Tree Genetics and Molecular Breeding 2025, Vol.15, No.5, 176-184 http://genbreedpublisher.com/index.php/tgmb 177 mainly glucose and fructose, while organic acids include tartaric acid, malic acid and citric acid. When the fruit ripens, sugar gradually accumulates while organic acids decrease, which directly affects the sweetness, acidity and overall flavor of the fruit (Wang et al., 2022a). Proteins, soluble solids and vitamin C, etc. also play important roles in the nutrition and flavor of fruits (Filimon et al., 2017; Deng et al., 2025). Meanwhile, environmental conditions such as temperature, precipitation and soil nutrients can also significantly affect the accumulation of these substances (Biasi et al., 2019; Li et al., 2024). 2.2 Secondary metabolites: anthocyanins, tannins, and aroma-related compounds Secondary metabolites are also very important in grape quality. Anthocyanins, tannins and polyphenols not only determine the color and taste of fruits, but are also related to antioxidant activity and health functions (Gascuel et al., 2017; Wang et al., 2022a). Anthocyanins are the main source of fruit peel color, while tannins bring astringency and a sense of structure. Aroma-related substances, such as terpenoids, alcohols and esters, determine the unique flavors of grapes and wines (Laucou et al., 2018; Wang et al., 2022a). The accumulation of these substances is influenced by multiple aspects such as genetics, light, temperature and cultivation management (Wang et al., 2022a; Ganugi et al., 2023). 2.3 Texture, firmness, seedlessness, and consumer-preferred traits The texture and hardness of the fruit will affect its storage and transportation performance as well as its eating taste. Fruit size, peel thickness, pulp firmness and seedlessness are important goals in table grape breeding (Ban et al., 2016; Kui et al., 2020; García-Abadillo et al., 2024). Among them, seedlessness is particularly popular in the market. Currently, related QTL and candidate genes have been identified (Laucou et al., 2018). In addition, traits such as fruit appearance (such as color and shape), crack resistance and disease resistance are also often included in the indicators for quality improvement (Vafaee et al., 2017; García-Abadillo et al., 2024). 3 Traditional Breeding Approaches 3.1 Germplasm collection and phenotypic selection The diversity of germplasm resources is the foundation for improving the quality of grapes. By collecting and preserving wild relatives, local varieties and traditional strains of grapes, breeders can obtain rich genetic variations. These variations provide a gene pool for the improvement of traits such as fruit flavor, nutritional components and stress resistance. Phenotypic selection relies on systematic evaluation of traits such as fruit appearance, flavor, sugar-acid ratio and disease resistance, from which individuals with outstanding performance are selected as breeding materials. Although long-term domestication and selection have reduced genetic diversity, there are still obvious differences in fruit traits among the existing germplasms. Through systematic phenotypic identification and genetic analysis, these differences can be utilized to provide a genetic basis for responding to climate change and market demand (Gascuel et al., 2017; Bigard et al., 2018; Bigard et al., 2020). 3.2 Hybridization for seedlessness, disease resistance, and quality Hybrid breeding is the core means for improving the quality of grapes. Through sexual hybridization, breeders combine superior fruit qualities (such as large size, good flavor, and balanced sugar and acid) with target traits like seedlessness and disease resistance. For instance, grape breeding has developed new varieties such as ‘Shine Muscat’ through diploid and tetraploid hybridization. These varieties are both large and seedless, and have disease resistance (Yamada and Sato, 2016). In addition, by using intraspecular hybridization and repeated backcrossing of grapes, traits such as cold resistance and disease resistance can be gradually introduced while maintaining high-quality fruits, avoiding the problem of fruit quality decline after hybridization of wild varieties (Wang et al., 2021; Wang et al., 2022b; Wang et al., 2024). Hybrid breeding is also widely used in the cultivation of seedless grapes, often combined with techniques such as embryo rescue to increase the yield of seedless offspring. 3.3 Clonal selection for stabilizing desirable fruit quality traits Clonal selection is an important method for maintaining and stabilizing excellent fruit traits. Grapes are mostly propagated asexually. Excellent individual plants can be cloned and expanded through methods such as cuttings and grafting to ensure the stable inheritance of fruit quality, flavor and yield. Clonal selection not only maintains the superior traits of varieties but also discovers new quality variations by screening natural mutants (such as bud
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