Isotopic discrimination of zinc in higher plants

The extent of isotopic discrimination of transition metals in biological processes is poorly understood but potentially has important applications in plant and biogeochemical studies. Using multicollector inductively coupled plasma (ICP) mass spectrometry, we measured isotopic fractionation of zinc (Zn) during uptake from nutrient solutions by rice (Oryza sativa), lettuce (Lactuca sativa) and tomato (Lycopersicon esculentum) plants. For all three species, the roots showed a similar extent of heavy Zn enrichment relative to the nutrient solution, probably reflecting preferential adsorption on external root surfaces. By contrast, a plant-species specific enrichment of the light Zn isotope occurred in the shoots, indicative of a biological, membrane-transport controlled uptake into plant cells. The extent of the fractionation in the shoots further depended on the Zn speciation in the nutrient solution. The observed isotopic depletion in heavy Zn from root to shoot (-0.13 to -0.26parts per thousand. per atomic mass unit) is equivalent to roughly a quarter of the total reported terrestrial variability of Zn isotopic compositions (c. 0.84parts per thousand. per atomic mass unit). Plant uptake therefore represents an important source of isotopic variation in biogeochemical cycling of Zn.