Biochemical and physiological responses of soybean [Glycine max (L.) Merrill] to nickel toxicity

Despite the crucial role of nickel (Ni) in the plant metabolism, small increases in its concentration can cause leaf tissues injury. This study identified the highest dose of Ni foliar-applied that does not cause toxicity to soybean plants. Plants were sprayed with five Ni doses (0, 30, 60, 120, and 240 g·ha-1). At 1, 3, and 5 days after spray (DAS), the malondialdehyde (MDA), superoxide (O2 -), hydrogen peroxide (H2O2), and photosynthetic pigments concentrations, antioxidant enzymes activities, and gas exchange and chlorophyll (Chl) a fluorescence parameters were determined. Symptoms of Ni toxicity started at 120 g·ha-1 Ni and intense foliar necrosis occurred at 3 DAS. The concentrations of O2 -, H2O2, and MDA were significantly higher by 49% at 3 DAS, 47% at 3 DAS, and 19% at 5 DAS, respectively, for plants sprayed with 120 g·ha-1 Ni and by 48% at 3 DAS, 48% at 3 DAS and 18% at 5 DAS, respectively, for plants sprayed with 240 g·ha-1 Ni. Higher antioxidant enzymes activities and lower Chl a and Chl b concentrations occurred for plants sprayed with either 120 and 240 g·ha-1 Ni compared to the other Ni doses. Decrease on energy destined to photochemical process [Y(II)] (8 and 8% at 5 DAS) and increase on the dissipation of energy by the nonregulated process [Y(NO)] (15 and 15% at 5 DAS) occurred for plants sprayed with 120 and 240 g·ha-1 Ni, respectively. The Ni doses above 120 g·ha-1 promoted oxidative stress to the plants and affected the functionality of their photosynthetic apparatus. Doses below 60 g·ha-1 had a low risk of toxicity to plants without causing any biochemical or physiological damage.