respiratory costs of nitrogen fixation in soyabean, cowpea, and white clover. II. Comparisons of the cost of nitrogen fixation and the utilization of combined nitrogen

Plants of soyabean, cowpea, and white clover were grown singly in pots in Saxcil growth cabinets at 23/18 °C, 30/24 °C, and 20/15 °C, respectively, until seed maturation or for 85 d (white clover). Two populations were produced within each species: one population effectively nodulated and wholly dependent for nitrogen on fixation in the root nodules, and a second population completely lacking nodules but receiving abundant nitrate nitrogen. In each species, the two populations were compared in terms of rate of gross photosynthesis, rate of shoot respiration, and rate of root respiration. Source of nitrogen had little or no effect on rate of photosynthesis or shoot respiration. In contrast, the rate of respiration of the nodulated roots of plants fixing their own nitrogen was greater, sometimes two-fold greater, than that of equivalent plants lacking nodules and utilizing nitrate nitrogen. This superiority in terms of rate of root respiration was generally confined to the period of intense nitrogen fixation. An analysis of the magnitude of this respiratory burden in terms of daily photosynthesis indicates that, in all three legumes, plants fixing their own nitrogen respire 11—13% more of their fixed carbon each day than equivalent plants lacking nodules and utilizing nitrate nitrogen.