Quantitative X-ray microanalysis of solutes in individual plant cells: a comparison of microdroplet and in situ frozen-hydrated data.

Two different approaches to X-ray microanalysis were tested and compared. These were the analysis of sap droplets extracted from individual cells (plants grown and analysed in Bangor, U.K.), and the analysis of cells in situ in frozen tissue (plants grown and analysed in Hannover, Germany). The data suggest that both these methods can produce quantitative data accurately reflecting in vivo concentrations in cereal leaf epidermal cells. The relative merits of the two procedures are discussed with reference to possible sources of error and their application to other cell types. Bulk wheat leaf tissue concentrations of K and Cl did not differ significantly between the two locations, but Ca concentration was significantly higher in the plants grown in Hannover. Microdroplet analysis invariably yielded linear responses in the range of concentrations found in plant tissue (r2 for Ca > 0.97, r2 for K, Cl > 0.99), and interference of other components of the sap was minimal. The calibration curves for the frozen-hydrated material were typically linear in the same range of concentrations (r2 for K, Ca, Cl > 0.95), and the results for K and Cl concentration in these samples were highly consistent with those measured in the microdroplet experiments. In wheat, for example, the cellular Cl concentration varied between 12 mM and 119 mM, but no significant differences were found between the two techniques of measurement. The results for cellular Ca differed in a manner which might be predicted from the results of the bulk tissue analyses.