CONTINUOUS EXPOSURE OF NODULATED-ROOTS OF SOYBEAN TO ACETYLENE INHIBITS IN VIVO NITROGENASE ACTIVITY (C2H2 REDUCTION) AND PHOTOSYNTHESIS

摘要

In order to investigate the effects of photosynthesis on nitrogen firing nodules, gas exchanges of shoot and nodulated-root were measured under continuous exposure of the modulated root to 10 % C2H2 during 24 h. This exposure to C2H2 induced inhibitions of photosynthesis (P) and nodule C2H2 reduction activity (ARA), the latter differing from the well-known short-term C2H2-induced decline of nitrogenase by a longer lag before ARA decreased. Photosynthesis, transpiration and ARA were inhibited by 50, 25 and 20 %, respectively, 2, 4 and 6 hours after the beginning of the exposure to C2H2. The inhibition of P was also observed with 5 and 2 % C2H2, and with 50 mu L L(-1) C2H4, but in these cases there was no subsequent inhibition of nodule ARA. On the other hand, inhibition of P by C2H2 was also observed with non-nodulated plants. These results suggest that C2H2 may have a C2H4-hormone-like action, although with a 2x10(3) times lower efficiency than C2H4, and that the inhibition of ARA was not a direct consequence of the inhibition of P. It is suggested that the ARA assay under C2H2 concentrations as low as 5 % may be one way to avoid the above described inhibition of in vivo nitrogenase activity.