Russeting is an economically important surface disorder in apple (Malus × domestica Borkh). Indirect evidence suggests an irregular skin structure may be the cause of the phenomenon. The objective of this study was to characterize epidermal and hypodermal cell morphology and the mechanical properties of the skins of apple cultivars of differing russet susceptibility. Dimensions of epidermal and hypodermal cells were determined using microscopy. Stiffness (S), maximum force ($F_{max}$), and maximum strain (${\mathsf{\epsilon }}_{max}$) at failure were quantified using uniaxial tensile tests of skin strips. Particularly during early fruit development, epidermal cells (EC) and hypodermal cells (HC) in russet non-susceptible cultivars occurred in greater numbers per unit area than in russet-susceptible ones. The EC and HC were lower in height, shorter in length, and of reduced tangential surface area. There were little differences in S or $F_{max}$ between non-susceptible and susceptible cultivars. However, the ${\mathsf{\epsilon }}_{max}$ were higher for the skins of non-susceptible cultivars, than for those of susceptible ones. This difference was larger for the young than for the later growth stages. It is concluded that russet-susceptible cultivars generally have larger cells and a wider distribution of cell sizes for both EC and HC. These result in decreased ${\mathsf{\epsilon }}_{max}$ for the skin during early fruit development when russet susceptibility is high. This increases the chances of skin failures which is known to trigger russeting.

Black soybean is an important functional crop valued for its high anthocyanin content and associated health benefits, attracting increasing interest in plant breeding and mutation-based approaches to enhance its nutritional and agronomic traits. Here, we investigated a DT26 black seed-coat mutant (DT26BS) derived from the Vietnamese cultivar DT26 following gamma irradiation. Genetic analysis of F₂ populations indicated that the phenotype is controlled by a single recessive mutation, with additional epistatic interactions observed in other genetic backgrounds. MutMap analysis based on whole-genome sequencing of pooled F₂ individuals identified a candidate region on chromosome 8 corresponding to the I locus. A large deletion (~176 kb) was identified in this region, affecting multiple Chalcone Synthase (CHS) gene repeats, and which may disrupt RNAi-mediated silencing of CHS, thereby triggering anthocyanin restoration in the seed coat. PCR-based markers confirmed tight linkage between this deletion and the black seed-coat phenotype. Crosses with elite lines and further selection until F7 generations showed that the mutation has no adverse effects on major agronomic traits and is useful as a donor for developing improved lines with black seed coat, shorter maturity, high yield, and enhanced anthocyanin content for nutritional improvement. These results demonstrate the utility of MutMap for detecting irradiation-induced structural variants and extend its application to a non-reference elite tropical soybean background, providing a useful genetic resource for black soybean breeding.