A NEW METHOD TO DETECT THE PRESENCE OF CONTINUOUS GAS-FILLED PATHWAYSFOR OXYGEN DIFFUSION IN LEGUME NODULES

A new method is presented which evaluates the contribution of gas-fill ed and water-filled pathways across the barrier which restricts O-2 di ffusion into infected cells of nodules. Since O-2 will move through ai r about 10(4) times faster than through water any continuous gas-fille d pathways which traverse the cortex would form a major route for O-2 transport. However, microscopic evidence for the existence of such dir ect connections is ambiguous. On theoretical grounds, O-2 should diffu se through a He atmosphere about 3.7 times faster than through air or Ar, but O-2 flux across a liquid barrier should be unaffected by chang es in the background mixing gas. Thus, if O-2 influx to the infected c ells is increased when the ambient gas phase is changed from air to He /21% O-2 this is evidence for a continuous pathway of gas-filled pores across the whole width of the cortical barrier. This theoretical appr oach was validated by measuring the rates of diffusion of O-2 through millipore filters in background atmospheres of either air, Ar or He. T hese membranes were used dry, to simulate the 'open pore' model for no dule diffusion, or wetted with water or gum Arabic (which is similar t o the glycoproteins associated with the barrier) to simulate the 'clos ed pore' situation. Knowing the diffusion constants for the gas- and w ater-filled pathways that may be involved in the nodule barrier, we th en evaluated the contribution of these components to the total diffusi on resistance by examining the effect of Ar/O-2 and He/O-2 gas mixture s on H-2 production and respiration of nodules. The results indicate t hat, in unstressed soyabean nodules, about half of the O-2 flux to inf ected cells is via inter-connected gas-filled pores, which 'close' to produce a liquid-filled barrier as the diffusion resistance increases in response to stress. In unstressed lupin nodules all of the O-2 flux crosses a liquid-filled barrier.