CURRENT NITROGEN-FIXATION IS INVOLVED IN THE REGULATION OF NITROGENASE ACTIVITY IN WHITE CLOVER (TRIFOLIUM-REPENS L)

Previous studies have shown that nitrogenase activity decreases dramat ically after defoliation, presumably because of an increase in the O-2 diffusion resistance in the infected nodules. It is not known how thi s O-2 diffusion resistance is regulated. The aim of this study was to test the hypothesis that current N-2 fixation (ongoing flux of N-2 thr ough nitrogenase) is involved in the regulation of nitrogenase activit y in white clover (Trifolium repens L. cv Ladino) nodules. We compared the nitrogenase activity of plants that were prevented from fixing N- 2 (by continuous exposure of their nodulated root system to an Ar:O-2 [80:20] atmosphere) with that of plants allowed to fix N-2 (those expo sed to N-2:O-2, 80:20). Nitrogenase activity was determined as the amo unt of H-2 evolved under Ar:O-2. An open flow system was used. In expe riment I, 6 h after complete defoliation and the continuous prevention of N-2 fixation, nitrogenase activity was higher by a factor of 2 com pared with that in plants allowed to fix N-2 after leaf removal. This higher nitrogenase activity was associated with a lower O-2 limitation (measured as the partial pressure of O-2 required for highest nitroge nase activity). In experiment II, the nitrogenase activity of plants p revented from fixing N-2 for 2 h before leaf removal showed no respons e to defoliation. The extent to which nitrogenase activity responded t o defoliation was different in plants allowed to fix N-2 and those tha t were prevented from doing so in both experiments. This leads to the conclusion that current N-2 fixation is directly involved in the regul ation of nitrogenase activity. It is suggested that an N feedback mech anism triggers such a response as a result of the loss of the plant's N sink strength after defoliation. This concept offers an alternative to other hypotheses (e.g. interruption of current photosynthesis, carb ohydrate deprivation) that have been proposed to explain the immediate decrease in nitrogenase activity after defoliation.