Foliar zinc supplementation as a mitigation strategy for reducing cadmium in cacao

Abstract

Cacao cultivation is an agricultural activity of great economic and social importance in tropical regions worldwide. However, in these regions, the presence of potentially toxic elements in the soil, such as cadmium (Cd), which bioaccumulate in different plant organs, has become a growing concern regarding the future of cacao exports in South America, particularly in Colombia, Ecuador, and Peru. It has been demonstrated that foliar zinc (Zn) application can inhibit Cd uptake in some crops by reducing its accumulation through the regulation of both root absorption and the transport of the metal from roots to shoots. In addition, Zn competes for the binding sites of protein transporters; thus, high Zn concentrations occupy these sites, preventing Cd transport to the phloem and favoring Zn translocation. With the aim of contributing to the solution of the cadmium bioaccumulation problem in cacao, this study sought to evaluate Cd bioavailability in the soil and in the plant through foliar Zn application as a mitigation strategy. To assess Cd inputs from phosphate sources and the effect of foliar Zn application, a completely randomized block design with a 4 × 3 factorial arrangement was used, in which the blocking factor corresponded to amendment application, and the interaction factors were the application of phosphate sources with contrasting Cd contents and the application of different foliar Zn doses. It was observed that juvenile cacao plants of the CCN 51 genotype exhibited root uptake and foliar accumulation of Cd, which increased in soils without amendment and with higher Cd concentrations in the soil. This response resulted in reduced photosynthesis and gas exchange, decreased chlorophyll content and PSII quantum efficiency, as well as alterations in the plant antioxidant enzymatic system. However, the application of a Zn dose of 8 g L⁻¹ increased leaf Zn concentration by up to sevenfold and reduced foliar Cd content by up to 26.9%, generating a positive effect reflected in the mitigation of adverse physiological effects and in the reduced likelihood of Cd accumulation in agronomically important organs, such as cacao fruits. These results indicate Zn as a practical and functional strategy for Cd mitigation in cacao cultivation in tropical and subtropical regions.