Department of Agronomy and Plant Genetics, University of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, Minnesota 55108.
Plant Physiol. 1987 Sep;85(1):283-9. doi: 10.1104/pp.85.1.283.
The dependence of alfalfa (Medicago sativa L.) root and nodule nonphotosynthetic CO(2) fixation on the supply of currently produced photosynthate and nodule nitrogenase activity was examined at various times after phloem-girdling and exposure of nodules to Ar:O(2). Phloemgirdling was effected 20 hours and exposure to Ar:O(2) was effected 2 to 3 hours before initiation of experiments. Nodule and root CO(2) fixation rates of phloem-girdled plants were reduced to 38 and 50%, respectively, of those of control plants. Exposure to Ar:O(2) decreased nodule CO(2) fixation rates to 45%, respiration rates to 55%, and nitrogenase activities to 51% of those of the controls. The products of nodule CO(2) fixation were exported through the xylem to the shoot mainly as amino acids within 30 to 60 minutes after exposure to (14)CO(2). In contrast to nodules, roots exported very little radioactivity, and most of the (14)C was exported as organic acids. The nonphotosynthetic CO(2) fixation rate of roots and nodules averaged 26% of the gross respiration rate, i.e. the sum of net respiration and nonphotosynthetic CO(2) assimilation. Nodules fixed CO(2) at a rate 5.6 times that of roots, but since nodules comprised a small portion of root system mass, roots accounted for 76% of the nodulated root system CO(2) fixation. The results of this study showed that exposure of nodules to Ar:O(2) reduced nodule-specific respiration and nitrogenase activity by similar amounts, and that phloem-girdling significantly reduced nodule CO(2) fixation, nitrogenase activity, nodule-specific respiration, and transport of (14)C photoassimilate to nodules. These results indicate that nodule CO(2) fixation in alfalfa is associated with N assimilation.
对苜蓿（Medicago sativa L.）根和根瘤非光合 CO2 固定对当前光合作用产物供应和根瘤固氮酶活性的依赖性进行了研究，在韧皮部环割和暴露于 Ar:O2 后不同时间进行了研究。韧皮部环割在实验开始前 20 小时进行，暴露于 Ar:O2 在 2 到 3 小时前进行。与对照植物相比，韧皮部环割植物的根瘤和根 CO2 固定率分别降低到 38%和 50%。暴露于 Ar:O2 使根瘤 CO2 固定率降低到对照的 45%，呼吸率降低到 55%，固氮酶活性降低到对照的 51%。暴露于（14）CO2 后 30 至 60 分钟内，根瘤 CO2 固定的产物通过木质部输送到地上部分，主要以氨基酸的形式存在。与根瘤不同的是，根输送的放射性物质很少，大部分（14）C 以有机酸的形式输出。根和根瘤的非光合作用 CO2 固定率平均为总呼吸率的 26%，即净呼吸和非光合作用 CO2 同化的总和。根瘤以根的 5.6 倍的速度固定 CO2，但由于根瘤占根系质量的一小部分，因此根占根系统 CO2 固定的 76%。这项研究的结果表明，暴露于 Ar:O2 使根瘤的特定呼吸和固氮酶活性降低的幅度相似，韧皮部环割显著降低了根瘤 CO2 固定、固氮酶活性、根瘤特定呼吸和光合产物向根瘤的运输。这些结果表明，苜蓿根瘤的 CO2 固定与氮同化有关。
