Effects of exogenous phytohormone 5-ALA on fruit quality and volatile metabolites of ' Meizao ' sweet cherry.

5-Aminolevulinic acid (5-ALA) is an endogenous amino acid and a precursor of chlorophyll synthesis. Studies have shown that it can promote fruit development and improve quality characteristics such as sweetness, acidity, and flavor. Therefore, in this study, 'Meizao' sweet cherry trees were used as test materials, and different concentrations of 5-ALA (50, 100, 150 mg·L⁻¹) were sprayed during the fruit expansion period to determine their effects on fruit quality. Based on principal component analysis, the fruit quality under 5-ALA treatment was comprehensively evaluated, and the best concentration for improving the fruit quality of 'Meizao' sweet cherry was selected to explore its effect on fruit volatile metabolites. The results showed that exogenous spraying of 100 mg·L⁻¹ 5-ALA had the best effect on improving cherry fruit quality. The contents of sucrose, fructose, glucose, and soluble sugar in cherry fruit were significantly increased by 203.33%, 174.12%, 109.32%, and 54.20% compared with the control, while the contents of malic acid and ascorbic acid were decreased by 56.10% and 40.61%. At the same time, the anthocyanin content decreased significantly by 64.47% compared with the control, whereas the contents of flavonoids, total phenols, and tannins increased by 114.62%, 7.80%, and 1.67%, respectively. The application of 5-ALA significantly increased the mass fraction of volatile metabolites in 'Meizao' cherry fruits, mainly elevating the levels of aliphatic hydrocarbons and ketones. This enrichment enhanced related aroma attributes such as wood, fruit, vanilla, and sweet flavors. The highest enrichment was found in the plant secondary metabolite pathway. Key compounds annotated in this pathway included: N-heptylidene-2,8-diazaspiro[4.5]decane; 2,8-dimethyl-2,8-diazaspiro[4.5]decane; 6-methyl-7-oxa-8-azabicyclo[4.2.1]non-8-ene; 1-(3,5-dimethyl-1H-pyrazol-4-yl)ethanone; octanal; N-heptylidene-methanamine; and 7-methyl-4-indancarboxylic acid. Consequently, the 'Meizao' sweet cherries developed a special aroma with woody, fruity, herbal, citrus, sweet, oily, plant-like, and floral characteristics.