Efficiency of respiration and energy requirements of N assimilation in roots of Pisum sativum

The function of alternative path respiration in roots was investigated in pea plants (Pisum sativum L. cv. Rondo). Plants were grown in symbiosis with Rhizobium leguminosarum (strain PF2), completely dependent on N2 fixation, or non‐nodulated, receiving nitrate or ammonium at the same rate as N2 was fixed in symbiosis. Under these conditions, relative growth rates of plants grown with N2, NO‐3 or NH+4 were the same. This facilitated interpretation of the effect of the N source on the efficiency of root respiration, as determined by the relative activity of the non‐phosphorylating alternative path. The ‘wasteful’ oxidation of carbohydrate via this pathway was defined as the glucose equivalent of the difference between the amounts of ATP (mol O2)‐1 produced in cytochrome and alternative path respiration. ‘Wasteful’ carbohydrate oxidation maximally amounted to 4% (N2), 15% (NO‐3) and 25% (NH+4) of the daily carbohydrate oxidation in the roots. It is concluded that the ‘wasteful’ oxidation of carbohydrate via the alternative path is of minor importance for the adaptation of root respiratory metabolism to different energy requirements of N assimilation. The total carbohydrate import by roots fixing N2 was ca 60 and 30% higher than the import by roots assimilating NO‐3 or NH+4, respectively. Two factors are shown to account for these differences: the high carbohydrate cost of N2 fixation, and the small contribution (30%) of the roots to NO‐3 reduction by the plant. The high carbohydrate requirements of roots fixing N2 were met by higher rates of photosynthesis as compared with plants utilizing NO‐3 or NH+4.