Differential effects of ozone on in vivo nitrate reduction in soybean cultivars. I. Response to exogenous sugars

Two cultivars of soybeans (Glycine max (L.) Merr.) differing widely in their resistance to ozone were exposed to 0.5 μl/ℓ ozone for 2 h in growth chambers. In vivo nitrate reduction was depressed by more than 50% in the primary leaves of Dare, the ozone-sensitive cultivar, but was not significantly altered in Hood, the ozone-resistant cultivar. Sucrose, up to 1.5% (w/v), added to excised seedlings of the Dare cultivar during exposure to ozone eliminated the ozone depression of in vivo nitrate reductase activity and also reduced foliar injury. Addition of two glycolytic intermediates, glyceraldehyde-3-phosphate and fructose-1,6-diphosphate, to the infiltration medium recovered some in vivo nitrate reduction in treated Dare leaves. The levels of extractable nitrate reductase and glyceraldehyde-3-phosphate dehydrogenase in the primary leaves of both cultivars were unaltered by ozone fumigations. These observations led to the conclusion that ozone depression of in vivo nitrate reduction is not due to ozone inactivation of nitrate reductase or of the enzymes coupling nitrate reduction to glycolysis, but may be caused by an inadequate supply of photosynthetic sugars. It was also noted that ozone depression of in vivo nitrate reduction only occurred with treatments which subsequently caused the development of visible foliar injury.