Combined UV-B and methyl jasmonate treatments enhance postharvest pigmentation of “Fuji” apples

The red color of postharvest apples is an important quality factor. Mixed colors cause a decrease in consumer purchase, even if other quality factors between apples are similar. The purpose of this study was to determine how well UV-B and surface colorant treatments (MeJA: methyl jasmonate; EP: ethephon) induce color changes in mixed-color apples after harvest. Partially less-pigmented apples were developed by artificial sun-blocking, and they were harvested during the regular harvest period. The harvested apples were irradiated with UV-B with/without colorants. UV-B irradiation alone or combined with EP increased cyanidin-3-O-galactoside (idaein) content in the less-pigmented skin to 50% of the level found in naturally pigmented skin. On the other hand, irradiation with UV-B and MeJA for 48 h had synergistic effects on idaein accumulation that produced almost 100% of the level found in naturally pigmented skin. Genes of the anthocyanin pathway, such as MdUFGT and MdDFR, were synergistically overexpressed in the treatment group that received UV-B irradiation and MeJA together. MdDFR2 was overexpressed to a level similar to MdDFR1 at 24 h and was higher at 48 h. The expression of other flavonoid biosynthetic genes, MdMYB10, MdCHS, MdCHI, MdF3H, MdFLS, MdLDOX, and MdUFGT, was induced by 24 h of UV-B irradiation, regardless of colorant treatment. The quercetin glycosides, except for quercetin-3-O-rutinoside, increased significantly when UV-B and colorant treatments were applied. These results suggest that short-term combination treatment with UV-B and MeJA is an efficient method for improving postharvest apple coloration.