Effect of 5-Aminolevulinic Acid on Polyphenol Content, PAL Activity and PAL Gene Expression in Young Apple Fruits

The purpose of this study was to find out the effect of 5-aminolevulinic acid(ALA) application on phenolic substances and phenylalanine ammonia lyase(PAL) activity and PAL gene expression in young apple fruits. Apple fruits were treated with various concentrations of ALA before thinning, and then over a subsequent period of 12 days, total phenolic contents and PAL activity were measured by UV spectrophotometry and the expression of PAL gene by fluorescent quantitative real-time polymerase chain reaction(RT-qPCR). The experiment results indicated that as the ALA concentration increased up to 300 mg/L, the PAL activity, total phenolic content and PAL gene expression levels all showed an increasing trend, while the opposite pattern was observed for these three parameters when the ALA concentration was further raised to 400 mg/L. At identical ALA concentrations, all these parameters became significantly higher with prolonged treatment time, reaching the maximum levels on the 9th day, and began to fall on the 12th day. Thus, it can be concluded that treatment of young apple fruits with ALA at concentrations around 300 mg/L may result in a variety of desired physiological changes, such as markedly increasing phenolic substances and improving the activity level and gene expression of PAL.