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Environmental and Stress Physiology

Early Inhibition of Photosynthesis during Development of Mn Toxicity in Tobacco ¹

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

Early physiological effects of developing Mn toxicity in young leaves of burley tobacco (Nicotiana tabacum L. cv KY 14) were examined in glass-house/water cultured plants grown at high (summer) and low (winter) photon flux. Following transfer of plants to solutions containing 1 millimolar Mn²⁺, sequential samplings were made at various times for the following 9 days, during which Mn accumulation by leaves increased rapidly from 70 on day 0 to 1700 and 5000 microgram per gram dry matter after 1 and 9 days, respectively. In plants grown at high photon flux, net photosynthesis declined by 20 and 60% after 1 and 9 days, respectively, and the onset of this decline preceded appearance (after 3 to 4 days) of visible foliar symptoms of Mn toxicity. Intercellular CO₂ concentrations and rates of transpiration were not significantly affected; moreover, the activity of the Hill and photosystem I and II partial reactions of chloroplasts remained constant despite ultimate development of severe necrosis. Though the activity of latent or activated polyphenol oxidase increased in parallel with Mn accumulation, neither leaf respiration nor the activity of catalase [EC 1.11.1.6] and peroxidase [EC 1.10.1.7] were greatly affected. These effects from Mn toxicity could not be explained by any changes in protein or chlorophyll abundance. Additionally, they were not a consequence of Mn induced Fe deficiency. Therefore, inhibition of net photosynthesis and enhancement of polyphenol oxidase activity are early indicators of excess Mn accumulation in tobacco leaves. These changes, as well as leaf visual symptoms of Mn toxicity, were less severe in plants cultured and treated at low photon flux even though the rates of leaf Mn accumulation at high and low photon flux were essentially equivalent.

¹ Supported by the United States Department of Agriculture-Agricultural Research Service and the Department of Energy DE-FG05-86ER13533 (G. M. C.). This paper (87-3-211) is published with approval of the Director of the Kentucky Agricultural Experiment Station.

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