Laser ablation ICP-MS reveals patterns of copper differing from zinc in growth zones of cucumber roots

Laser ablation coupled with inductively coupled plasma-mass spectrometry was used to find Cu and Zn concentration in surface tissue along a longitudinal developmental gradient with meristem, rapidly elongating tissue, and nongrowing tissue in a model system of seedling roots of Cucumis sativus L. (cucumber). Tissue metal accumulation was determined for roots of seedlings growing on cellulosic germination paper treated with nutrient solution (controls), and also treated with concentrations of Zn (40 ppm) and Cu (10 ppm) that reduced growth. Cu content of all roots is highest at the apex and falls sharply to lower values by 2 mm from the root tip. In contrast, at moderate Zn availability (0.07 ppm), Zn content rises from the apex to 2 mm then falls throughout the remainder of the growth zone. At high external Zn the spatial pattern resembles that of Cu. Cucumber root growth zones accumulate more of each metal with higher external availability. Metal deposition rates were calculated using a continuity equation with data on local metal content and growth velocity. Deposition rates of both metals are generally highest in the rapidly elongating region, 1.5-3.5 mm, even where metal concentration is decreasing with position and root age and even when the accumulation is inhibitory to growth.