Ultrastructural Changes of Tobacco Cell Walls Following Sequential Extraction

The role of calcium in plants has been the subject of research for many years. Calcium has been postulated to cover a wide variety of functions which have major and minor influences on the plant's metabolism. Calcium interaction with pectin has been postulated as a major source of cell wall stability, however, no direct measurements of this interaction have been made. In this study, a sequential extraction method has been utilized to fractionate the various forms of calcium present in cured bright and Burley tobacco. The extraction method uses water, potassium chloride, lanthanum chloride and hydrochloric acid with emphasis on the lanthanum chloride extraction which appears to preferentially replace the structural calcium. Extraction data in conjunction with light microscopy (LM) and transmission electron microscopy (TEM) data have been used to predict the role of structural calcium in the cell wall. Oxalate and calcium analyses have been made at each of the extraction steps for bright tobacco. It is shown that the major portion of calcium extracted is not associated with oxalate except for the hydrochloric acid step. The data show that approximately 20 % of the calcium is structurally related and that calcium oxalate utilizes a maximum of another 20 % of the total calcium. The remaining 60 % is non-structural and non-oxalate and is probably inorganic salts and salts of organic acids. Combining these two techniques provides a means of obtaining quantitative information that either technique used independently could not provide. The techniques used in this study are applicable to the investigation of other plant types and may be useful in furthering the general knowledge of the role of calcium in plant materials.