ZINC TOLERANCE AND THE BINDING OF ZINC AS ZINC PHYTATE IN LEMNA-MINOR - X-RAY MICROANALYTICAL EVIDENCE

Electron probe X-ray microanalysis of fracture faces of quench frozen hydrated bulk samples of Lemna minor fronds exposed to high levels of Zn (300-mu-M) has shown the presence of globular deposits up to 1-mu-m in diameter containing Zn, K and P in parenchyma cells of mature fronds, and Zn, K, Mg and P in immature parenchyma cells of daughter fronds (or turions). Although this method of direct analysis of bulk samples does not lend itself to a quantitative comparison of number and composition of globules in Zn-tolerant and Zn-sensitive clones of Lemna minor, it has the advantages of simplicity of specimen preparation, rapid detection of Zn-P-containing globules, and avoidance of diffusional artifacts. Comparisons with prepared K Mg phytate and Zn standards, and additional data obtained by X-ray microanalysis of freeze-substituted thin sections, established the elemental proportions of Zn, Mg, K and Ca relative to P, which indicate that generally up to 8 (molar ratio of Zn to phytic acid = 4) and sometimes up to 12 (molar ratio of Zn to phytic acid = 6) valencies can be occupied by Zn and up to 4 by K and/or Mg. Some of the Ca that appears to be associated with globules may result from elemental redistribution during the process of freeze-substitution especially when Ca oxalate crystals are present in neighbouring raphide cells. X-ray microanalysis provided evidence for the absence of Zn from these raphides.