Electron microscopic investigation of water occlusions in intercellular spaces in the inner cortex of lucerne nodules

It is unclear to what extent oxygen diffusion pathways through the cortex o f the nitrogen-fixing zone of indeterminate nodules are liquid filled and w hether a blockage of these pathways is involved in varying nodule oxygen pe rmeability to control nitrogenase activity. We examined the proportion of w ater-filled intercellular spaces of lucerne (Medicago sativa L.) nodules wi th cryo-scanning electron microscopy. This technique allows for direct obse rvation of water accumulation. Thirty percent of all intercellular spaces i n the inner cortex of lucerne nodules were liquid filled. Decreasing the no dule oxygen permeability by detopping of the plant or by increasing the rhi zospheric oxygen partial pressure to 80 kPa had no statistically significan t effect on the water distribution in the intercellular spaces. Therefore, the hypothesis of a continuous aqueous diffusion barrier in the inner corte x could not be supported. The abundance of glycoproteins in intercellular s paces of the inner cortex was investigated with immunoelectron microscopy. No alteration due to detopping or after increase of the rhizospheric oxygen partial pressure was observed. Therefore, our results do not support the h ypothesis of a short-term regulation of oxygen permeability by blockage of diffusion pathways through morphological changes in the cortex region of th e nitrogen-fixing zone of lucerne nodules. (C) 1999 Academic Press.