Anthocyanins in red grape skin have a positive effect on fruit color and human health. The effect of foliar potassium application on anthocyanin accumulation in grape skin is not well understood. The study aimed to better understand the mechanism of anthocyanin accumulation in grape skin in response to foliar sprays of K₂SO₄ and KH₂PO₄. In this study, we investigated the effects of foliar application of KH₂PO₄ (T2), K₂SO₄ (T1) and distilled water (CK) on the skin color of ‘Kyoho’ grapes at mid-ripe and mature stages. At 90 and 110 days after full bloom (DAFB), T2 had the greatest total soluble solids (TSS), flavonoid and total anthocyanin contents, followed by T1 and CK. At two stages, the titratable acid content decreased and the juice pH increased under T2 treatment relative to CK. T1 and T2 had lower lightness (L*) than CK, and the color index of red grapes (CIRG) under T1 and T2 increased at two stages compared to CK. KEGG metabolic pathway analysis revealed that flavonoid biosynthesis was the most significantly enriched pathway in CK vs. T2 at 90 and 110 DAFB. At 90 DAFB, T2 had higher expressions of phenylalanine ammonia-lyas (PAL), cytochrome P450 CYP73A100 (CYP73A), 4-coumarate: CoA ligase (4CL), chalcone synthase (CHS), flavanone 3-dioxygenase-like (F3H) and UDP glucose: flavonoid 3-o-glucosyl transferase (UFGT) than CK and T1. Foliar application of potassium fertilizer may accelerate anthocyanin accumulation by altering the transcript levels of PAL, CYP73A, 4CL, CHS, F3H, and UFGT of the flavonoid biosynthesis.

As a widely distributed fruit, grapes are susceptible to oxidative damage during storage and transportation, resulting in declining quality and commodity value. This study aimed to investigate the effects of preharvest application of different concentrations of multi-walled carbon nanotubes (MWCNTs) on the postharvest quality of ‘Flame Seedless’ grapes. The results showed that low-concentration (25 and 50 mg L⁻¹) MWCNTs treatments maintained the comprehensive quality index, firmness, soluble sugar, titratable acid, pH value, and ascorbic acid (AsA) content of grapes. MWCNTs at 25 and 50 mg L⁻¹ increased the activities of peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), and ascorbic acid (APX). Furthermore, MWCNTs reduced the malondialdehyde (MDA) content and decreased the accumulation of excessive reactive oxygen species (ROS) in grape peel and pulp tissues. In addition, transmission electron microscopy (TEM) images demonstrated that MWCNTs were absorbed by parenchymal cells in the grape peel and pulp through the epidermal cell layer. MWCNTs with a specific concentration can be used as a new inducer for the biosynthesis of antioxidants to reduce oxidative damage in grapes during storage.

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) 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 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.