbate peroxidase （APX） in shoots and underground organs were also significantly increased. The order of drought resistance of the four treatments under drought stress was MT₅₀>MT₁₀>MT₁₀₀>MT₀. Exogenous application of 50 μmol·L^-1 melatonin effectively regulated the activities of antioxidant enzymes and osmotic regulation in the aboveground and underground parts of maize seedlings， reduced the MDA content， increased the photosynthetic capacity， and beneficially regulated the soluble protein content， ascorbate （AsA） content， glutathione （GSH） content， SOD activity， POD activity， APX activity and CAT activity in the aboveground and underground parts of maize seedlings. The biomass and elongation of maize seedlings were increased， the detrimental effect of drought stress on root configuration was reduced， and the drought resistance of maize seedlings was improved. These results provide a theoretical basis for revealing the mechanism of melatonin alleviation of drought stress.

Key words: melatonin, maize, drought stress, root configuration, physiological and biochemical characteristics