Nitrate and ammonium proportion plays a key role in copper phytoextraction, improving the antioxidant defense in Tanzania guinea grass.

Various nitrate and ammonium proportions (NO₃^-/NH₄⁺) in the growth media can increase metal phytoextraction compared to supplying solely NO₃^-. However, there are no studies showing these effects in plants under copper (Cu) contamination as well as their consequences in plant stress tolerance. The objective was to evaluate the effect of NO₃^-/NH₄⁺ proportions in Cu phytoextraction by Panicum maximum cv. Tanzania and its consequence in the oxidative stress, photosynthesis, and antioxidant system under Cu stress. The experiment was carried out in a randomized complete block design, by using a 3 × 4 factorial with six replications. Three NO₃^-/NH₄⁺ proportions (100/0, 70/30, and 50/50) were combined with four Cu rates (0.3, 250, 500, and 1000 µmol L^-1) in the nutrient solution. It was found that the largest Cu accumulation in the shoots occurred at the first harvest of the plants supplied with 70/30 NO₃^-/NH₄⁺ and Cu 1000 µmol L^-1. Such plants also displayed high concentrations of proline in the shoots as well as high superoxide dismutase activity in the roots. Malondialdehyde concentration was high in the plant parts at the Cu rate of 1000 µmol L^-1. Hence, transpiration rates, stomatal conductance, quantum efficiency of photosystem II, electron transport rate, and net photosynthesis were all low at the Cu rate of 1000 µmol L^-1. Catalase, guaiacol peroxidase, ascorbate peroxidase, and glutathione reductase activities in the roots were high when plants were exposed to Cu 1000 µmol L^-1. In conclusion, the combination of NO₃^- with NH₄⁺ increases copper phytoextraction that causes oxidative stress, but also favors the antioxidant system of Tanzania guinea grass in attempt to tolerate such stress.