Novel ultrastructural observations of pea (Pisum sativum) root nodule cells by high-pressure freezing and propane-jet freezing techniques

Pea (Pisum sativum) root nodule cells infected by the diazotroph Rhizobium leguminosarum have been well characterized by chemical fixation techniques. Propane-jet freezing and high-pressure freezing were used in this study to compare rapidly frozen and chemically fixed pea root nodule cells. Cells t hat had been incubated in 2-(N-morpholino)ethanesulfonic acid buffer and fr ozen with the propane-jet freezer were better preserved than cells that had been chemically fixed or frozen with the high-pressure freezer. Rapidly fr ozen infected nodule cells showed that the rough endoplasmic reticulum had a high frequency of associations with the peribacteroid membrane and the in fection thread. The peribacteroid space also varied in size depending on th e method of preservation; however, it was most reduced in size and devoid o f inclusions in the propane-jet frozen tissue. The biological significance of these observations is discussed.