Understanding Cadmium Uptake by Cocoa Plants via Cd Isotope Analyses

Cadmium (Cd) is a trace metal that is highly toxic even at low concentrations. The relatively high Cd abundances often found in cocoa are a serious problem for the cocoa producers of developing nations, as the European Union plans to implement strict new upper limits for Cd in cocoa products in 2019. This development has recently spurred research into the causes of Cd enrichment in cocoa. Such work has mainly focused on the impact of external factors, such as soil type, pH and cultivation practices, on the Cd content of cocoa but until now, these investigations have failed to provide conclusive results. Hence, it has been suggested that Cd accumulation may vary across different cocoa varieties but this possible link has yet to be investigated.

The current project constitutes collaborative research by Prof M Rehkamper, an isotope geochemist at Imperial College London, and Prof J Dunwell, a plant physiologist at the University of Reading. Previous work of Rehkamper has shown that coupled Cd concentration and stable isotope analyses, initially developed for planetary science research, can provide unique insights into Cd uptake and translocation by plants. Furthermore, a 2015 pilot study of Rehkamper and Dunwell revealed that such analyses are also promising for cocoa.

In the current 6-month project, the results of the 2015 pilot experiment will be verified, to ensure that coupled Cd concentration and isotope data indeed provide a reliable measure for Cd uptake by and translocation within plants. Furthermore, a suite of different cocoa varieties will be analysed to characterise the overall variability of Cd uptake and accumulation amongst different types of this plant. Together, the results will enable an initial identification of cocoa varieties that are particularly suitable for cultivation because they feature minimal Cd accumulation in beans, due to low uptake of Cd by the plant and reduced transport from the roots to the fruit.

As such, the results of this study can make an important contribution to the development of sustainable methods for the production of cocoa beans with safe levels of Cd, thereby ensuring a reliable source of income for cocoa producers in developing nations and boosting the economies of rural communities.

This study will provide an improved understanding of cadmium (Cd) accumulation in cocoa and an initial assessment of the variability of Cd uptake amongst different cocoa varieties. As such, our results are of direct societal and economic relevance. In particular, a better understanding of how different cocoa varieties and cultivation practices impact the Cd content of cocoa means that farmers can adopt practices, which produce market-acceptable cocoa with safe levels of Cd, thereby sustaining local economies in rural communities. In addition, our work also helps to secure a reliable supply of cocoa for the chocolate industry (including local producers in developing countries) and is ultimately of benefit to all consumers of cocoa products.

Given the possible societal and economic impact of our work, relevant stakeholder will be briefed directly on the results. Engagement of cocoa stakeholders will thereby be developed through the existing links of Project Partner Dunwell. In fact, our 2015 pilot study was initiated in response to requests to Dunwell from cocoa growers and industry to identify new solutions for the problem of Cd accumulation in cocoa. Notably, the results of our pilot investigation were already communicated to the stakeholders in January 2016, and they subsequently agreed to release resources from an existing grant to fund the UR research component of the present study.

Direct briefing of stakeholders on the findings of this project and their implications for cultivation practices to reduce Cd accumulation is, therefore, of key importance. Of particular significance in this context are Dunwell's close links to national cocoa research institutes, especially in South & Central America, where the problem of Cd accumulation appears to be most severe and the institutes are already involved in studies to reduce Cd accumulation in cocoa.

By providing our findings on suitable genotypes to such research centres, including CATIE in Costa Rica, INIAP in Ecuador, and CORPOICA in Columbia (and others), they can be incorporated into on-going breeding programs, which in turn routinely support and supply local farmers. In addition, existing contacts with industry, such as local chocolate producers (e.g., Casa Luker, Colombia) and cocoa exporters (e.g., ANECACAO), larger estates (e.g., Las Canas, Ecuador), as well as governmental (e.g., ICCO) and trade associations (e.g., ABICAB Brazil) will be utilized, to raise awareness for our work and support adoption of revised practices.

Ultimately, our research is poised to support the rural cocoa producers of developing nations, by aiding the identification of cocoa genotypes and/or cultivation practises that enable sustainable production of cocoa beans with low Cd contents. The current project, however, cannot provide a comprehensive response to this challenge, because only a small number of genotypes (in comparison to the known genetic diversity) can be screened. Hence, we intend to carry out a comprehensive follow-up investigation that involves coupled Cd concentration and isotope analyses for a much larger suite of genotypes and samples. Funding for such a follow-up study can be sought from a number of sources. In particular, we envision a Global Challenge Research Fund application because such follow-up work is ODA eligible, as it focuses on the needs of developing nations and is of particular significance to poorer communities. In addition, we will utilize Dunwell's links with cocoa stakeholders to discuss and encourage further funding for a follow-up investigation (particularly from industry) and consider the feasibility of RCUK research grant applications.