Ascorbic acid alleviation of manganese-induced toxicity in Vallisneria natans (Lour.) Hara

We investigated the growth and physiological responses of the submerged aquatic plant species Vallisneria natans (Lour.) Hara to 80 mg L^-1 manganese (Mn) with different doses of ascorbic acid (AsA 0, 5, 10, 20, 50, 100, 200 mg L^-1) after 21 days of treatment. Mn stress significantly reduced the final leaf number and superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) activities of V. natans, while increased the malondialdehyde (MDA), hydrogen peroxide (H₂O₂) and proline contents, and peroxidase (POD) activity, with no significant differences in plant relative growth rate (RGR) and photosynthetic pigment contents. With increasing doses of AsA supplementation (≤ 50 mg L^-1), the MDA content gradually decreased, while the proline, soluble protein, and photosynthetic pigment contents, antioxidase (except POD) activities, and RGR of V. natans increased. AsA levels ≥ 100 mg L^-1 exacerbated Mn toxicity in V. natans by significantly reducing the antioxidase activities and photosynthetic pigment contents and even triggering plant lethal effects. These results suggest that the Mn stress induced in this investigation could bring about oxidative stress and influence the growth of V. natans. Moreover, the optimal AsA dose that can alleviate Mn-induced oxidative stress was 41.37-50.25 mg L^-1 according to the regression analysis based on plant growth and enzymatic responses.