Mitigation of Pb toxicity by Mn in seedling of the cacao clonal CCN 51 genotype grown in soil: physiological, biochemical, nutritional and molecular responses

Authors: MUÑOZ, JOSE JULIAN APR - University Of Santa Cruz - Brazil; ALMEIDA, ALEX-ALAN - University Of Santa Cruz - Brazil; PIROVANI, CARLOS - University Of Santa Cruz - Brazil; AHNERT, DARIO - University Of Santa Cruz - Brazil; Baligar, Virupax

Submitted to: Ecotoxicology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2021
Publication Date: 2/2/2021
Citation: Muñoz, J.A., Almeida, A.F., Pirovani, C.P., Ahnert, D., Baligar, V.C. 2021. Mitigation of Pb toxicity by Mn in seedling of the cacao clonal CCN 51 genotype grown in soil: physiological, biochemical, nutritional and molecular responses. Ecotoxicology. https://doi.org/10.1007/s10646-021-02348-y.
DOI: https://doi.org/10.1007/s10646-021-02348-y

Interpretive Summary: Soils in cacao growing regions of South America are invariable infertile and acidic in nature and in some areas may contain high levels of lead. Lead is very toxic to growth and development of cacao there by it affects its sustainability and yield potentials. In this paper we report that addition of manganese to lead contaminated soils reduces the uptake of toxic lead, improves mineral nutrition, enhances photosynthesis and growth of cacao. This information will be useful to scientist and extension workers to develop manganese-based soil amendment technology to reduces lead toxicity and thereby improve cacao sustainability and quality of cocoa beans. Cocoa beans with low concentration of lead enhances their marketability in international trade thereby it could improve the economic wellbeing of resource poor cacao farmers.

Technical Abstract: Lead (Pb) is a highly toxic metal for plants, even at low concentrations in the soil. The world annual production of cocoa beans is approximately 4 million tons and most of these beans are used in the manufacture of chocolate. Recent work has shown that the concentration of Pb in these beans has exceeded the critical level (1 mg kg-1 16 DM), thereby reducing their marketability in international trade. The objective of this study was to evaluate the influence of Mn on mitigation of Pb toxicity in young clonal cocoa genotype CCN 51 through physiological, biochemical, molecular and nutritional responses. Plants were cultivated in soil with different concentrations of Mn+Pb, Pb, and Mn [0.3 g Mn kg-1 soil + 0.5 g Pb kg-1 soil, 0.15 g Mn kg-1 soil + 0.75 g Pb kg-1 soil, 0.45 g Mn kg-1 soil + 0.25 g Pb kg-1 soil, 1.0 g Pb kg-1 soil and 0.6 g Mn kg-1 soil,] and control without addition of Pb and Mn. The increasing soil Pb concentrations promoted (i) increase in translocation and accumulation of Pb in the aerial part of the plants, (ii) alterations in mineral nutrient absorption by competition or synergism, (iii) changes in photosynthetic activity caused by reduction in chloroplastidic pigment content and damage to the photosynthetic machinery, evidenced by the Fv/Fm ratio and expression of the psbA gene, and (iv) increased GPX and SOD activity in the leaf. Addition of 1.0 g Pb kg-1 soil there was repression of the expression of the majority of the evaluated genes. The additions of higher levels of Mn to the soil can be an effective treatment for reducing Pb toxicity since Mn has the same chemical valence, thus generating an antagonism in the absorption Pb. High levels of added Mn did not create Mn toxicity, thus confirming its remedial potential in the cultivation of cocoa in areas where Pb concentrations in soils are too high.