Keywords: 
• glyphosate /
• cadmium /
• maize seedling /
• antioxidant enzymes /
• chlorophyll content /
• OJIP

Abstract: In order to investigate the underlying mechanisms of effects on crops growth (maize seedling) under the combined stress of glyphosate (PMG) and cadmium (Cd), the pot experiments in greenhouse were carried out, including a single treatment with one of the PMG concentration sets (0, 1.25, 2.5, 5, 10 and 20 mg·kg^-1) and a combined treatment together with one of the concentration sets of PMG (the concentration of 0, 1.25, 2.5, 5, 10, 20 mg·kg^-1, respectively) and 5 mg·kg^-1 of Cd₂₊. The maize seedling antioxidant enzymes (peroxidase POD, catalase CAT and superoxide dismutase SOD), malondialdehyde (MDA) content, chlorophyll fluorescence, dynamic curve and related parameters were determined by using spectrophotometry and a continuous excitation fluorometer. Under the condition of either the single exposure of PMG (1.25-5 mg·kg^-1) or the combined exposure (1.25-2.5 mg·kg^-1 PMG and Cd), the results showed that the activities of the antioxidant enzymes (SOD, POD, CAT) were increased in maize seedlings, the accumulation of reactive oxygen species were decreased, and the rates of the synthesis of chlorophyll and photosynthesis were also enhanced. In addition, under the single exposure of PMG (5-20 mg·kg^-1) or the combined exposure (2.5-20 mg·kg^-1 PMG and Cd), the contents of lipid peroxidation (MDA) in maize seedlings were higher than the other treatments, the antioxidant system were destroyed, and the synthesis of chlorophyll content were suppressed accompanied with the damaged functions of PSII (M_O, φP_O, Ψ_O, φ_EO, φ_DO, ABS/RC, TR_O/RC, ET_O/RC, DI_O/RC, PI_ABS). All these changes finally restrained the photosynthesis of maize seedlings and inhibited the seedling growth. It was concluded that there were two phases for the effects of single PMG or combined with Cd stress on enzyme activity and photosynthesis in maize seedlings, in which a promotion at the low concentration and an inhibition at the high concentration. Compared with the single treatment, Cd₂₊ aggravated the damage of PMG treatment alone, and decreased the sensitive concentration of maize seedling to PMG stress from 5 mg·kg^-1 to 2.5 mg·kg^-1.