Determining whether isotope fractionation occurs during root water uptake is a prerequisite for using stem or xylem water isotopes to trace water sources. However, it is unclear whether isotope fractionation occurs during root water uptake in gramineous crops. We conducted prevalidation experiments to estimate the isotope measurement bias associated with cryogenic vacuum distillation (CVD). Next, we assessed isotope fractionation during root water uptake in two common agronomic crops, wheat (Triticum aestivum L.) and maize (Zea mays L.), under flooding after postdrought stress conditions. Cryogenic vacuum distillation caused significant depletion of 2 H but negligible effects on 18 O for both soil and stem water. Surprisingly CVD caused depletion of 2 H and enrichment of 18 O in root water. Stem and root water δ18 O were more than soil water δ18 O, even considering the uncertainty of CVD. Soil water 18 O was depleted compared with irrigation water 18 O in the pots with plants but enriched relative to irrigation water 18 O in the pots without plants. These results indicate that isotope fractionation occurred during wheat and maize root water uptake after full irrigation and led to a heavy isotope enrichment in stem water. Therefore, the xylem/stem water isotope approach widely used to trace water sources should be carefully evaluated.
Keywords: agronomic crop; cryogenic vacuum extraction; isotope fractionation; maize (Zea mays L.); root water uptake; water conditions; wheat (Triticum aestivum L.).
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