Research progress on peeling characteristics of kiwifruit

Kiwifruit (Actinidia spp.) is a globally important horticultural crop, valued for its unique flavor and outstanding nutritional properties, especially its high vitamin C content. Fruit quality is the core focus of postharvest research and is assessed by a complete set of external (e.g., size, color, shape) and internal (e.g., flavor, sugar- acid ratio, soluble solids, nutritional components) attributes. However, peelability, that is, the difficulty level of peeling the fruit skin, has relatively few systematic studies. Current research is mainly focused on easily peeled fruits such as citrus and bananas. For kiwifruit, poor peelability is a common issue among commercially dominant cultivars (e.g., A. chinensis var. chinensis and A. chinensis var. deliciosa). Manual peeling with these cultivars often causes flesh damage and juice loss, which impairs consumer experience and may reduce consumers’repurchase intention. As consumer demand for convenient, ready-to-eat fruit increases, enhancing peelability has emerged as a crucial breeding objective to improve industry efficiency. Actinidia eriantha, a unique Chinese kiwifruit species characterized by dense white fruit trichomes, is a key genetic resource for breeding due to its remarkably easy-peeling phenotype. This review synthesized recent research on the peeling characteristics of kiwifruit, with particular emphasis on A. eriantha, to construct a preliminary theoretical framework and provide a basis for elucidating its underlying mechanisms. The peelability of fruit is a complex trait influenced by the interplay of structural, physiological, and molecular factors. Anatomically, kiwifruit peel exhibits considerable interspecific diversity. A. arguta (hardy kiwifruit) has a smooth, edible skin with a relatively simple structure comprising a cuticle, epidermis, and hypodermis. In contrast, the peels of A. chinensis and A. deliciosa are more structurally complex, featuring a periderm (formed from collapsed epidermal and hypodermal cells, layers of thick-walled sclereids, and embedded condensed phenolics—all of which contribute to their toughness and adhesion to the flesh. Actinidia eriantha peel is structurally distinct: it is covered with abundant trichomes and a layer of non-collapsed dead surface cells, which overlie a well-developed hypodermis consisting of multiple layers of thick-walled cells. Interestingly, comparative studies between easy-peeling and difficult-peeling genotypes of A. eriantha have revealed no consistent, distinct histological separation layer at the peel-pulp interface, suggesting that peelability is not primarily governed by gross anatomical differentiation. Evaluation of peelability has evolved from subjective metrics such as“number of peeling strokes”and“peeling score”(based on separation cleanliness) to more objective measurements of peeling force obtained using mechanical force gauges. An integrated assessment system utilizing these three parameters has been established, which clearly distinguishes easy-peeling phenotypes (requiring low force, few strokes, and clean separation) from difficult-peeling ones. From a physiological perspective, peelability is regulated by multiple factors. Genetic background is the fundamental determinant, and considerable variation has been observed both within and across species. This is clearly demonstrated by the high diversity of peeling scores among wild A. eriantha germplasm. Plant growth regulators and calcium play modulating roles. Ethylene and abscisic acid (ABA) promote fruit softening, which is a prerequisite for easy peeling in many fruits, while auxins such as NAA can increase peel thickness and delay softening. Calcium ions strengthen cell walls by cross-linking with pectin, which can potentially increase peel integrity. The most critical physiological determinant appears to be the dynamic remodeling of the cell wall, especially the pectin matrix. During fruit ripening, the degradation of cell wall polysaccharides by enzymes such as polygalacturonase (PG), pectin methylesterase (PME), and β-galactosidase leads to loosening of the cell wall and reduced cell-to-cell adhesion. In citrus, easy-peeling cultivars have a more porous albedo (the white inner layer) with abundant intercellular spaces, whereas difficult- peeling cultivars have a more compact albedo. For kiwifruit, research indicates that peelability is closely associated with specific changes in pectin chemistry and distribution, rather than extensive cell wall degradation. Immunolocalization studies show that the peel of difficult-to-peel A. deliciosa Hayward is rich in lowesterified pectin (which forms strong calcium bridges) throughout the entire tissue. In contrast, in an easy- peeling hybrid (A. deliciosa × A. eriantha), such pectin is concentrated only near the epidermis. This pattern may allow the peel to stay intact while separating cleanly from the flesh. Furthermore, easypeeling A. eriantha genotypes exhibit less overall degradation of cell wall components during softening than difficult- peeling A. chinensis and A. deliciosa. At the molecular level, existing research on this mechanism remains nascent, but has yielded revealing insights. Proteomic analysis revealed limited differential protein expression between easy-peeling and difficult-peeling A. eriantha genotypes. Notably, while PG genes (PGC1, PGC2) were expressed at higher levels in easy-peeling types, no corresponding PG enzyme activity was detected. This finding suggests that pectin solubilization is limited, which may help maintain peel integrity during peeling. A key finding is the significantly higher activity of xyloglucan endotransglucosylase/hydrolase (XTH) in easy- peeling genotypes. XTH remodels hemicellulose networks, which can potentially enhance the flexibility and elasticity of fruit peels and prevent tearing during the peeling process. Genes encoding β-galactosidase (Adβgal-1, Adβgal-2) and pectin methylesterase have been implicated in kiwifruit softening. More recently, integrated transcriptomic and metabolomic analyses of A. eriantha‘Ganlü 1’during fruit peeling have identified candidate genes associated with the peeling process-including AeGAE6, AebHLH35-like, and AeBGAL5-and linked these genes to metabolomic shifts in sugars and sugar acids related to cell wall metabolism. In conclusion, unlike ab-scission processes or the structured separation layers in citrus, the peelability of A. eriantha likely arises from a subtle, chemically regulated weakening at the peel-pulp junction. This process involves specific modifications to pectin esterification and hemicellulose remodeling, which are coordinated by hormonal signals. Future research should employ multi-omics approaches combined with physiological assays to fully delineate the genetic networks and metabolic pathways that govern this valuable trait. A deeper understanding of the easy-peeling mechanism in A. eriantha will provide essential theoretical support and molecular tools for the targeted breeding of novel, consumer- friendly kiwifruit varieties, thereby enhancing the competitiveness and sustainability of the kiwifruit industry.