Effects of Acetylene and External Oxygen Concentration on the Respiratory Quotient (RQ) of Nodulated Roots of Soyabean and White Clover

Abstract

Carbon dioxide output and oxygen uptake by intact nodulated roots of soyabean and white clover were determined over a range of external oxygen concentrations (10 to 60%) in the presence and absence of 10% acetylene. The responses of both sets of root systems were in agreement with the known characteristics of the variable oxygen diffusion resistance of the nodules. These measurements were used to calculate the respiratory quotient (RQ, C02/02) for both total root respiration and nitrogenase-linked respiration values for each external oxygen concentration. In the absence of acetylene, the RQ values of soyabean total root respiration showed no significant response to external oxygen, with a mean of 1 -06, but differences in values for white clover roots were significant at P=005, with a mean of 109. The same trend was shown for RQ values of nitrogenase-linked respiration, with mean values of 110 and 112 for soyabean and clover, respectively. The addition of acetylene caused a small but significant initial reduction in RQ of total root respiration with both soyabean and white clover, as did the subsequent reduction of external oxygen to 10%. For white clover, increases in external oxygen concentration to 50% and 60% also produced significant increases in the RQ of total root respiration compared to that in air. These variations were more pronounced for RQ values of nitrogenase linked respiration. Results are discussed in relation to the use of RQ values of unity in calculations of nodular oxygen diffusion resistance and the involvement of physical and biochemical processes in the regulation of oxygen supply to bacteroids.