The wall of the infected cell may be the major diffusion barrier in nodules of Myrica galE L

Abstract

Nitrogenase activity (C 2H 2 reduction) by single- or two-lobed nodules of Myrica gale L. with long nodule roots was measured at 6 and 8 kPa O 2 in a flow-through system after various treatments. Neither excision of nodule roots nor covering the cut surface with silicone grease after nodule root removal had a significant effect on C 2H 2 reduction, while covering the entire nodule with grease eliminated C 2H 2 reduction. Likewise, covering the nodule lobe while leaving nodule roots uncovered had little effect on C 2H 2 reduction. These results suggest that the diffusion resistance to gases is low at the surface of the nodule lobe and the nodule roots. Nodule slices ( ca 100 μm thick) retained high C 2H 2 reduction rates which were O 2-limited up to 18 kPa, indicating that the diffusion barrier is within the slices. The O 2 gradient from the gas phase to the infected cells in the slices was calculated to be ca 0.65 kPa out of a total gradient of 20 kPa. The addition of 2 kPa CO had little effect on nodule respiration (CO 2 evolution), suggesting that the diffusion barrier is outside the cytoplasm of the infected cells. Thus the major diffusion barrier may be in the wall of the infected cells.