THE IMPACT OF OZONE ON THE N-15 INCORPORATION AND NITROGEN ASSIMILATION OF WHEAT AND MAIZE

Young wheat (C3) and maize (C4) plants were exposed to near-ambient co ncentrations of ozone in open-top chambers in order to investigate the possible effects of ozone on nitrogen metabolism. Nitrogen was suppli ed to the plants by adding N-15-labelled tracer substances via the soi l substrate. Enzyme activities (NADH nitrate reductase, nitrite reduct ase, glutamine synthetase and NADH glutamate dehydrogenase) and the in corporation of N-15 were determined. The findings show that nitrogen m etabolism was affected by O-3, however, there were distinct difference s between the two species. In plants treated with O-3, NADH nitrate re ductase activity in maize leaves was reduced, while NR activity in whe at leaves only slightly declined. Only minor changes were observed wit h respect to the activities of nitrite reductase, glutamine synthetase and NADH glutamate dehydrogenase. Feeding experiments using (NO3-)-N- 15 showed that the incorporation of nitrate nitrogen in wheat plants e xposed to ozone remains virtually unchanged, whereas in maize plants r educed incorporation rates were observed for nitrate nitrogen. The inc orporation of ammonium nitrogen was distinctly increased in wheat and maize by the impact of ozone. When investigating pigment contents, red uced levels of chlorophyll a and b and carotenoids were observed, wher eas the pigment content of wheat leaves remained unchanged. These resu lts indicate that young maize plants are more susceptible than wheat p lants to short-term ozone exposure.