A freeze-fracture study of mesophyll cell membranes in the embryonic Phaseolus leaf, in the dry seed and during the initial stages of germination.
Using rapid-freezing and freeze-fracturing techniques, we have examined cellular organisation within mesophyll cells in primary leaves of Phaseolus, in the dry seed and over a period of 48 hr from the onset of germination. The rapidly-frozen, unetched image reveals a dynamic cytoplasm with membrane shape and role able to change and fluctuate with the increase in cell metabolism. The ER in this tissue is highly mobile, becoming progressively associated with protein and lipid bodies, then plastids, mitochondria and nuclei as cytoplasmic requirements change. The ER thus provides an efficient transport of substrates to metabolic sites within the cytoplasm. Nuclei and plastics change shape in early germination and the protein body membrane changes in role and character to that of a vacuolar tonoplast. The freeze-fractured image emphasizes the considerable movement that occurs within the cytoplasm during germination, with flexible and mobile membranes of the various organelles able to associate and interact with others. We have also measured the density of intramembrane particles on most of the membranes within the mesophyll parenchyma cell, during this period of early germination. Using our method of rapid-freezing, we find consistently higher IMP density on the protoplasmic face of each membrane. The counts provide a set of comparative values for membranes of a specific tissue, and for a specific cryofixation technique.
McLean
B
B
Department of Plant Sciences, University of Oxford, Oxford, UK.
Humphries
J
J
Juniper
B E
BE