Silicon improves photosynthetic activity and induces antioxidant enzyme activity in Tanzania Guinea grass under copper toxicity

Copper (Cu) toxicity reduces plant photosynthetic activity and causes oxidative stress in plants, which impairs plant growth. A possible solution to overcome such toxicity is the supply of silicon (Si), which may promote higher tolerance of plants to Cu stress. The present study was designed to examine the potential protective roles of Si against Cu toxicity in Panicum maximum cv. Tanzania. The experiment was set as a complete randomized block design with three replicates containing three Si rates (0, 1, and 3 mmol L-1) and three Cu rates (0.3, 250, and 500 mu mol L-1). The grass was grown under hydroponic conditions for 62 days in a greenhouse, with a total of 27 pots. Plants were evaluated during two growth periods: first for 32 days after seedling transplantation and the second for 30 days after the first harvest. In the first growth period, plants were exposed to Cu for seven days, 25 days after being transplanted. The second harvest had the objective of evaluating the regrowth of the grass under the residual Cu rates. Gas exchange parameters were determined in the diagnostic leaves (DL) at the end of each growth period. Concentrations of stress indicators malondialdehyde (MDA), hydrogen peroxide (H2O2) and proline, and activities of antioxidant enzymes were determined in shoots and roots. Silicon rates positively and Cu rates negatively affected gas exchange parameters in the first growth. In the second growth period, a eustress event was observed where the highest values of gas exchange parameters and lowest H2O2 concentrations were observed in the combinations with high Cu rates. Antioxidant system enzymes had their activities considerably reduced by the raising in Cu rates. Superoxide dismutase (SOD) and guaiacol peroxidase (GPX) activities were positively affected by Si supply to plants in the second growth period. Catalase (CAT) activity increased with the addition of Si to the nutrient solution in both growth periods. The results provide evidence of the positive influence of Si supplementation on the photosynthetic parameters and antioxidant enzyme activities of Tanzania guinea grass under stress by Cu toxicity.