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ABSTRACI

Early physiological effects of developing Mn toxicity in young leaves of burley tobacco (Nicotiana tabacum L. cv KY 14) were examined in glasshouse/water cultured plants grown at high (summer) and low (winter) photon flux. Following transfer of plants to solutions containing I 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 1.11.1.61 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. Mn TOXICITY EFFECTS ON PHOTOSYNTHESISsupply on the development of Mn toxicity was studied by providing plants with either 30 or 100 ,uM FeEDTA on transfer to nutrient solutions. All treatments were imposed in triplicate.During the summer, the maximum PAR was approximately 1400 ,umoV/s.m2 (14 h photoperiod). During the winter, plants were grown with supplementary light from high pressure Na lamps (1500 ,umol/s.m2 of PAR, 15 h photoperiod) or only natural light with maximum PAR of approximately 900 ,umol/s.m2 (10 h photoperiod).In all experiments, at least one plant was harvested from each container at the time Mn treatments were imposed (designated d 0) and on each day up to 9. Measurements were made on the leaf which was the third youngest on d 0 and which, independent of Mn treatment, increased in length from 15 to 25 cm and was the fourth youngest leaf on d 9. All the results reported represent the mean of two experiments (each with three replicates).Gas Exchange Measurements. Net photosynthesis and transpiration were determined on attached leaves using an open gas exchange system which employed: (a) a Plexiglas clamp-on chamber (21) that enclosed a 16 cm2 section of a leaf (midway along on the leaf and avoiding the midvein), (b) an ...