1 Introduction

Table grapes (Vitis vinifera L. and allied varieties) are amongst the most important fresh fruit crops of the world with excellent economic and nutritional importance. Global consumer needs are increasingly emphasizing parameters such as berry size, even color, sweetness, texture, and storage life. Among these, sugar deposition and general fruit quality are key parameters determining market competitiveness and consumer acceptability. Management of these problems demands an extremely accurate vineyard practice which optimizes environmental conditions and plant physiological responses.

The removal of leaves is a widespread viticultural practice used to modify canopy structure, increase penetration of light, and regulate the microclimate of the fruit. Through the alteration of the source-sink relationship, the practice may influence photosynthetic efficiency, berry growth, and metabolite accumulation (VanderWeide et al., 2021). Leaf removal has been used traditionally in vineyards for increased cluster exposure, defense against diseases, and better fruit ripening. Its application in table grape production has been emphasized over the last few years because it has the ability to contribute to berry appearance, sweetness, and storage quality. Leaf removal effects vary vastly depending on timing, severity, and location of defoliation and cultivar response (Bakhsh et al., 2021).

Despite extensive research on canopy management, there is still limited knowledge of the systematic physiological and molecular foundation of the effect of defoliation on fruit quality, particularly for table grape varieties destined for fresh-market. The majority of current studies are focused on wine grapes when there is an existing lacuna of knowledge regarding table grapes, where organoleptic quality and sugar composition are prioritized over secondary metabolites like phenolics. Moreover, the interactions of defoliation with environmental factors, cultivar characteristics, and other horticultural procedures remain yet to be fully elucidated. These gaps underscore the need for a more integrated approach combining physiological, biochemical, and molecular perspectives to account for the role of defoliation in the control of fruit quality and sugar accumulation (Doğan, 2025).

This study summarizes the current application status of leaf picking, in the production of fresh table grapes, with a focus on discussing its impact on fruit quality and sugar accumulation. The content covers physiological, biochemical and molecular mechanisms, as well as the practical effects of different leaf-picking strategies. This study further emphasizes the importance of optimizing leaf-picking measures in table grape cultivation, and provides a reference for precise vineyard management and fruit quality improvement to meet the constantly changing market demands.

2 Fundamentals and Methods of Leaf Removal

2.1 Definition and common approaches of leaf removal

Leaf removal, also referred to as defoliation or thinning of the leaves, is a farming technique in which there is selective removal of leaves from the grapevine canopy, but within the fruiting zone. The technique is primarily used to improve the microclimate surrounding the grape clusters for improved sunlight exposure and air movement, leading to improved quality of the fruit and reduced incidence of diseases. It can be mechanically or manually removed and is generally applied to basal leaves that cover up clusters, with the specific practice varying depending on purpose and vineyard conditions considered (VanderWeide et al., 2021; Doğan, 2025).

2.2 Straw yield

The removal rate is generally classified as light, moderate, or heavy, depending on the percentage of leaves taken off the canopy. Light removal takes off a few leaves and causes minimal change in the canopy structure. Moderate removal typically takes off several leaves per shoot, especially those immediately above the fruit zone, and is commonly used to maximize fruit exposure and vine vigor. Heavy removal refers to the removal of a significant number of leaves, potentially significantly altering the source-sink regime of the vine. Although moderate removal is most commonly associated with optimal improvement in grape quality, heavy removal might subject vines to overexposure, sunburn, or reduced yield, and hence should be exercised with extreme caution (Aipperspach et al., 2020).

2.3 Effects of different removal positions

The date of leaf removal is significant in determining its effect on grape quality. Basal leaf removal, or removal of leaves close to the base of shoots and surrounding the clusters, is the most popular practice and is shown to be successful in facilitating sunlight exposure, increase phenolic and anthocyanin accumulation, and reduce disease pressure. Removal of middle or upper canopy leaves is not often utilized but may be an option on some training systems or to even further open the canopy. Cluster-zone leaf thinning, for the leaves immediately surrounding the fruit clusters, has also been shown to be particularly effective in improving microclimate conditions for berry growth, promoting sugar uptake, and boosting synthesis of aroma and pigment compounds while exercising care to avoid over-exposure (Tarricone et al., 2020; Quartacci et al., 2022).

2.4 Timing of leaf removal

When to drop the leaves is a crucial aspect in modifying its impact on grape quality and growth. Early defoliation, practiced before or at flowering, can potentially lower compactness of bunches and disease pressure, and overall be associated with more sugar and phenolics, though yield will be lowered through less fruit set (Sivilotti et al., 2016; VanderWeide et al., 2021). Leaf removal during berry growth (fruit set to veraison) may enhance color and volatile development with less risk of fruit loss, while leaf removal later during ripening (veraison to harvest) may enhance color and phenolics with more risk of sunburn and less effect on sugar content. The choice timing should be adjusted according to the corresponding cultivar, climate, and production target (Yue et al., 2020a; Quartacci et al., 2022).

3 Effects of Leaf Removal on Table Grape Quality

3.1 Fruit appearance traits

Removing the leaves significantly enhances several appearance traits of table grapes. Studies show that the process makes the berries and clusters heavier, longer, and wider, resulting in fruits that are larger and better appearing. There are also enhancements in rind color, which has increased lightness and chroma values, correlated with increased anthocyanin deposition due to greater exposure to sun. Better coloration is particularly important for commercial quality, since brighter color is often associated with quality in the consumer's mind. Although less frequently reported, increased skin rupture force and berry detachment force after leaf removal are signs of a tougher, more resistant peel, beneficial to appearance and handling at harvest and transportation (Tarricone et al., 2020; Yue et al., 2020b; Yao et al., 2024).

3.2 Physicochemical indicators

Leaf removal exerts a beneficial effect on principal physicochemical indexes of grape quality. The general improvement in content of soluble solids (SSC, Brix), a measure of increased deposition of sugar in the berries. Sugar-acid ratio and pH rise, titratable acidity decreases, with a resulting sweeter and more palatable fruit. All these are caused through improved microclimate conditions around the clusters which enhance photosynthesis efficiency and transport of sugar to the berries. These improvements in acid and sugar equilibrium are very crucial for both fresh fruit intake and processing quality (Bakhsh et al., 2021; Lanati et al., 2021; VanderWeide et al., 2021; Wang et al., 2021).

3.3 Flavor-related compounds

Leaf removal leads to extreme elevation in flavor volatiles, including aroma volatiles and phenol compounds. Greater sunlight initiates biosynthesis of anthocyanins, flavonols, and other phenolics that contribute to color depth, antioxidant activity, and complex flavor. Accumulation of aroma volatiles like monoterpenes and norisoprenoids is also increased, which improves sensory quality of the grapes. These changes have been linked with the upregulation of important genes in phenolic and aroma biosynthesis pathways, making defoliation a powerful mean to enhance both sensory and nutritional qualities of table grapes (Figure 1) (Alessandrini et al., 2018; Quartacci et al., 2022; Li et al., 2023).

Figure 1 Phenolic compound loading profiles after grape leaf removal; LR-BF: Leaves removed 10 days before flowering, LR-AF: Leaves removed 35 days after flowering, LR-V: Leaves removed at veraison (Adopted from Li et al., 2023)

3.4 Postharvest storage ability and market value

Berry firmness, skin rupture strength, and detachment force following leaf removal enhance the potential for postharvest storage since more firm berries are less likely to get damaged during transport and handling. Higher antioxidant activity and phenolics could further guarantee higher shelf life through lesser oxidative spoilage. All together, these quality advancements build market value for table grapes because they are delivering what consumers demand in terms of appearance, taste, and storage life and reducing postharvest loss for producers (Tarricone et al., 2020; Quartacci et al., 2022).

4 Mechanisms of Sugar Accumulation Influenced by Leaf Removal

4.1 Photosynthesis and assimilate supply

Leaf removal directly alters the photosynthetic pathway of grapevines by reducing the total leaf area available for carbon assimilation. Modest removal within the cluster zone can assist in optimizing light penetration and the microclimate, with the photosynthetic efficiency of the retained leaves being improved and the supply of assimilates to developing berries being increased. Yet, over-removal or excessive reduction of the leaf area-to-fruit weight ratio can limit overall photosynthetic production, retard sugar accumulation, and reduce final sugar concentrations in the fruit (Silva et al., 2017; Wang et al., 2021; Assefa et al., 2025). The balance between sufficient photosynthetic supply and improved microclimate is of utmost significance for optimal sugar accumulation.

4.2 Source-sink relationship regulation

Removal of leaves regulates source-sink balance by transforming the ratio of photosynthetic sources (leaves) to sinks (fruits). Partial leaf area reduction may enhance assimilate distribution towards the berries, promoting sugar accumulation, whereas severe leaf area reduction may result in source limitation, constraining sugar availability to the berries. The duration and severity of leaf excision are also critical: early or moderate treatments induce greater sugar translocation, while severe or late leaf excision may reduce the pool size of assimilates and retard fruit ripening (Silva et al., 2017; Arrillaga et al., 2021; O'Brien et al., 2021) (Figure 2). Besides, berry anthocyanin capacity per berry (expressed as equal malvidin-3-glucoside, A/berry) has been shown to be related closely with leaf-to-fruit ratio and grape yield. These correlations can be modeled with non-linear and linear polynomial models (Figure 2A: A/berry vs. leaf-to-fruit ratio; Figure 2B: A/berry vs. grape yield), which indicate that source-sink balance regulation in the dynamic form affects not only sugar accumulation, but also largely contributes to anthocyanin production.

Figure 2  Relationship between grape leaves and fruit yield (Adopted from Arrillaga et al., 2021) Image caption: EMG: equivalent malvidin glucoside. Non-lineal-regression and linear polynomic model were fitted; A: anthocyanin potential in berry (A/berry) as a function of leaf to fruit ratio; B: anthocyanin potential in berry (A/berry) as a function of yield per vine (Adapted from Arrillaga et al., 2021)

4.3 Regulation of carbohydrate metabolism-related enzymes and genes

Leaf removal, which influences gene expression and enzymatic activity of carbohydrate metabolism in grape berries, was found to increase the transcript level of sugar unloading-related genes such as VvHT3, VvHT5, VvSUC11, VvSUC12, VvSS, and VvcwINV, all of which were positively correlated with reducing sugar and soluble sugar content of berries. These molecular changes facilitate enhanced hexose (glucose and fructose) accumulation, which helps achieve improved fruit quality. The regulation of these enzymes and genes is among the main molecular mechanisms of leaf removal effects on sugar accumulation (Wang et al., 2021).

4.4 Sugar transport pathways and fruit accumulation mechanisms

Sugar deposition in grape berries depends on efficient transport from source leaves to the sink tissues. Leaf removal, particularly in the cluster zone, triggers sugar transporter genes and enhances sugar transporter activity, allowing unloading and deposition of sugars in the fruit. The increased expression of genes such as VvSUC11 and VvSUC12, as well as sucrose metabolic enzymes, is conducive to the quick movement and hydrolysis of sucrose to hexoses within the berry. The sugar transport and metabolism process that is regulated organizes the delivery and storage of the assimilates effectively within the developing fruit, thus directly reaping the advantages of higher sugar content (Wang et al., 2021).

5 Key Factors Affecting the Outcomes of Leaf Removal

5.1 Genetic differences among grape cultivars

Efficacy in leaf removal is, to a great extent, based on genetic difference among grape varieties. Differences in cultivar and rootstock can lead to diverse responses in yield, fruit setting, as well as factors such as sugar content and disease resistance (Yu et al., 2016; Cataldo et al., 2021). Other cultivars are also more susceptible to early defoliation, with larger yield losses or greater increases in soluble solids, while others react minimally. Genotype x management interaction is therefore a critical factor influencing the success of leaf removal practices (Lanati et al., 2021; VanderWeide et al., 2021).

5.2 Environmental conditions

Environmental factors such as light, temperature, and water regime significantly impact the consequences of defoliation. Anthocyanin and flavonol production may be encouraged through higher light exposure caused by defoliation but can cause sunburn or degradation of delicate compounds under high heat. Water status also comes into play with leaf removal, as water stresses can augment or diminish the effects on yield and berry quality. Environmental stresses at various points prior to and following leaf removal will hence alter both the risks and benefits of such an operation (Cincotta et al., 2021; Yu et al., 2021; Yao et al., 2024; Verdenal et al., 2025).

5.3 Combination of timing and intensity of leaf removal

When and how much to drop the leaves are critical factors that determine its impact on grapevine physiology and fruit quality. Early drop, e.g., pre-bloom or flowering, can restrict cluster compactness and disease but, in turn, reduce yield if excessive. Sooner removal nearer to veraison has lesser impact on yield but can also improve berry composition. The intensity of leaves removed must be properly controlled; moderate removal is most often optimal between enhancing quality and reducing risk, with excessive removal likely harming sugar accumulation and enhancing vulnerability to environmental stress (Aipperspach et al., 2020; Verdenal et al., 2025).

5.4 Coordination with other orchard management practices

Leaf removal impacts also depend on how the practice is combined with other orchard management techniques such as cluster thinning, pruning, and irrigation. Combination of leaf removal with yield control techniques can attain an optimal leaf-to-fruit ratio, increasing fruit quality and reducing disease incidence. Alteration in canopy height or pruning levels can counterbalance removed leaf area and maintain vine equilibrium and productivity. Integrated management techniques that consider the interaction of multiple practices play a significant role in maintaining consistent and desirable results in grape quality and yield (Verdenal et al., 2024).

6 Research Progress and Regional Comparisons in Leaf Removal Studies

6.1 Major findings and case studies from international research

Worldwide studies ever give evidence that leaf removal, especially pre-bloom, reduces bunch rot disease and °Brix total soluble solids in grapes and improves fruit quality. Meta-analysis of global research determined that pre-bloom leaf removal lowered bunch rot by 61% and Brix by 5.2%, and the most beneficial effects were found in cultivars and rootstocks that were sensitive to the practice. However, the impact on pH, titratable acidity, and content of phenols is inconsistent, with anthocyanin increases reported in a few studies, and there being no consistent differences in others. The effectiveness of leaf removal is determined by the timing, the variety, and local weather, but the contribution of climate is usually less than the contribution of genes. European, North American, and South American case studies cite the success of this method in disease control and quality assurance but with yield loss as a common compromise (VanderWeide et al., 2021).

6.2 Practical experience and limitations in domestic studies

In past decades, studies in China and other domestic regions have shown that leaf removal at the fruit-zone can improve anthocyanin and flavonols contents in grape cultivars such as Cabernet Sauvignon and Marselan, while inducing limited effects on soluble solids content and berry weight (Lu et al., 2021; Li et al., 2023; Yao et al., 2024). In Jieshishan in eastern China, where there is rainy summer weather, leaf removal was found to lower titratable acidity of grapes but had no big effect on soluble solids content. At the same time, this treatment consistently enhanced anthocyanin and flavonol accumulation in berries and, consequently, grape color potential. However, in Cabernet Sauvignon, early defoliation negatively affected the synthesis of carotenoid-derived compounds (e.g., β-damascenone), showing varietal effects.

The findings also underscore the importance of adapting defoliation operations to local conditions, since too much sun exposure and heat could lead to sun scald of berries or loss of desirable volatile compounds. The constraints are fluctuating effects based on vintage and cultivar and the need for further optimization of defoliation timing and severity in different grape varieties and climatic conditions (Figure 3).

Figure 3 Meteorological conditions and grape physicochemical parameters; daily maximum, average, and minimum temperatures (lines) and precipitation (histograms) recorded from 1 July to 1 October at the Chateau Langes weather station in 2021 (A) and 2022 (B). Time axis of Cabernet Sauvignon and marselan berry development and ripening in 2021 (C) and 2022 (D). (Adopted from Yao et al., 2024)

6.3 Varietal and regional differences in outcomes

Varietal and regional difference is also crucial in the decision of leaf removal effects. For example, in Italy and China, studies confirmed that anthocyanin and flavonol accumulation response to leaf removal is highly genotype-dependent, with some increase in flavonols in cultivars and no change in anthocyanins. In semi-arid conditions like those of Xinjiang, distal (upper canopy) leaf removal is used to retard ripening and mitigate global warming, resulting in increased flavonol content and improved aroma but sometimes at the expense of postponed sugar accumulation. In humid conditions, however, early leaf removal would be more useful in managing disease and controlling yields. These findings emphasize the need for region- and variety-specific leaf removal protocols to maximize grape quality and production performance (Pastore et al., 2017; Arrillaga et al., 2021; Lu et al., 2022).

7 Applications of Leaf Removal in Table Grape Cultivation

7.1 Case studies of improved fruit appearance and quality

Recent research demonstrates that leaf removal, especially when it is combined with other summer pruning, significantly enhances table grape quality and appearance. For example, research on the Alphonse Lavallée variety shows that leaf removal enhances cluster and berry weight, length, and width, along with lightness and chroma parameters. These changes result in more attractive fruit with thicker skins and increased handling resistance, a requirement for marketability. Leaf removal also improves the overall phenolic content and antioxidant activity, adding another dimension to fruit quality and consumer attraction (Doğan, 2025). For the Michele Palieri variety, leaf removal and its combination with topping at various phenological stages are recommended for the realization of quality table grapes because the operations maximize the exposure of leaves and improve yield and berry attributes (Korkutal et al., 2022).

7.2 Strategies for enhancing sugar accumulation and flavor

Leaf removal is a major method of enhancing sugar accumulation and flavor in table grapes. Early leaf removal, particularly before or at bloom, increases total soluble solids (Brix), pH, and the maturity index but decreases titratable acidity. This makes the fruit sweeter and more flavorful. The practice also improves the accumulation of phenolic compounds and aroma volatiles that are responsible for flavor complexity. Combining leaf removal with reflective mulch or bunch tipping also allows more sunlight to penetrate the canopy, which results in improved skin color and higher sugar content. The most ideal method of improving sugar and flavor is often with combined approaches, e.g., combining early leaf removal with reflective mulch or bunch tipping (Kok, 2022).

7.3 Synergy with other vineyard management practices

Removal of leaves plays a synergistic role with other vineyard practices to optimize table grape quality. When combined with fruit thinning or cluster thinning, leaf removal provides further emphasis to berry size, sugar content, and phenolic composition enhancement, as well as reduced disease incidence such as bunch rot. Synthesis of leaf removal with pest management resources such as targeted insecticide applications will also repress pest populations and disease pressure, supporting sustainable production. Leaf stripping period and intensity, if coordinated with other management practices like topping or reflective mulches, allow growers to tune canopy microclimate, maximize fruit quality, and minimize yield loss (Prazaru et al., 2023).

8 Concluding Remarks

Leaf removal is an essential viticultural process that has a significant effect on table grape quality and sugar deposition. Research indicates that well-timed and precisely placed defoliation can enhance cluster light exposure, enhance the microclimate of the fruit, and regulate source-sink relations to produce enhanced berry color, higher sugars, and enhanced flavors. Mechanistically, leaf removal affects photosynthetic efficiency, sugar metabolism, and gene expression associated with sugar transport and accumulation. The responses are environment-specific, cultivar-specific, and dependent on the timing and intensity of defoliation, which points to its specificity and delicacy of effects.

Leaf removal in table grape cultivation provides real benefits to the growing process, as well as to quality improvement. It results in more uniform ripening, reduced disease development, and improved postharvest performance. Through its alteration of berry physiological and biochemical activity, leaf removal is an inexpensive and environmentally friendly means of improving fruit quality, making it easier for growers to meet market specifications for sweetness, appearance, and high-quality table grapes.

Future research in leaf removal methods will probably focus on precision management, with cultivar-specific response, environmental monitoring, and mechanized or automated defoliation systems being integrated. The synthesis of physiological, molecular, and digital vineyard data has the promise to enable the optimized schedule of leaf removal to improve quality and efficiency. These practices have the potential for extensive use in commercial table grape production, with benefits for sustainable vineyard management, fruit quality improvement, and economic return across the industry.

Acknowledgments

The authors sincerely acknowledge the research team for their invaluable assistance and support throughout the study's development and documentation process, whose contributions provided crucial support for the successful completion of this research. Additionally, the authors extend gratitude to the two anonymous peer reviewers for their insightful comments and suggestions during the manuscript review process.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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