Efficiency and regulation of root respiration in a legume: Effects of the N source

A comparison was made of energy metabolism of nodulated N 2 fixing plants and non‐nodulated NO 3 ‐fed plants of Lupinus albus L. Growth, N‐increment, root respiration (O 2 uptake and CO 2 production) and the contribution of a SHAM‐sensitive oxidative pathway (the alternative pathway) in root respiration were measured. Both growth rate and the rate of N‐increment were the same in both series of plants. The rate of root respiration, both O 2 uptake and CO 2 production, and the activity of the SHAM‐sensitive pathway were higher in NO 3 ‐fed plants than in N 2 fixing plants. The rate of ATP production in oxidative phosphorylation was computed also to be higher in NO 3 ‐fed plants. It is concluded that both carbohydrate costings and ATP costings for synthesis + maintenance of root material were lower in N 2 fixing than in NO 3 ‐fed plants. The respiratory quotient of root respiration was 1.6 in N 2 ‐fixing plants and 1.4 in NO 3 ‐fed plants. These values were slightly higher than the values calculated on the basis of CO 2 output due to N‐assimilation and the experimental values of O 2 uptake, but showed the same trend: highest in N 2 fixing plants. Root respiration of NO 3 ‐fed plants showed a diurnal pattern (both O 2 uptake, CO 2 production and the activity of the SHAM‐sensitive pathway), whilst no diurnal variation in root respiration was found in N 2 fixing plants. However, C 2 H 2 reduction did show a diurnal rhythm, which is suggested to be related to the diurnal variation in transpiration. Addition of NO 3 to N 2 fixing plants increased the rate of root respiration and the activity of the alternative pathway. This treatment did not decrease C 2 H 2 reduction and H 2 evolution within 4 days. Withdrawal of NO 3 ‐supply from NO 3 ‐fed plants decreased the rate of root respiration but had no effect on the relative activity of the alternative pathway. It is suggested that the higher rate of root respiration and the higher activity of the SHAM‐sensitive pathway in NO 3 ‐fed plants is due to a larger supply of carbohydrates to the roots, partly due to a better photosynthetic performance of the shoots and partly due to a higher capacity of the roots to attract carbohydrates.