Abstract

The soil is among the main contamination sources of Pb in cocoa beans, which carries potential risks to human health from ingesting contaminated cocoa products. Therefore, the Pb contents in cocoa beans depend not only on the genotype, but also on the geographic location. Pb toxicity in plants is highly modified by increasing the Zn/Pb ratio. Pb uptake by the roots decreases with the increase in the Zn content in hyperaccumulator plant species of Pb/Zn, as well as in non-accumulator species, clearly indicating that the inflow of Pbis largely attributed to Zn transporters, with a strong preference for Zn at the Pb detriment. The objective of this study was to evaluate the influence of Zn on mitigation of Pb toxicity in young plants of the cacao clonal CCN 51 genotype grown in soils with different doses of Pb, Zn and Zn+Pb, through physiological, biochemical, molecular and nutritional responses. Young plants of the clonal cacao genotype CCN 51 grown in soils with high Pb, Zn and Zn + Pb contents accumulate these heavy metals in the roots and leaves. The uptake of Pb and Zn by the roots and their transport to the aerial part promoted significant physiological alterations in variables such as photosynthesis, gas exchange, carboxylation efficiency, nutritional balance, antioxidant metabolism and the genetic expression especially of PsbA and PsbOgenes related to photosystem two efficiency (PSII). The increased activity of the SOD enzyme and the proline content in the leaves contributed to mitigate the toxicities of Pb and Zn at the highest doses of these metallic elements applied in the soil. Furthermore, the adequate doses of Zn + Pb applied in the soil mitigated the toxicity of Pb in the plants. On the other hand, the doses of Zn + Pb and Zn applied to the soil, induced the death of young plants of the clonal cocoa CCN 51 genotype 15 days after the application of the treatments, the application of Zn in adequate doses to the soil. It can be used to mitigate the toxicity of Pb in young plants of the CCN 51 clonal cocoa genotype that grows in contaminated soils.

Keywords

metal toxicity, photosynthesis, antioxidant metabolism, gene expression, mineral nutrients, natural conditions, metal mining and smelting, industrial waste, fertilizers, pesticides, biological function, physicochemical parameters, chloroplast, mitochondria, nucleus and membranes