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

Abstract

AbstractA comparison was made of energy metabolism of nodulated N2 fixing plants and non‐nodulated NO3‐fed plants of Lupinus albus L. Growth, N‐increment, root respiration (O2 uptake and CO2 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 O2 uptake and CO2 production, and the activity of the SHAM‐sensitive pathway were higher in NO3‐fed plants than in N2 fixing plants. The rate of ATP production in oxidative phosphorylation was computed also to be higher in NO3‐fed plants. It is concluded that both carbohydrate costings and ATP costings for synthesis + maintenance of root material were lower in N2 fixing than in NO3‐fed plants.The respiratory quotient of root respiration was 1.6 in N2‐fixing plants and 1.4 in NO3‐fed plants. These values were slightly higher than the values calculated on the basis of CO2 output due to N‐assimilation and the experimental values of O2 uptake, but showed the same trend: highest in N2 fixing plants.Root respiration of NO3‐fed plants showed a diurnal pattern (both O2 uptake, CO2 production and the activity of the SHAM‐sensitive pathway), whilst no diurnal variation in root respiration was found in N2 fixing plants. However, C2H2 reduction did show a diurnal rhythm, which is suggested to be related to the diurnal variation in transpiration.Addition of NO3 to N2 fixing plants increased the rate of root respiration and the activity of the alternative pathway. This treatment did not decrease C2H2 reduction and H2 evolution within 4 days.Withdrawal of NO3‐supply from NO3‐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 NO3‐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.