Endodermal Ca and Sr partitioning in needles of the European larch ( Larix decidua (L.) Mill.)

Abstract

Summary The aim of this study was to find out whether the needle endodermis of young larch trees contributes to the regulation of needle Ca ++ content as well as the retention of toxic minerals such as Sr ++ from the surrounding mesophyll. For this, plants were hydroponically grown under controlled conditions in nutrient solutions without (‹control›) and with Sr ++ (Ca ++ /Sr ++ ratio: 1; ‹mixed-treatment›). EDX-analyses of deep frozen needle segments revealed that in control plants the endodermal Ca-concentrations were much higher than in the vacuoles of mesophyll cells but they coincided with the mean Ca-concentration found in the whole needle. The discrepancy between endodermal and mesophyll Ca-concentrations can be explained by the presence of intracellular Ca-oxalate crystals localized in the parenchyma tissue of the vascular bundle. When Ca ++ and Sr ++ were supplied in equal concentrations in the nutrient solution, Ca ++ translocation into the needles occurred in preference to Sr ++ . In contrast, more Sr ++ than Ca ++ was accumulated in the vacuoles of the endodermis cells. Furthermore, endodermal Sr ++ concentrations were much higher than in the mesophyll. More than 19 % of the total needle Sr ++ was stored in the endodermis, which occupies only 3–4% of the needle volume. A function for retaining harmful substances in the endodermal vacuoles is discussed.