Effect of Source of Nitrogen on the Growth of Fiskeby Soya Bean: the Carbon Economy of Whole Plants

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Abstract
Fiskeby V soya bean was grown from seed germination to seed maturation with two contrasting patterns of nitrogen metabolism: either wholly dependent on dinitrogen fixation, or with an abundant supply of nitrate nitrogen, but lacking root nodules. The carbon and nitrogen economies of the plants were assessed at frequent intervals by measurements of photosynthesis, shoot and root respiration, and organic and inorganic nitrogen contents. Plants fixing atmospheric nitrogen assimilated only 25–30 per cent as much nitrogen as equivalent plants given nitrate nitrogen: c. 40 per cent of the nitrogen of ‘nitrate’ plants was assimilated after dinitrogen fixation had ceased in ‘nodulated’ plants. The rates of photosynthesis and respiration of the shoots of soya bean were not markedly affected by source of nitrogen; in contrast, the roots of ‘nodulated’ plants respired twice as rapidly during intense dinitrogen fixation as those of ‘nitrate’ plants. The magnitude of this respiratory burden was calculated to increase the daily whole-plant res piratory loss of assimilate by 10–15 per cent over that of plants receiving abundant nitrate. It is concluded that ‘nodulated’ plants grew more slowly than ‘nitrate’ plants in these experiments for at least two reasons: firstly, the symbiotic association fixed insufficient nitrogen for optimum growth and, secondly, the assimila tion of the nitrogen which was fixed in the root nodules was more energy-demanding in terms of assimilate than that of plants which assimilated nitrogen by reducing nitrate in their leaves.

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