Molybdenum and sulfur nutrition effects on nitrate reduction in Burley tobacco

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Abstract
Nicotiana tabacum L. cv. ''Ky-14'' plants were grown in an intermittently irrigated gravel culture system in a greenhouse. The nutrient variables were either Mo (0-0.66 .mu.eq/l) or Mo (0 and 0.33 .mu.eq/l) plus SO42- (0-0.50 meq/l). About 45 days after initiating the treatments, the plants were harvested and separated into leaves, midribs, stalk epidermis, stalk xylem-phloem, and stalk pith for Mo, S, NO3-, total N and nitrate reductase (NR) (EC 1.6.6.1) activity assay. Other plants were grown in complete Hoagland''s solution (i.e., 0.66 .mu.eq Mo/l and 0.50 meq SO42-/l) for all nutrient elements and used for determinations of canopy and seasonal profiles of NR activity. Mo concentration in all plant parts increased with increasing solution Mo concentration. Sulfate drastically reduced the concentration of Mo, and the inhibition was essentially equal in all plant parts. Greatest reduction of Mo occurred with the 1st increment of SO42- (0.05 meq/l), with little effects from further additions of SO42-. The data suggest that the inhibitory effects of SO42- on Mo content occur primarily during the absorption process, with some antagonistic mechanism involved during translocation from roots to leaves. Mo was not inhibitory to S accumulation. Mo and S content in leaves was greater than in any other plant part. NR activity generally increased from bottom to top leaves. About 60% of the NR in the plant occurred in the leaves. The canopy profiles suggest that the recent, completely open leaf (7-9 in. long) and the 2nd and 3rd leaves below it could be sampled for maximum NR activity for NO3- reduction experiments. Sulfate nutrition increased NR activity, as a result of increased NR synthesis, while Mo nutrition was without effect on NR in these experiments. The NO3- content was not affected by SO42-, but NO3- increased in the absence of MO.

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