A freeze-fracture electron microscopic study of Frankia in root nodules of Alnus incana grown at three oxygen tensions

ABEYSEKERA, R. M., NEWCOMB, W . , SILVESTER, W. B., and TORREY, J. G. 1990. A freeze-fracture electron microscopic study of Frankia in root nodules of Alnus incana grown at three oxygen tensions. Can. J. Microbiol. 36: 97-108. Nodulated plants of Alnus incana ssp. rugosa and ssp. incana were grown with the roots exposed to 5,21, and 40 kPa 02. The nodules were studied by freeze-fracture transmission electron microscopy to determine the effect of varying O2 tension on the numbers of lipid laminae in the Frankia envelope. Lipid laminae were present in the cell envelopes of hyphae, stalks, and symbiotic vesicles. The mean number of lipid laminae in hyphal envelopes varied from five to nine. Stalks of symbiotic vesicles contained mean numbers of 35-59 lipid laminae over the range of p02's studied. Symbiotic vesicle envelopes showed mean numbers of lipid laminae varying from 48 to 94. The numbers of lipid laminae wcreobserved to increase significantly in the distal regions of the symbiotic vesicles in response to raised p02 while the numbers on the proximal portions remained unchanged. The increase in the numbers of lipid laminae in response to raised pOz was not sufficient to account for the expected increase in resistance to O2 required at the symbiotic vesicle envelope if lipid laminae formed the exclusive diffusion barrier to 0 2 . These results suggest that lipid laminae surrounding symbiotic vesicles may not constitute the only O2 protection mechanism in Alnus nodules.