Investigation of phloem transport of foliar-absorbed Cd in wheat

The transport pathway of foliar absorbed Cd in wheat has not been clarified. Hydroponically raised wheat seedling stems were steam girdled to disrupt phloem transport, then the leaves were exposed to Cd solutions (40 or 60 mg L⁻¹) containing 20 mg L⁻¹ Rb (a phloem transport indicator) for 16 h. Plants were harvested 7 days later and divided into roots and leaves, while the stem was divided into two parts above and below the girdling location, with phloem sap collected from the upper part. Samples were analyzed for Cd, Rb, Cu, Zn and Mn concentrations; root glutathione concentration was also analyzed to investigate Cd transport and its effect on other metals. Girdling increased leaf Cd concentrations by >2 folds, reduced root Cd and Rb concentrations by ∼50 %, reduced root glutathione concentration by 21.4 %, and phloem sap Rb concentrations of the girdled plants were 30.8–63.1 % greater than the ungirdled treatments. Root to leaf Cd concentration ratios were in the range of 0.064–0.443, and girdled treatments had lower values than the ungirdled treatments. Negative correlations were found between root Cd and leaf Mn and Cu concentrations in the ungirdled treatments, but not in the girdled treatments. These results indicate that phloem transport plays an important role in the transport of leaf-absorbed Cd in wheat, with leaf absorbed Cd having high mobility. These findings are important for discerning Cd uptake and transport in plants found living in Cd-contaminated conditions.