Physiological, ultrastructural, biochemical and molecular responses of young cocoa plants to the toxicity of Cr (III) in soil.

The objective of this study was to evaluate Cr toxicity in young plants of the CCN 51 Theobroma cacao genotype at different concentrations of Cr ³⁺ in the soil (0, 100, 200, 400 and 600 mg kg ^-1 ) through physiological, ultrastructural, antioxidant and molecular changes. Doses of 400 and 600 mg Cr ³⁺ kg ^-1 soil severely affected foliar gas exchange, promoted by damages in photosynthetic machinery evidenced by the decrease in CO ₂ fixation. Decreased expression of psbA and psbO genes, changes in enzymatic activity and lipid peroxidation also affected leaf gas exchange. A hormesis effect was observed at 100 mg Cr ³⁺ kg ^-1 soil for the photosynthetic activity. As a metal exclusion response, the roots of the cocoa plants immobilized, on average, 75% of the total Cr absorbed. Ultrastructural changes in leaf mesophyll and roots, with destruction of mitochondria, plasmolysis and formation of vesicles, were related to the oxidative stress promoted by excess ROS. The activity of the antioxidant enzymes SOD, APX, GPX and CAT and the amino acid proline coincided with the greater expression of the sod cyt gene demonstrating synchronicity in the elimination of ROS. It was concluded, therefore, that the tolerance of the cocoa plants to the toxicity of Cr ³⁺ depends on the concentration and time of exposure to the metal. Higher doses of Cr ³⁺ in the soil promoted irreversible damage to the photosynthetic machinery and the cellular ultrastructure, interfering in the enzymatic and non-enzymatic systems related to oxidative stress and gene expression. However, the low mobility of the metal to the leaf is presented as a strategy of tolerance to Cr ³⁺ .
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