Study On The Effects And Mechanism Of UV-C Irradiation On Ripening In Postharvest Cherry Tomato Fruit

Short-wave ultraviolet light (UV-C,190-280nm) irradiation is considered as a potentially new and environment-friendly physical technology to prolong the shelf-life for fresh fruit and vegetables. UV-C irradiation can help to improve tomato fruit quality and delay softening, which has great significance on regulating quality of fruits and vegetables. Cherry tomato is a kind of people’s favourite fruit due to its small size and abundant nutrition. The objective of this research was to investigate the effects of UV-C irradiation on postharvest cherry tomato fruit quality, softening and color changes. Besides, the physiological, biochemical and molecular mechanisms were also studied.Mature green cherry tomato fruits were harvested and treated with UV-C irradiation at a predetermined dose of4.2kJ m-2and stored at18℃for35days. UV-C (4.2kJ m-2) irradiation inhibited expression of ACC synthase gene and ACC oxidase genes, and thereby reduced ACC synthase and ACC oxidase activity, which leads to an inhibition of ethylene production. It has been clearly shown that UV-C treatment could delay fruit ripening. However, in this study, UV-C treatment resulted in a general down-regulation of genes involved in ethylene signal transduction (only CTR4was up-regulated), and it was not consistent with ethylene negative regulation theory, suggesting that there might be a feedback regulation mechanism involved in tomato.By measuring DPPH radical scavenging ability and FRAP, we found that UV-C irradiation (4.2kJ m-2) improved the ability of DPPH radical scavenging and FRAP values. UV-C irradiation improved catalase activity, and inhibited H2O2content in tomato fruit. CAT might clear excessive amount of hydrogen peroxide (H2O2), and thus protects the tomato tissue against active oxygen damage. MDA is a byproduct of membrane lipid peroxidation. The MDA content in UV-C treated fruit was lower than that in control fruit, indicating that UV-C increased the antioxidant capacity of tomato fruit, and thus delayed the senescience of tomato fruit.The effects of UV-C treatment on firmness, cell wall compositions (pectin and cellulose), cell wall ultra-structure, level of ethylene production and activities of cell wall degrading enzymes in the fruit were investigated during storage. Major genes involved in the cell wall degradation (PME2.1, Cel1, PG cat and Exp1) were also checked. The firmness was better maintained in UV-C treated fruit corresponding with higher contents of cellulose and acid-soluble pectin. Transmission electron microscopy indicated that UV-C irradiation retarded the cell wall disassembly in pericarps. The ethylene production was significantly inhibited by UV-C treatment. The UV-C treatment also suppressed the transcriptional expression of PME2.1, Cel1, PGcat and Exp1genes, and inhibited the pectin methylesterase (PME; EC3.1.1.11), polygalacturonase (PG; EC3.2.1.15) and cellulase (EC3.2.1.4) activities during the whole storage. These results suggested that the inhibition of ethylene production, which in turn down-regulates expression of genes encoding cell wall degrading enzymes, might be one of the possible mechanisms of UV-C in delaying tomato fruit softening.The effects of UV-C treatment on the color changes, pigment contents, and the expressions of major genes involved in carotenoids metabolism, including Psy1, Pds, Lcy-β, and Lcy-ε, encoding phytoene synthase, phytoene desaturase, lycopene β-cyclase and lycopene ε-cyclase respectively, were examined. The UV-C treated fruit developed a pink red color in contrast with the normal orange red color of control fruit. Lycopene accumulations during ripening in UV-C treated fruit were significantly inhibited but its final content was not affected. However, both β-carotene accumulation and final content of β-carotene were significantly suppressed in UV-C treated fruit. The lower content of β-carotene, leading to higher lycopene to β-carotene ratio, is probably responsible for the altered color phenotype in UV-C treated fruit. Psy1, a major gene involved in lycopene synthesis was inhibited by UV-C irradiation. Significantly suppressed expression of Lcy-β gene was also observed in UV-C treated fruit. Thus it is possible that the lower transformation to carotenes contributed to the relatively stable content of lycopene.