Salt stress increases the respiratory cost of nitrogen fixation

The effects of NaCl stress (0, 3,000, 6,000 ppm) on the nitrogen ixation, respiration, and respiratory cost of nitrogen fixation of detached alfalfa nodules were investigated. Acetylene reduction activity and respiration of the nodules decreased to 96 and 84% of the control at 3,000 ppm NaCl, and to 48 and 68% at 6,000 ppm, respectively. The respiratory cost of nitrogen fixation (R f ) and the maintenance and growth respiration (R mg ) were estimated using the linear regression of the nodule respiration on the acetylene reduction under 10 and 20% po 2 conditions. In the alfalfa nodules, the R f and R mg values were 2.23 and 24.01 μmol CO 2 /(g(f.w.)·h) in the control, respectively. NaCl stress at 6,000 ppm increased the R f value and decreased the R mg value significantly, though no significant effect was observed at 3,000 ppm NaCl. It is assumed that the increase of the R f value was due to the overflow of energy by unbalance between energy supply of nitrogen fixation respiration and energy consumption by nitrogenase system due to the more pronounced suppression of the nitrogenase activity than of the nitrogen fixation respiration.