Oxygen Supply Limits Nitrogenase Activity of Clover Nodules After Defoliation

The aim of this study was to test the effect of oxygen partial pressure as a possible limiting factor of nitrogen fixation following defoliation. The response of nitrogenase activity (C2H2-reduction) of defoliated and undefoliated white and red clover plants (Trifolium repens L. and Trifolium pratense L.) to either 19 kPa oxygen or 55 kPa oxygen was investigated. Prior to defoliation, white clover plants were grown for five weeks in a growth chamber, and red clover plants for 7 or 11 weeks in a glasshouse. The results included measurements of 16N2-uptake. Increasing oxygen partial pressure from 19 to 55 kPa severely restricted nitrogenase activity of undefoliated white clover plants; however, 2 h after complete defoliation, the same treatment caused a significant increase. A fivefold increase in nitrogenase activity upon exposure to the elevated oxygen partial pressure was found at the end of a 24 h period. This beneficial effect decreased gradually from 1 to 5 d after defoliation. The response of recently defoliated red clover plants to 55 kPa oxygen partial pressure was similar to that of white clover, independently of plant age. The gradual recovery of nitrogenase activity during three weeks of regrowth was associated with a simultaneous change in the response to increased oxygen partial pressure, leading again to the response of undefoliated plants. These data suggested that lack of oxygen at the site of nitrogen fixation, resulting from a dramatic increase in oxygen-diffusion resistance, is the main factor limiting nitrogenase activity following defoliation.