Economy of Carbon and Nitrogen in Nodulated and Nonnodulated (NO3-grown) Cowpea [Vigna unguiculata (L.) Walp.]

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The response of non-nodulated cowpea (Vigna unguiculata (L.) Walp. cv Caloona) to a wide range of NO3 levels in the rooting medium was studied 40 days after sowing by in vitro assays of plant organs for NO3 reductase (EC 1.6.6.1) and analyses of root bleeding (xylem) sap for nitrogenous solutes. Plants fed 1, 5, 10, 20, and 40 millimolar NO3 showed, respectively, 64, 92, 94, and 91% of their total reductase activity in shoots and 34, 30, 66, 62, and 58% of the total N of their xylem sap as NO3. These data, and the absence in the plants of significant pools of stored NO3, indicated that shoots were major organs of NO3 assimilation, especially at levels of NO3 (10 to 40 millimolar) that maintained plant growth at near maximum rates. Partitioning and utilization of C and N were studied in nodulated, minus NO3 plants and non-nodulated plants fed 10 or 20 millimolar NO3, the levels of NO3 which gave rates of growth and N assimilation closest to those of the symbiotic plants. The conversion of the C of net photosynthate to dry matter was similar in nodulated plants (67%) and NO3-grown plants (64%), but greater proportions of photosynthate were translocated to below ground parts of nodulated plants (37%) than of NO3-fed plants (23 to 26%). Greater photosynthate consumption by nodulated roots was associated with proportionately greater root growth and respiration and 2-fold greater export of C in xylem than in the NO3-fed plants. Theoretical considerations suggest that the elevated CO2 output of nodulated roots was due not only to CO2 loss associated with nodule function, but also to a much greater nonassimilatory component of respiration in the supporting root of the nodulated plant compared to roots of the NO3-fed plants. Data are compared with previously published information from other legumes.

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