Magnesium-isotope Fractionation During Plant Growth

Magnesium is an essential nutrient, which activates more enzymes than any other mineral element and, thus, plays an important role in biogeochemical cycles. With three stable isotopes naturally abundant (Mg-24, 78.992%; Mg-25, 10.003%; Mg-26, 11.005%), magnesium stable isotope fractionation may provide insights into these cycles. Here, we detail for the first time the magnesium stable-isotope distribution in a higher plant, wheat (Triticum aestivum L.), during its growth cycle. Wheat plants were grown in a limiting nutrient supply hydroponically, some being left to mature through senescence and others detopped at maturity for collection of exudates. Measurements of the magnesium isotopic composition of chlorophylls, seeds, shoots, roots, leaves, exudates, and the limiting nutrient solution over time show that the plant appears to establish an isotopic equilibrium with the nutrient available to it and that the plant (in particular, the seeds and exudates) becomes enriched in the heavy isotopes of magnesium in a mass-dependent relationship as the plant reaches maturity. The preference of the plants for heavy magnesium isotopes suggests that a difference might exist in the bioavailable magnesium of agricultural and natural soils due to the periodic removal of heavy magnesium isotopes by harvest.