Calcium oxalate crystals in cacao trees and their interactions with cadmium

Cadmium (Cd) concentrations in cacao beans from Latin America often exceed limits for trading. A better understanding of the mechanisms of Cd accumulation in Theobroma cacao L. trees is necessary to advance mitigation strategies. Recent studies on a high Cd accumulating cultivar of cacao revealed that calcium oxalate (CaOx) crystals were involved in Cd accumulation in the branches. The purpose of this study was to quantify soluble and crystalline oxalate in cacao compartments, to characterize their morphology and distribution in the tissues, and to evaluate the relationship between CaOx and Cd total concentrations in each plant compartment. Two representative cultivars from Latin America (CCN-51 and Nacional) were studied. CCN-51 trees grew on soils with low and high total Cd contents (0.120 ± 0.002 and 2.59 ± 0.48 mg kg⁻¹, respectively), and Nacional trees grew on soils with low Cd content (0.188 ± 0.005 mg kg⁻¹). Oxalate was present in all organs of the two cultivars. In mature leaves, oxalate content exceeded the limit of 5% per dry weight used to define extreme oxalate accumulators. The crystalline form predominated in branches and mature leaves (82-92%), whereas the soluble form predominated in nibs (67-82%). Calcium oxalate crystal size varied from <1 μm (generally agglomerated as crystal sand) to a few tens of μm (faceted crystals). Log CaOx and Cd concentrations were positively correlated in branches (R² = 0.77, p = 0.002) and roots (R² = 0.71, p = 0.005), whereas in nibs, the oxalate content was almost constant among conditions. The possible roles of CaOx crystals in the cacao plant, including Ca regulation, protection against herbivory, tissue stiffening and Cd detoxification are discussed.