Comparison of cadmium pathways in a high Cd accumulating cultivar versus a low Cd accumulating cultivar of Theobroma cacao L.

Abstract

Understanding cadmium (Cd) pathways in cacao trees is critical for developing Cd mitigation strategies. This study investigates whether Cd uptake and translocation mechanisms differ between a low and a high Cd-accumulating cacao cultivar. We sampled three replicate trees of each cultivar, and a grafted cultivar that shared the same scion as the low Cd accumulator but had a different rootstock. All cultivars grew in the same field with similar bioavailable soil Cd. We utilized Cd stable isotope analyses to trace Cd pathways within the trees, complemented by micro-scale imaging of Cd distribution in leaves and branches, and nutrient and Cd quantification across tree organs. The high Cd accumulator exhibited 2.9-fold higher Cd uptake than the low Cd accumulator, while the grafted cultivar showed 1.7-fold higher uptake. These differences matched Mn uptake. The δ^114/110Cd values of organs increased in the order: roots ≤ nibs, young leaves ≤ branches ≤ pod husks < mature leaves for the high and grafted cultivar, and nibs ≤ roots ≤ branches ≤ pod husks < mature leaves for the low cultivar. The enrichment in heavy isotopes correlated with a progressive retention of Cd compared to Mn along the xylem pathway from roots to branches to leaves. The differences in Cd isotope compositions between cultivars indicate that there are differences in translocation processes, yet they did not affect the relative Cd internal distribution. Cd mass balances and internal translocation factors (ITF) corroborated that differences in nib Cd concentrations among cultivars were primarily due to uptake rather than translocation.