Use of 13C and 15N isotopes to investigate O3 effects on C and N metabolism in soybeans. Part II. Nitrogen uptake, fixation, and partitioning

摘要

Short- and long-term N uptake/partitioning dynamics were studied using stable isotope techniques to investigate the uncertain mechanism(s) of O3 action on plant yield and photosynthate partitioning. Soyabeans cv. Clark were grown in 15N enriched soil within open-top chambers and exposed to one of three O3 regimes: half-ambient, ambient, or 2X ambient. The seasonal 7 h average O3 concentrations were 25, 43, and 76 nl litre-1, respectively. 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 seed. 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 seed, O3 significantly decreased seed yield and all N parameters except N fixed/soil N, but the responses were dependent upon year. Results suggested 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. It is concluded 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, soyabeans damaged by the O3exposures 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 indicated a significant reduction in photosynthate and total N translocated tonodules, and an increased reliance on soil N.