A SHORT-TERM DECREASE IN NITROGENASE ACTIVITY (C2H2 REDUCTION) IS INDUCED BY EXPOSURE OF SOYBEAN SHOOTS TO THEIR CO2 COMPENSATION POINT

Photosynthesis and nitrogenase acetylene-reducing activity (ARA) were measured in soybeans (Glycine max [L.] Merr.) in which the shoots were exposed for 48 h to 60 mu L L(-1) CO2, a value corresponding to their CO2 compensation point. Six hours after the beginning of the light pe riod at low CO2, the ARA started to decrease, reaching a rate of 50% o f the control rate in 14 to 24 h and 20% of the control rate in 34 to 38 h after the beginning of the CO2 treatment. At these times, there w as no net photosynthesis, and the transpiration rate was 20% lower tha n that in the control plants. An increase in the partial pressure of O -2 around the nodules alleviated this inhibition of ARA. The maximal A RA achieved at 40 kPa O-2 was 3 times higher than that at 20 kPa O-2 a nd similar to the maximal ARA of the control plants. It was argued tha t the decrease in ARA of soybean exposed to the CO2 compensation point was due to a decrease in the nodule's permeability to O-2 diffusion.