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 delta K-41 values of the roots from the three plant species were similar, and have an overall average of -0.55 +/- 0.24 parts per thousand 2s, while the overall average (delta Ca-44 for roots is -0.67 +/- 0.44. For leaves, the overall average of delta K-41 is -0.97 +/- 0.4 parts per thousand, compared to an overall average leaf delta Ca-44 of -0.83 +/- 0.09 parts per thousand. In the case of the soybean plants, the lightest K and Ca occurs in the stems with average (delta K-41 of -1.31 +/- 0.40 parts per thousand 2s and average delta Ca-44 of -1.20 +/- 0.19 parts per thousand 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 similar to 0.8 parts per thousand 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.