| Plant calcium oxalate crystals occur within cells called crystal idioblasts. Important aspects of this calcification phenomenon have not been characterized. This dissertation examines some of the aspects of this ubiquitous type of calcification including (1) characterization of ultrastructural features of developing crystal idioblasts, (2) determination of the relationship of specialized ultrastructural features of the idioblasts to transport of compounds and mechanisms of crystal deposition, and (3) the biochemical relationship between ascorbic acid metabolism and production of oxalic acid used for crystal formation.; Structural and cytochemical studies revealed that crystal idioblasts have dense cytoplasm, modified plastids, enlarged nuclei, extensive endoplasmic reticulum, numerous dictyosomes and vesicles, and a bundle of raphide crystals in their vacuoles. Small vesicles and a flocculent material are abundant in the vacuole. The crystals are formed in membranous chambers which are connected together by unit membranes in ladder-like chains. The connecting membranes are formed first from flocculent material and the crystal chambers form subsequently between them. Deposits of phosphatase reaction products were prominent in the vesicles in both vacuole and cytoplasm, along the membranes connecting crystal chambers and in the chamber membranes. Ca was most abundant in endoplasmic reticulum, dictyosome-derived vesicles, crystal chamber membranes and the flocculent material within the vacuole. A mechanism for Ca transport and crystal precipitation is proposed, based on these results.; There is a strong and dynamic relationship between Ca concentration and oxalic acid produced for crystal formation, where increasing Ca level in the growth medium lead to increased total and insoluble oxalate in the plant. Calmodulin antagonists reduced oxalic acid production. Compared to glycolic acid, ascorbic acid was found to be major precursor for oxalic acid production in Lemna minor. The glycolate oxidase inhibitor, HPMS, partially reduced oxalic acid production. Addition of unlabeled glycolic or glyoxylic acids slightly reduced the percent of label converted to oxalic acid from (1-{dollar}sp{lcub}14{rcub}{dollar}C) ascorbic acid, while addition of unlabelled ascorbic acid, galactonolactone or erythorbic acid had no significant effect on label conversion from (1-{dollar}sp{lcub}14{rcub}{dollar}C) glycolic acid. The possible routes from ascorbic acid to oxalic acid are discussed based on these results. |
