Potassium and Calcium Isotopic Fractionation by Plants (Soybean [Glycine max], Rice [Oryza sativa], and Wheat [Triticum aestivum])

We conducted hydroponic experiments growing soybean ( Glycine max ), rice ( Oryza sativa ), and wheat ( Triticum aestivum ) under K and Ca replete conditions to establish the degree of K isotopic fractionation by plants, and compare the isotopic fractionation of Ca and K. Each of the test plants displays fractionation relative to the growth solution favoring the light isotopes of K and Ca. The average δ 41 K values of the roots from the three plant species were similar, and have an overall average of −0.55 ± 0.24‰ 2s, while the overall average δ 44 Ca for roots is −0.67 ± 0.44. For leaves, the overall average of δ 41 K is −0.97 ± 0.4‰, compared to an overall average leaf δ 44 Ca of −0.83 ± 0.09‰. In the case of the soybean plants, the lightest K and Ca occurs in the stems with average δ 41 K of −1.31 ± 0.40‰ 2s and average δ 44 Ca of −1.20 ± 0.19 ‰ 2s. We present a simple box model involving the relative fluxes of K and its isotopic fractionation that reproduces our K isotopic observations and suggests a fractionation of ∼0.8‰ with K uptake from solution by roots. Directly comparing the per amu fractionation of K and Ca reveals an average factor of 2.05 ± 0.50 2s greater fractionation of K isotopes which may reflect their different roles and behaviors in plants.