Assessing the roles of roots and shoots in the accumulation of cadmium in two sweet potato cultivars using split-root and reciprocal grafting systems

Abstract

The root cadmium (Cd) concentrations of sweet potato (Ipomoea batatas [L.] Lam.) cultivars vary greatly. In this study, we explored the role of shoots and roots in Cd accumulation as well as the related underlying physiological mechanisms using the previously identified high-Cd cultivar X16 and low-Cd cultivar N88. We used the split-root technique and reciprocal grafting to determine the role of shoots and feeder roots in edible root Cd concentration. Two hydroponic experiments—one time-dependent and one dose-dependent—were also carried out to investigate the differences in the root-to-shoot Cd translocation between the two cultivars. Furthermore, we determined the influence of transpiration rate on Cd accumulation in the shoots and roots of the two cultivars using a hydroponic system. The Cd accumulation in the shoots and edible roots of split-root sweet potatoes increased with the soil Cd concentration on either side; however, the feeder root Cd concentration was only related to the soil Cd level on the growth side, but not to that on the other side. The Cd concentrations in all the tissues of the grafts with X16 scion and N88 rootstock were higher than those of the non-grafted N88. Yet, the Cd concentrations in all the tissues of the grafts with N88 scion and X16 rootstock were lower than those in the non-grafted X16. Furthermore, X16 had a higher root-to-shoot Cd translocation and transpiration rate, which affected Cd translocation but not Cd uptake by the root. Cd was accumulated in the edible roots of sweet potato from both shoots and feeder roots. X16 had a higher ability of Cd translocation from the roots to the shoots via the xylem than N88. Therefore, the inter-cultivar variations in edible root Cd accumulation in sweet potato are related not only to Cd translocation from feeder to edible roots, but also to translocation from feeder roots to shoots.