Phenolics Profile and Phenol-Related Enzyme Activities in Cucumber Plants Under Ni Stress

Ni phytotoxicity has been attributed to its multidirectional detrimental effects on plant cell structure and function. However, relatively little is known about Ni's impact on phenolic metabolism in plants. The objective of our study was to obtain insight into the effect of Ni treatment on phenolic compound composition, phenol-related enzyme activities, and lignin accumulation in cucumber plants. Besides growth reduction, the chlorophyll a and carotenoid contents as well as the chlorophyll a fluorescence parameters, namely, the maximum photochemical efficiency of PS II and non-photochemical quenching, were significantly decreased in the Ni-treated cucumber plants. Application of Ni resulted in changes in the phenolic acid and flavonoid profiles; however, the total content of the detected phenolic compounds remained unchanged in the leaf and slightly decreased in the root. The Ni-induced release of free phenolic acids from their conjugates was found in the leaf. Ni treatment led to a marked increase in leaf peroxidase activities assayed with various phenolic substrates, while it did not influence phenyl ammonia lyase and polyphenol oxidase activities. Increased lignin deposition was observed in the leaf blade of Ni-exposed plants. Neither lignin accumulation nor induction of peroxidase activities were found in the root. Our results indicate that the Ni effect on phenolic compound composition and related enzyme activities is organ-specific. The observed changes in the content of individual compounds might result rather from the metal-triggered conversions of the compounds constitutively present in the cucumber tissues than from de novo synthesis.