Ammonium nutrition mitigates cadmium toxicity in rice (Oryza sativa L.) through improving antioxidase system and the glutathione-ascorbate cycle efficiency.

Nitrogen (N) forms not only affect cadmium (Cd) accumulation in plants, but also affect plant resistance to Cd toxicity. However, few researches have been reported underlying the mechanism of the relationship between nitrogen forms and plant resistance under Cd exposure. Here, we explored the mechanism on how different NO₃^-/NH₄⁺ ratios affect antioxidase system and the glutathione-ascorbate cycle under five different ratios of NO₃^-/NH₄⁺ (1:0, 2:1, 1:1, 1:2, 0:1) and three dosages of Cd exposure (0, 1, 5 μmol L^-1 Cd) in rice (Oryza sativa L.). The results showed that high NO₃^- and high Cd exposure both significantly inhibited tissue growth of rice plants, and this inhibiting trend was mitigated with increasing NH₄⁺ ratios as proved by the increased biomass and the decreased concentrations of malonaldehyde (MDA) and hydrogen peroxide (H₂O₂), as well as the levels of Cd contents in rice tissues. Additionally, high NH₄⁺ ratios elevated the SOD activities in rice tissues, especially at high Cd treatment. However, other two antioxidases (CAT and APX) were insensitive to changes of NO₃^-/NH₄⁺ ratios (except the full NO₃^-). Furthermore, high NH₄⁺ ratios induced increasing of the efficiency of glutathione-ascorbate cycle (GSH-AsA) under two levels of Cd exposure, as evidenced by increasing concentrations of GSH and AsA and the activities of GR and DHAR in rice tissues. Overall, these results revealed that ammonium nutrition caused an enhancement resistance to Cd stress in rice plants was responsible for increasing of partial antioxidase system and the efficiencies of GSH-AsA cycle.