Anthocyanins, soluble sugars, and organic acids play a vital role in the color and flavor of grape berries. N and KH2PO4 are essential nutrients for grape growth and development. However, the research on the effects of foliar spraying of KH2PO4 on the skin color and flavor of grapes under different N levels were not systematic. In this study, “Flame seedless” grapes were used as the test material. There were six treatments in this experiment, including low nitrogen (LN), low nitrogen + KH2PO4 (LNK), moderate nitrogen (MN), moderate nitrogen + KH2PO4 (MNK), high nitrogen (HN), and high nitrogen + KH2PO4 (HNK). Foliar spraying of KH2PO4 on grapes significantly increased total K, anthocyanin contents, and the color index of red grapes (CIRG) in LN, MN, and HN. In the N and KH2PO4 treatments, foliar spraying of KH2PO4 significantly increased the content of methylated, acetylated, and coumarylated anthocyanins under MN treatment. The glucose and fructose contents of MNK were the highest compared to other treatments. The sole use of N showed the highest glucose and fructose contents with MN application. Anthocyanin had a significant positive correlation with soluble sugars; and showed a significant negative correlation with organic acids. Overall, foliar spraying of 0.5% KH2PO4 improved the color and flavor of “Flame seedless” grapes under all N levels, with the most significant effect at MN.
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.
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