REGULATION OF NITROGEN-FIXATION IN INFECTED-CELLS OF LEGUMINOUS ROOT-NODULES IN RELATION TO O-2 SUPPLY

Respiration and nitrogen fixation in legume root nodules is considered to be limited by the rate at which O-2 from the atmosphere can enter nodules. A thin diffusion barrier in the inner cortex, restricts acces s to the central tissue where there is a high demand for and low conce ntration of O-2. Observed variations in rates of nodule activities in response to imposed stresses, are often attributed to variations in th e diffusion resistance of the barrier. In the present work, alternativ e or supplementary metabolic mechanisms are considered. Aspects of nod ule structure and of metabolism underlying nodule activities are revie wed in terms of components of the symbiotic system, the nature of stea dy states and in relation to homeostasis of low concentration of O-2 w ithin the bacteroid-filled host cells. It is suggested that variations in O-2-demand of both mitochondria and bacteroids, serve to preserve nitrogenase activity by poising O-2 concentration within 'safe' limits . Further, data from isolated soybean bacteroids suggest that nitrogen ase is converted to a less active but more robust form, in the presenc e of O-2 in excess of about 70 nM, thus protecting nitrogenase from ir reversible inactivation by excess O-2. This regulation is rapidly-reve rsible when O-2 concentration falls below about 0.1 mu M. Respiration by large numbers of host mitochondria in the periphery of infected nod ule cells, adjacent to gas-filled intercellular spaces, is considered to play an important part in maintaining a steep gradient of O-2 conce ntration in this zone. Also, it is possible that variations in nodule O-2 demand may be involved in the apparent variations in resistance of the diffusion barrier. It is concluded that there are many biochemica l components which should be considered, along with possible changes t o the diffusion barrier, when the effects of imposed stresses on nodul e activities are being analysed.