Mitigation of cadmium toxicity by zinc in young cacao plants

Authors: SANTOS, M - University Of Santa Cruz - Brazil; ALMEIDA, A-A - University Of Santa Cruz - Brazil; SILVA, N - University Of Santa Cruz - Brazil; SILVA, J - University Of Santa Cruz - Brazil; BARROSO, J - University Of Santa Cruz - Brazil; SOUZA, J - University Of Santa Cruz - Brazil; AHNERT, D - University Of Santa Cruz - Brazil; Baligar, Virupax

Submitted to: Environmental and Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2020
Publication Date: 8/2/2020
Citation: Santos, M.L., Almeida, A., Silva, N.M., Silva, J.V., Barroso, J.P., Souza, J.O., Ahnert, D., Baligar, V.C. 2020. Mitigation of cadmium toxicity by zinc in young cacao plants. Environmental and Experimental Botany. https://doi.org/10.1016/j.envexpbot.2020.104201.
DOI: https://doi.org/10.1016/j.envexpbot.2020.104201

Interpretive Summary: Cacao in South America is invariably grown on soils that have high cadmium (Cd) . Cacao is efficient in transporting soil Cd to aerial parts and into the cocoa beans. Cd toxicity inhibits growth and production of cacao by interfering in the physiological and biochemical processes. Presence of high levels of Cd in cocoa beans have an impact on international trade of beans due to stricter regulations imposed by importing countries. In this paper, we report that increasing levels of soil Zn reduced uptake and transport of Cd by cacao. The roots immobilized much of the Cd in their tissues, and it appears that cacao using this as a tolerance strategy, reduced transport of toxic Cd to aerial parts. Findings of this research will be useful in cacao improvement programs to select cacao genotypes that accumulate large amounts of Cd in roots and thereby transport low levels of toxic Cd to shoots. Such genotypes could be useful as rootstock to graft high yielding cacao scion to reduce Cd transport to shoots. Farmers could apply Zn-fertilizers to soil that are high in Cd to reduce Cd in cacao beans thereby protecting their marketability. This information will be used by researchers, plant breeders, extension staff and farmers to improve cacao production.

Technical Abstract: Cacao is invariably grown on soils high in Cadmium (Cd) and Cd taken up by cacao leads to toxicity and zinc (Zn) in plant known mitigate the Cd toxicity. On the other hand, Zn is an essential nutrient and plays an important metabolic function in plants. The study of the interaction between a Zn nutrient and a nonessential element Cd may be important for understanding, analyzing and improving the defense strategies presented by plants. The main objective of this work was to evaluate the mitigation of Cd toxicity by Zn in young plants of the CCN 51 cocoa genotype, grown in soil with different concentrations of Cd, Zn and Cd + Zn. Plants physiological, biochemical, molecular and micromorphological responses were evaluated. High concentrations of Zn, Cd and Zn + Cd in the soil promoted alterations in the enzymatic and non-enzymatic antioxidative metabolism, and in the expression of genes involved in photosynthesis and antioxidative metabolism by increasing the activity of the antioxidative enzymes and the proline content and the reduction of the lipid peroxidation. Leaf gas exchanges were affected at the highest concentrations of Cd (0.8 mmol Cd kg-1 soil) combined with different concentrations of Zn (0.4, 0.8, 1.2 and 1, 6 mmol kg-1 soil), resulting in a decrease in CO2 fixation. The higher concentration of Cd, together with the intermediate concentrations of Zn (0.8 Cd + 0.4 or 0.6 Zn mmol kg-1 soil), promoted reduction of the thickness of the leaf mesophyll and, consequently, decreased the leaf gas exchanges. It was observed a hormesis effect due to high photosynthetic activity in low concentration of Cd. The increase in Cd concentration in the soil altered the uptake of Cd and Zn by the roots of the CCN 51 cocoa genotype. The increase of Zn concentration in the soil promoted the decrease of the Cd uptake by the root system of the CCN 51 cacao genotype and reduced the transport of Cd to the leaves. The roots immobilized much of the Cd in their tissues, as a tolerance strategy, avoiding that part of Cd was transported to aerial part. The increase of Zn + Cd concentration in the soil did not influence the accumulation of Zn in the leaves of the young plants of the CCN 51 cocoa genotype.