USE OF C-13 AND N-15 ISOTOPES TO INVESTIGATE O-3 EFFECTS ON C AND N METABOLISM IN SOYBEANS .2. NITROGEN UPTAKE, FIXATION, AND PARTITIONING

Short- and long-term N uptake/partitioning dynamics were studied using stable isotope techniques to investigate the uncertain mechanism(s) o f O-3 action on plant yield and photosynthate partitioning. Glycine ma x [L.] Merr.(soybean) plants were grown in N-15 enriched soil within o pen-top chambers and exposed to one of three O-3 regimes: half-ambient , ambient, or 2 X ambient, The seasonal 7 h average O-3 concentrations (nl l(-1)) were 25, 43, and 76 nl l(-1), respectively. Nitrogen fixat ion was estimated using the N-15 isotope dilution method utilizing a n on-nodulating soybean isoline as the control. Macro-kjeldahl technique was used for determining N concentration. Short-term plant responses were investigated by evaluating the following parameters: % N, total N , total N fixed, total N fixed per organ dry weight, the proportion of N-fixed/soil N, and the fraction of N derived through rhizobial N-fix ation on an individual organ (leaves, stems, roots, pods, and nodules) and whole plant basis at two reproductive growth stages. Long-term pl ant responses were investigated by characterizing the same N parameter s of the mature grain. Ozone significantly affected both short- and lo ng-term N uptake/partitioning dynamics. Ozone exposure reduced the amo unt of N derived from N-fixation, but did not significantly affect tot al N or % N for organs and whole plants. For mature grain, O-3 signifi cantly decreased seed yield and all N parameters except N-fixed/soil N , but the responses were dependent upon year. Our results suggest that total nodule activity was affected rather than specific activity. Tot al N uptake was maintained despite significant decreases in % N-fixed and N-fixed/soil N. We conclude that N-fixation was inhibited by reduc ed photosynthate translocation to nodules. The photosynthate transloca ted was sufficient to maintain moderate rates of soil N uptake, but no t adequate to maintain high rates of N-fixation, the latter costing mo re energy. Thus, soybeans damaged by the exposures imposed here, relie d more heavily on soil N to meet their total N requirements when photo synthate translocation was inhibited. The long-term negative effects f or mature seed also indicate a significant reduction in photosynthate and total N translocated to nodules, and an increased reliance on soil N. In summation, these findings and those of our companion carbon stu dy, support the hypothesis that the mechanism of chronic O-3 action in volves an inhibition of carbon translocation from leaves to other orga ns.