Effect of chlorate, nitrogen source, and light on chlorate toxicity and nitrate reductase activity in soybean leaves

Abstract

Growth chamber studies were conducted to assess the relationship between nitrate reductase (NR) activity and development of chlorate (KClO3) toxicity symptoms in leaflets of soybeans [Glycine max (L.) Merr.]. Fourteen day‐old soybean seedlings, grown in NO3 ‐ or urea‐nutrient solutions, were exposed to various KClO3 concentrations (0 to 2.0 mM) and light levels (100, 67, 33 and 0% of full light which was 750 μE m−2s−1) for 24 h. Visual KClO3 toxicity symptoms were noted and NR activity was measured.Toxicity symptoms (interveinal chlorosis) were evident within 24 h following addition of 0.5 mM KClO3 to the nutrient solution, regardless of N nutrition, and symptom severity increased with increased KClO3 concentration (up to 2.0 mM). Leaflet NR activity was lower following 24 h KClO3 treatments at concentrations of 0.5 mM and higher, indicating that ClO3 ‐ or some reduction product of ClO3 ‐ likely ClO2 ‐ was detrimental to enzyme functionality. The light study supported involvement of NR activity in KClO3 toxicity in that comparison of control and KClO3 treated plants exposed to decreased light levels revealed a decrease in NR activity of control plants parallel to a decrease in severity of KClO3 toxicity symptoms of treated plants. Urea‐grown plants, which have an apparent constitutive NR enzyme, were used to verify that the KClO3 toxicity symptoms were not simply N starvation symptoms due to competition of ClO3 ‐ and NO3 ‐ for uptake and reduction. In vivo NR assays also ruled out that ClO3 ‐ was decreasing NR activity through competition with NO3 ‐ for reduction sites. The close relationship between KClO3 toxicity symptoms and NR activity, in response to light treatments, suggested that KClO3 toxicity symptoms were associated with reduction of ClO3 ‐ to ClO2 ‐ by the NR enzyme. However, the possibility that a more direct photochemical reaction occurred in the presence of KClO3 to produce the toxicity symptoms could not be ruled out.