UV-C hormesis in broccoli florets: Preservation, phyto-compounds and gene expression

The effect of pre-storage exposure to ultra-violet radiation (UV-C) on preservation of broccoli (Brassica oleraceae var. Italica) florets and glucosinolates, phenolic acids and their precursor amino acids as well the expression of genes related to the biosynthetic pathways of glucosinolates and phenolic compounds in broccoli stored at 4 degrees C and 90-95% HR was investigated. The UV-C dose of 1.2 kJ m(-2) was found to be hormetic in delaying the yellowing and in lowering the weight loss of broccoli florets during storage. The time-averages over the storage period of both ascorbic acid titer and ORAC (oxygen radical absorbance capacity) value of the tissue exposed to hormetic dose of 1.2 kJ Im(-2) or a high dose of 3.0 kJ m(-2) were lower. The overexpression of genes (phenylalanine N-hydroxylase, tryptophan N-hydroxylase, dihomo-methionine N-hydroxylase and flavonoid monooxygenase) in UV-C exposed broccoli, hours after exposure (0 d), and that of chalcone synthase and coumarate ligase was observed on day 0, 2 and 4. The titers of glucosinolate-precursor amino acids, methionine, tryptophan and phenylalanine in tissue were dose-dependent, where the doses of 1.2 and 3.0 kJ m(-2) UV-C caused a decrease in their concentrations compared to the control. Hormetic dose of UV-C significantly increased the concentration of total glucobrassicins and 4-hydroxyglucobrassicin. In addition, UV-C treated florets with the dose of 1.2 or 3.0 kJ m(-2) contained a higher level of hydroxycinnamic acids in broccoli compared to the control during the storage. The results suggest that the application of hormetic dose of UV-C can be beneficial in maintaining not only the quality of broccoli florets, but also in enhancing the phyto-compounds during the low-temperature storage.