From soil to cacao bean: Unravelling the pathways of cadmium translocation in a high Cd accumulating cultivar of Theobroma cacao L

Abstract

The research on strategies to reduce cadmium (Cd) accumulation in cacao beans is currently limited by a lack of understanding of the Cd transfer pathways within the cacao tree. Here, we elucidated the transfer of Cd from soil to the nib (seed) in a high Cd accumulating cacao cultivar. Here, we elucidated the transfer of Cd from soil to the nib (seed) in a high Cd accumulating cacao cultivar through Cd stable isotope fractionation, speciation (X-Ray Absorption Spectroscopy), and localization (Laser Ablation Inductively Coupled Plasma Mass Spectrometry). The plant Cd concentrations were 10-28 higher than the topsoil Cd concentrations and increased as placenta< nib< testa< pod husk< root< leaf< branch. The retention of Cd in the roots was low. Light Cd isotopes were retained in the roots whilst heavier Cd isotopes were transported to the shoots (Δ 114/110 Cd shoot-root = 0.27 ± 0.02 ‰ (weighted average ± standard deviation)). Leaf Cd isotopes were heavier than Cd in the branches (Δ 114/110 Cd IF3 leaves-branch = 0.18 ± 0.01 ‰), confirming typical trends observed in annual crops. Nibs and branches were statistically not distinguishable (Δ 114/110 Cd nib-branch = −0.08‰ ± 0.06 ‰), contrary to the leaves and nibs (Δ 114/110 Cd nib-IF3 leaves = -0.25‰ ± 0.05 ‰). These isotope fractionation patterns alluded to a more direct transfer from branches to nibs rather than from leaves to nibs. The largest fraction (57%) of total plant Cd was present in the branches where it was primarily bound to carboxyl-ligands (60-100%) and mainly localized in the phloem rays and phelloderm of the bark. Cadmium in the nibs was mainly bound to oxygen ligands (60-90%), with phytate as the most plausible ligand. The weight of evidence suggested that Cd was transferred like other nutrients from root to shoot and accumulated in the phloem rays and phelloderm of the branches to reduce the transfer to foliage. Finally, the data indicated that the main contribution of nib Cd was from the phloem tissues of the branch rather than from leaf remobilization. This study extended the limited knowledge on Cd accumulation in perennial, woody crops and revealed that the Cd pathways in cacao are markedly different than in annual crops.

Extraction of heavy metals from copper tailings by ryegrass (Lolium perenne L.) with the assistance of degradable chelating agents

“…The content of soil organic matter (SOM) was determined by the potassium dichromate external heating method 35 . The redox potential (Eh) was assessed with FJA-6 ORP depolarization automatic analyzer 36 , while the cation exchange capacity (CEC) of soil was estimated by hexamine cobalt trichloride leaching-spectrophotometry 37 . The available Cu and Cd contents in soil were determined using DTPA-CaCl 2 -TEA system and atomic absorption spectrometry 38 .…”

Atomic spectrometry update: review of advances in the analysis of clinical and biological materials, foods and beverages

“…Cocoa is an economically important crop, but as a natural product can introduce a risk of exposing consumers to toxic metals, such as Cd. To understand the mechanisms of Cd uptake in cocoa plants Blommaert et al 227 utilised MC-ICP-MS to look at 114 Cd : 110 Cd ratio. Cadmium species in the cocoa plant materials were measured by XAS and LA-ICP-MS was used to map Cd locations in branches.…”

Coupling metal stable isotope compositions and X-ray absorption spectroscopy to study metal pathways in soil–plant systems: a mini review

“…for trees). 20 This isotope composition for the whole plant represents the δ-value of incorporated metals into the plant, assuming that no biomass and metals were lost from the plant. Depending on the definition of ‘whole plant’ and ‘whole root’, root external metal precipitates and sorbed species in the root apoplast may be included or excluded by using chemical extractions (see section ‘The soil–soil solution–plant interfaces’).…”

“… 33 (9) The grain is a phloem sink where the majority of Cd is complexed to Cd–S. Figure layout based on references 126 and 20 .…”