The distribution of 67Zn and 207Pb applied to white spruce foliage at ambient concentrations under different pH regimes

Abstract

A stable isotope tracer technique was used to evaluate the translocation of trace metals applied to white spruce ( Picea glauca) foliage at ambient concentrations under different pH regimes. Simulated rainfall (0.1 ml) containing 67Zn (102 ng ml −1) or 207Pb (184 ng ml −1) was applied to needles, at the mid-point of 1-year-old shoots, twice daily for a 30-day period, in rainfall adjusted to pH 4.0 or 6.0. One week after the final application, the distribution of the excess 67Zn and 207Pb within the 3-year-old seedlings was assessed. All of the applied 67Zn was found in the application segment, in both pH treatments. The majority (∼99%) of the excess 207Pb in white spruce remained in the application segment, although translocation of Pb away from the application zone occurred, and was significantly enhanced when 207Pb was applied in rainfall at pH 4.0 compared with pH 6.0. Most of the translocated 207Pb moved towards the shoot tip, but the amount of excess 207Pb in plant tissues decreased sharply with increasing distance from the application area. Even though metal application rates were in the upper range of ambient levels recorded in rain or fog, very small increases in Pb concentration were found in any plant section away from the application zone, suggesting foliar uptake is not a major pathway of Zn and Pb accumulation in white spruce.