Fitorremediação de cádmio por Khaya ivorensis A. Chev. mitigação do estresse por silício nas modulações fisiológicas, bioquímicas e anatômicas.

Abstract

Among the heavy metals (MPs), cadmium (Cd) represents one of the biggest threats to the environment on a global scale, due to its high ecotoxicological potential and increasing soil indices, which contributes to the loss of carbon dioxide productivity and biodiversity of ecosystems. Fast-growing woody plants represent effective tools for Cd extraction in contaminated soil. In this sense, African mahogany (Khaya ivorensis A. Chev), a tree species belonging to the Meliaceae family, presents satisfactory characteristics for remediation given its accelerated growth, axial root system and high biomass production. However, the phytoremediation strategies and Cd tolerance mechanisms implemented by this species, associated with the improvement of nutritional status, are not clear. Given the above, this research aims to identify the modulations of silicon (Si), a beneficial micronutrient, in the physiology, biochemistry, antioxidant system and anatomy of African mahogany seedlings submitted to toxic levels of Cd and correlate these processes with increased phytoremediation potential to MP. The experimental design was a randomized block design in a 4x4 factorial scheme with five replications, and the factors constituted by increasing levels of MP (Cd 0,25, 50 and 75 mg L-1) and silicon doses (Si 0, 100, 150 and 300 mg L-1). Polynomial regression analysis, response surface was performed. Cd 50 and 75 mg L-1 concentrations were toxic and caused changes in biochemical processes with negative physiological and anatomical repercussions on K. ivorensis. There was photoinhibitory damage to PSII, where the variables of chlorophyll a, (ΦPSII), (qP) and (ETR) fluorescence, photosynthesis (A), gas exchange (gs), (Ci/Ca) and photosynthetic pigments , especially chlorophyll b, were the most affected by the highest level of the metal and presented respective minimum points by the regression adjustment of 0.05; 0.07; 0.15 µmol molar CO2 -1 ; 3.2 mmol CO2m -2 s -1; 0.1 mol H2O m-2s -1; 0.06 µmol molar CO2 and 0.18 mg g-1 and were 78%, 82%, 75%, 83%, 96%, 93% and 95% lower than the control. Disorganization in cytosolic metabolism caused lipid peroxidation. Malonoaldehyde content was increased by 69% and 63% in leaves and roots, which was accompanied by reductions in antioxidant enzyme activities. Above all, CAT had its expression decreased by 69% in the root system. Anatomical changes occurred in the tissues with marked decreases in epidermal thickness. The palisade parenchyma and root cortex were 84% and 72% less thick under that toxic Cd level. However, Si attenuated the stress promoted by the ion and modulated overexpression in those variables, which ensured the growth and development of K. ivorensis under conditions of MP contamination. Normal plant metabolism was resumed only in those submitted to treatment with Cd 50 mg L-1 and nourished with Si 150 mg L-1. Canonical analysis showed that the combination of Cd 45mg L -1 and Si 160 mg L-1 resulted in the maximum points of those parameters where the values came close to the control. Si modulation in antioxidant metabolism was confirmed by a 31% increase in SOD activity. These response patterns were related to the reduction of Cd translocation to the aerial parts due to its entrapment in the Si-induced root system. Thus, the micronutrient increased K. ivorensis tolerance and phytoremediation potential to Cd by MP phytostabilization which resulted from mechanisms of Si complexation to Cd, as well as ion restrictions imposed by the caspary striations, mediated by the beneficial mineral. Tolerance to Cd was due to improved water relations.