Modulation of cadmium-induced phytotoxicity in Cabomba caroliniana by urea involves photosynthetic metabolism and antioxidant status.

Urea is a widespread organic pollutant, which can be a nitrogen source, playing different roles in the growth of submerged macrophytes depending on concentrations, while high cadmium (Cd) concentrations are often toxic to macrophytes. In order to evaluate the combined effect of urea and Cd on a submerged macrophyte, Cabomba caroliniana , the morphological and physiological responses of C. caroliniana in the presence of urea and Cd were studied. The results showed that high concentrations of urea (400 mg L −1 ) and Cd (500 µmol L −1 ) had negative effects on C. caroliniana. There were strong visible symptoms of toxicity after 4 days of exposure under Cd-alone, 400 mg L −1 urea, and Cd+400 mg L −1 urea treatments. In addition, 400 mg L −1 urea and Cd had adverse effects on C. caroliniana 's pigment system. Significant losses in chlorophyll fluorescence and photosynthetic rates, as well as Rubisco activity were also observed under Cd-alone, 400 mg L −1 urea, and Cd+400 mg L −1 urea treatments. 400 mg L −1 urea markedly enhanced Cd toxicity in C. caroliniana , reflected by a sharp decrease in photosynthetic activity and more visible toxicity symptoms. The results of thiobarbituric acid reactive substances (TBARS) pointed to extreme oxidative stress in C. caroliniana induced under Cd or 400 mg L −1 urea exposure. Exogenous ascorbate (AsA) protected C. caroliniana from adverse damage in 400 mg L −1 urea, which further corroborated the oxidative stress claim under 400 mg L −1 urea. However, results also demonstrated that lower urea concentration (10 mg L −1 ) alleviated Cd-induced phytotoxicity by stimulating chlorophyll synthesis and photosynthetic activity, as well as activating the activity of catalase (CAT) and glutathione-S-transferase (GST), which may explain the alleviating effect of urea on C. caroliniana under Cd stress. [ABSTRACT FROM AUTHOR]