Using laser ablation ICP-MS to quantify copper and zinc in root growth zones and their rhizospheres

Laser ablation coupled with inductively coupled plasma-mass spectrometry (LA-ICP-MS) can provide information on copper and zinc in plant tissue with a spatial resolution of 20 microns and detection on the scale of  parts per million (ppm).  Metal concentration was determined within meristems, elongating cells, and tissue at the base of the growth zone in a model system of seedling roots of cucumber (Cucumis sativus)  growing on a cellulosic germination paper. Tissue accumulation was determined for samples grown on paper treated with  Hoaglands nutrient solution, and also treated with concentrations of Zn and Cu that reduced growth.  Laser scans were made from the root surface for 2 mm through the model rhizosphere. Tissue content (and therefore deposition rate) of both metals is generally highest in the elongating region Cucumber root accumulates both metals in proportion to external availability; zinc competes with copper for uptake.  Preliminary results suggest that the newly differentiated tissue supplies metal to the elongating cells; copper is taken up locally in the growth zone so that the rhizosphere is depleted in copper for perhaps 600 microns around the moving elongation zone.  Zinc is excreted from the root growth zones for high external concentrations, which makes zinc deposition near the root surface. A problem for quantitative analysis is a strong dependence of the apparent metal concentration on the moisture content of the substrate.