Evidence of cadmium transport via the phloem in cacao seedlings

Background and aims It is unclear if cadmium (Cd) is loaded into cacao beans directly from the roots and stem or rather via reallocation from leaves and stem via the phloem. Here, a split-root experiment with a stable isotope 108 Cd tracer was set up to determine the short-term circulation in the vegetative tissues of Cd in Theobroma cacao L. seedlings. Methods Roots of cacao seedlings were split into two parts, each growing in separate containers. The compartments contained an equal volume of nutrient solution with a low-dose Cd concentration of 20 nM, and were either (+) or not (-) enriched in stable 108 Cd. Seedlings were grown for two weeks and periodically harvested. Results On day 1 after labelling, there was a rise in the 108 Cd isotopic abundance (IA) in the roots in the labelled compartment whereas the 108 Cd IA was unaffected in other plant parts. After 7 days, about 75% of the tracer 108 Cd in the plant had been translocated to the shoots and the largest fraction of tracer was recovered in the stem. The 108 Cd IA in the roots in the non-labelled compartment rose after day 11, demonstrating that Cd was phloem mobile and reallocated to developing tissues. The isotopic abundance of 108 Cd in developing leaves was similar to that in the older leaves, suggesting that Cd originated from leaves rather than directly from roots. Conclusions We corroborated Cd pathways previously studied in mature cacao trees and demonstrated that Cd is reallocated to sink organs via the phloem.