Search

Phloridzin is a phenolic compound unique to apple (Malus domestica (Suckow) Borkh.) and its wild relatives. Since its discovery, phloridzin has been researched for its nutraceutical properties, including anti-diabetic, anti-cancer, and antioxidant activities, making phloridzin a potential target for nutritional improvement in new apple cultivars. However, phloridzin accumulates at significantly lower concentrations in fruit than in vegetative tissues and seeds. In ‘Golden Delicious’ and its sports, we observed higher phloridzin content in peels of sports with a cuticle disorder termed russet. In russeted apples, the smooth, waxy fruit cuticle is partially or entirely replaced by a corky layer, induced through environmental and genetic effects. To understand the variation of phloridzin content and fruit russet in apple fruit, we surveyed 108 accessions with variation in russeting from the USDA-ARS Malus germplasm collection in Geneva, NY. Russeting in apple fruit ranged from 0 to 100%, and phloridzin content ranged from 24.3 to 825.0 μg/g in peels. Mean phloridzin content varied significantly between russeting groups; in groups with light (0–5%), medium–high (70–80%), and high (90–100%) russeting mean phloridzin content was 115.2, 591.2, and 378.8 μg/g, respectively. We observed that genetic factors and russeting are strong predictors of phloridzin content in peels, but not fruit flesh or leaves. Conversely, other peel phenolics are negatively associated with russeting. We observed variable phloridzin content related to russet incidence during fruit development in ‘Golden Delicious’ (low to medium russet) and its sports, ‘Empress Spur’ (low russet), ‘Razor’ (complete russet), and ‘Sergeant Russet’ (medium to high russet).

Dihydrochalcones, beneficial phenolic compounds, are abundant in Malus Mill. species, particularly in vegetative tissues and seeds. Phloridzin (phloretin 2′-O-glucoside) is the primary dihydrochalcone in most Malus species including cultivated apple, Malus × domestica Borkh. A few species contain sieboldin (3-hydroxyphloretin 4′-O-glucoside) or trilobatin (phloretin 4′-O-glucoside) in place of phloridzin, and interspecific hybrids may contain combinations of phloridzin, sieboldin, and trilobatin. Proposed health benefits of phloridzin include anti-cancer, antioxidant, and anti-diabetic properties, suggesting the potential to breed apples for nutritional improvement. Sieboldin and trilobatin are being investigated for nutritional value and unique chemical properties. Although some of the biosynthetic steps of dihydrochalcones are known, little is known about the extent of variation within Malus germplasm. This research explores the genetic diversity of leaf dihydrochalcone content and composition in Malus germplasm. Dihydrochalcone content was measured using high performance liquid chromatography (HPLC) from leaf samples of 377 accessions, representing 50 species and interspecific hybrids from the USDA-Agricultural Research Service (ARS) National Plant Germplasm System Malus collection. Within the accessions sampled, 284 accessions contained phloridzin as the primary dihydrochalcone, one had only trilobatin, two had phloridzin and trilobatin, 36 had sieboldin and trilobatin, and 54 had all three. Leaf phloridzin content ranged from 17.3 to 113.7 mg/g with a heritability of 0.76 across all accessions. Beyond the potential of dihydrochalcones for breeding purposes, dihydrochalcone composition may be indicative of hybridization or species misclassification.

Apple (Malus × domestica Borkh.) is one of the top three US fruit crops in production and value. Apple production has high costs for land, labor and inputs, and orchards are a long-term commitment. Production is dominated by only a few apple scion and rootstock cultivars, which increases its susceptibility to dynamic external threats. Apple crop wild relatives, including progenitor species Malus sieversii (Ledeb.) M. Roem., Malus orientalis Uglitzk., Malus sylvestris (L.) Mill., and Malus prunifolia (Willd.) Borkh., as well as many other readily hybridized species, have a wide range of biotic and abiotic stress resistances as well as desirable productivity and fruit quality attributes. However, access to wild materials is limited and wild Malus throughout the world is at risk of loss due to human encroachment and changing climatic patterns. The USDA-ARS National Plant Germplasm System (NPGS) Malus collection, maintained by the Plant Genetic Resources Unit in Geneva, NY, US is among the largest collections of cultivated apple and Malus species in the world. The collection currently has 5004 unique accessions in the field and 1603 seed accessions representing M. × domestica, 33 Malus species, and 15 hybrid species. Of the trees in the field, 3,070 are grafted and are represented by a core collection of 258 individuals. Many wild species accessions are represented as single seedlings (non-grafted). The crop vulnerability status of apple in the US is moderate because although there are a few breeders developing new commercial cultivars who also access Malus species, threats and challenges include new diseases, pests, and changing climate combined with industry needs and consumer demands, with a limited number of cultivars in production.