Use of 13C and 15N isotopes to investigate O 3 effects on C and N metabolism in soybeans. Part II. Nitrogen uptake, fixation, and partitioning

Abstract

Short- and long-term N uptake/partitioning dynamics were studied using stable isotope techniques to investigate the uncertain mechanism(s) of O 3 action on plant yield and photosynthate partitioning. Glycine max [L.] Merr.(soybean) plants were grown in 15N enriched soil within open-top chambers and exposed to one of three O 3 regimes: half-ambient, ambient, or 2 × ambient. The seasonal 7 h average O 3 concentrations (nl l −1) were 25, 43, and 76 nl l −1, respectively. Nitrogen fixation was estimated using the 15N isotope dilution method utilizing a non-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-fixation on an individual organ (leaves, stems, roots, pods, and nodules) and whole plant basis at two reproductive growth stages. Long-term plant responses were investigated by characterizing the same N parameters of the mature grain. Ozone significantly affected both short- and long-term N uptake/partitioning dynamics. Ozone exposure reduced the amount of N derived from N-fixation, but did not significantly affect total N or % N for organs and whole plants. For mature grain, O 3 significantly 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. Total N uptake was maintained despite significant decreases in % N-fixed and N-fixed/soil N. We conclude that N-fixation was inhibited by reduced photosynthate translocation to nodules. The photosynthate translocated was sufficient to maintain moderate rates of soil N uptake, but not adequate to maintain high rates of N-fixation, the latter costing more energy. Thus, soybeans damaged by the exposures imposed here, relied more heavily on soil N to meet their total N requirements when photosynthate translocation was inhibited. The long-term negative effects for 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 study, support the hypothesis that the mechanism of chronic O 3 action involves an inhibition of carbon translocation from leaves to other organs.