Silicon modulates the metabolism and utilization of phenolic compounds in cucumber (Cucumis sativus L.) grown at excess manganese

Cucumber plants (Cucumis sativus L. cv. Chinese long) were grown in nutrient solution with increasing manganese (Mn) concentrations (0.5, 50, and 100 mu M) with (+Si) or without silicon (-Si) supplied as silicic acid at 1.5 mM. High external Mn supply induced both growth inhibition of the whole plant and the appearance of Mn-toxicity symptoms in the leaves. The application of Si alleviated Mn toxicity by increasing the biomass production. Although the total Mn concentration in the leaves did not differ significantly between +Si and -Si plants, symptoms of Mn toxicity were not observed in Si-treated plants. The concentrations of phenolic compounds, particularly in the leaf extracts of cucumber plants grown at high external Mn concentrations, differed from those of plants grown without Si. The increased tissue concentrations of phenols (e.g., coniferyl alcohol, coumaric and ferulic acids) were in agreement with enhanced enzymes activities, i.e., peroxidases (PODs) and polyphenol oxidases (PPO) in the tissues of -Si plants. The activities of both enzymes were kept at a lower level in the tissue extracts of +Si plants grown at high external Mn concentrations. These results suggest that Si nutrition modulates the metabolism and utilization of phenolic compounds mainly at the leaf level, most probably as a consequence of the formation of Si-polyphenol complexes.