Plant material, growth conditions, design and sampling

This experiment was carried out in the growth chamber at Agricultural College, Northeast Agricultural University in Harbin, China. The DCPTA and maize inbred line Chang 7–2 were obtained from China Zhengzhou Zhengshi Chemical Company, Ltd. and Henan Academy of Agricultural Sciences, China, respectively.

After the germination (for 6 days), uniform seedlings were transferred to half-strength Hoagland’s nutrient solution (10 L) and maintained in opaque plastic containers (inner length, width, height: 50 cm, 30 cm, 18 cm; contained 60 seedlings) under controlled conditions: temperature, 25/18°C (day/night); light, 16/8 h (light/dark) period, 400 μmol m^-2 s^-1; relative humidity, 60~70%.

Maize seedlings (three-leaf stage) were subjected to the following four treatments: (1) control, untreated; (2) PEG, 15% (w/v) PEG-6000-simulated drought stress; (3) DCPTA, pre-treatment with DCPTA (15 mg l^–1) added to the nutrient solution for 24 h under non-stress conditions (0% (w/v) PEG-6000); and (4) PEG+DCPTA, pre-treatment with DCPTA added to the nutrient solution for 24 h followed by exposure to 15% (w/v) PEG-6000-simulated drought stress. The doses of DCPTA (15 mg l^–1) and PEG-6000 (15%) were selected according to our previous work [19]. The experiment was repeated five times. The nutrient solution was renewed and the pH was regulated to 6.50 (±0.05) every day. The nutrient solution was aerated daily at 7:00~9:00, 11:00~13:00 and 15:00~17:00.

Whole plants were sampled from each treatment on the 7^th day of drought stress for measurements of growth parameters. The second fully developed leaves of the seedlings were harvested on the 7^th day after drought stress for ultramicroscopic observations and histochemical staining of O₂·⁻ and H₂O₂; at the 0, 1^st, 3^rd, 5^th and 7^th days after drought stress, portions of second fresh leaves were used to measure the electrolyte leakage (EL). The remaining leaves were immediately frozen in liquid nitrogen and stored at −80°C for later determination of other indicators.