Responses in root physiological characteristics of

We selected the submerged macrophyte Vallisneria natans (Lour.) Hara for investigating the effects of nutrient loadings (nitrogen (N)-phosphorus (P) in mg·L-1: (1) 0.5, 0.05; (2) 1.0, 0.1; (3) 5.0, 0.5; (4) 10.0, 1.0) on root physiological characteristics using sand culture during the growth season (June to October). Results showed that the best root growth was in macrophytes exposed to moderate nutrient conditions (N-P 2, in 1.0 and 0.10 mg· L-1), and high nutrient loadings induced declines in root growth. Analysis of root antioxidant enzyme (superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT)) activities, protein content and cell ultrastructure revealed that the root of V. natans was subject to the stress of senescence during the end of the growth season. Moreover, high nutrient loadings increased oxidative stress in aging roots of V. natans, and made the cell ultrastructure of roots vulnerable to damage during senescence. The results for the changes in CAT activity suggest that CAT can serve as an important component of antioxidant defense mechanism in aging roots of V. natans to protect against nutrient loading induced oxidative injury in the early period.