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Environmental and Stress Physiology

Evidence for Effects on the in Vivo Activity of Ribulose-Bisphosphate Carboxylase/Oxygenase during Development of Mn Toxicity in Tobacco ¹

Plant Physiology/Biochemistry Program, University of Kentucky, Lexington, Kentucky 40546-0091

The progressive decrease in net photosynthesis accompanying development of Mn toxicity in young leaves of burley tobacco (Nicotiana tabacum L. cv KY 14) is a result of effects on in vivo activity of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (rubisco, EC 4.1.1.39). This conclusion is supported by: (a) decrease in rates of CO₂ depletion during measurements of CO₂ compensation, (b) increase in leaf RuBP concentrations, (c) progressive decreases in rate-constants of RuBP loss (light to dark transition analyses) with progressive increases of leaf Mn concentrations, and (d) restoration of diminished rates of net photosynthesis to control rates by elevated CO₂ (5%). Moreover, elevated CO₂ (1100 microliters per liter) during culture of Mn-treated plants decreased elevated RuBP concentrations to control levels and alleviated foliar symptoms of Mn toxicity. These effects of Mn toxicity on in vivo activity of rubisco were not expressed by in vitro kinetic analyses of rubisco prepared under conditions to sequester Mn or to adsorb polyphenols or their oxidation products. Similarly, the in vitro activity of fructose bisphosphatase (EC 3.1.3.11) was unaffected by Mn toxicity.

¹ Supported by United States Department of Agriculture-Agricultural Research Service and the Department of Energy DE-FG05-86ER13533 (G. M. C.). This paper (87-3212) is published with approval of the Director of the Kentucky Agricultural Experiment Station.

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