Short‐term effects of manganese toxicity on ribulose 1,5 bisphosphate carboxylase in tobacco chloroplasts

Abstract

The short‐term effects of manganese (Mn) toxicity on ribulose 1,5 bisphosphate carboxylase EC 4.1.1.39 (Rubisco) activity and concentration in tobacco (Nicotiana tabacum L. ‘KY 17') chloroplasts were examined. The activity of the enzyme from both Mn‐treated and control plants was determined 6,12,18, 24, and 48 h after introduction of Mn (80 mg/L). Enzyme activity was determined by monitoring rates of carbon dioxide (¹⁴CO₂) fixation into acid stable products. A decrease in the enzyme's activity in experimental plants was noted after 48 h of exposure. Visible symptoms, such as chlorosis and decreased leaf size, were also observed after 48 h of Mn exposure in experimental plants. Using Rocket Immunoelectrophoresis, no appreciable differences between Rubisco concentration levels of the experimental plants and the control plants were noted indicating that the effect on Rubisco activity is a post‐translational phenomenon and that Rubisco is not being degraded at an accelerated rate. Manganese accumulated in the experimental plants to concentrations as high as 3282 μg/g dry wt as determined by atomic absorption spectrophotometry. A shuttling mechanism for Mn between young and old leaves was indicated by an observed decrease in the concentration of Mn in the young leaf tissue between 12 and 18 h after treatment.