ADAPTATION OF NODULATED SOYBEAN (GLYCINE-MAX L MERR) TO GROWTH IN RHIZOSPHERES CONTAINING NONAMBIENT PO2

Nodulated soybean (Glycine max L. Merr. cv White Eye inoculated with Bradyrhizobium japonicum strain CB 1809) plants were cultured in the absence of combined N from 8 to 28 days with their root systems maintained continuously in 1, 2.5, 5, 10, 20, 40, 60, or 80% O2 (volume/volume) in N2. Plant dry matter yield was unaffected by partial pressure of oxygen (pO2) and N2 fixation showed a broad plateau of maximum activity from 2.5 to 40 or 60% O2. Slight inhibition of nitrogenase activity occurred at 1% O2 and as much as 50% inhibition occurred at 80% O2. Low pO2 (less than 10%) decreased nodule mass on plants, but this was compensated for by those nodules having higher specific nitrogenase activities. Synthesis and export of ureides in xylem was maintained at a high level (70-95% of total soluble N in exudate) over the range of pO2 used. Measurements of nitrogenase (EC 1.7.99.2) activity by acetylene reduction indicated that adaptation of nodules to low pO2 was largely due to changes in ventilation characteristics and involved increased permeability to gases in those grown in subambient pO2 and decreased permeability in those from plants cultured with their roots in pO2 greater than ambient. A range of structural alterations in nodules resulting froM low pO2 were identified. These included increased frequency of lenticels, decreased nodule size, increased volume of cortex relative to the infected central tissue of the nodule, as well as changes in the size and frequency of extracellular voids in all tissues. In nodules grown in air, the inner cortex differentiated a layer of four or five cells which formed a band, 40 to 50 micrometers thick, lacking extracellular voids. This was reduced in nodules grown in low pO2 Comprising one or two cell layers and being 10 to 20 micrometers thick in those from 1% O2. Long-term adaptation to different external pO2 involved changes which modify diffusive resistance and are additional to adjustments in the variable diffusion barrier.