Breeders usually focus on yield and quality traits in flax breeding. Yield-related traits include plant height, length, number of branches, number of fruits per plant, grain weight per plant, 1000-grain weight, and number of fruit grains. The quality traits of flax include crude fat and fatty acids, lignin, flax gum, and protein (Zhang et al. 2017; Deng et al. 2014; Guo et al. 2019). DUS test traits are mainly morphological characteristics that are determined by a specific genotype or combination of genotypes and show sufficient consistency and reproducibility under specific environmental conditions (Wang et al. 2022; Ji et al. 2022; Deng et al. 2020; Wang et al. 2023b). The trait expression of varieties in specific ecological areas can be evaluated via DUS tests. The flax guidance (NY/T 2562 − 2014 (2014)) includes information not only on yield and quality traits but also on other traits. These include the number of sepal spots, petal size, anther color, filament color, style color, relative position of the petal, and capsule septum cilium. These traits have rarely been used in previous studies. Although these traits cannot provide information related to variety yield and quality, they can still exhibit variety diversity and genetic variation; can be clearly defined, accurately identified and clearly described; and can provide a more comprehensive, objective and accurate description and comparison of varieties. In this study, 30 flax varieties were identified using 18 DUS morphological characteristics. The variation coefficient ranged from 5.42–39.63%, which indicated that the traits were less affected by the environment and could be preferentially selected for breeding improvement.
SSR markers have the advantages of stable amplification, wide distribution, high polymorphism and simple manipulation and are widely used in crop genetic diversity studies. In this study, 20 pairs of SSR primers were used to analyze the polymorphisms of 30 flax varieties. A total of 90 polymorphic bands were amplified, of which the number of alleles per primer pair ranged from 2 to 10, with an average of 4.5 bands per primer pair. The PIC was correlated with allele frequency and allele number, which reflects the degree of variation among varieties. When the PIC was greater than 0.5, the loci exhibited high diversity. When the PIC was between 0.25 and 0.5, the diversity of the loci was moderate, and when the PIC was less than 0.25, the diversity of the loci was low. Therefore, the higher the PIC value is, the stronger the identification ability. The PIC values of the SSR markers selected in this study were estimated to be between 0.25 and 0.82, with an average of 0.51, indicating that the SSR markers had high identification ability for flax. These SSR markers can be applied to study the genetic diversity, variety identification and variety protection of flax germplasm resources.
Although molecular markers can play an important role in variety identification, the existing molecular markers cannot replace morphological characteristics. Morphological characteristics are still widely used for DUS testing. In other words, in DUS testing, it is impossible to use molecular markers to construct DNA fingerprints to completely replace morphological characteristics. No crop species has a trait association marker that can be used for all the traits described by DUS. Different varieties with different DNA fingerprints may exhibit the same morphological characteristics, whereas different varieties with the same DNA fingerprints may exhibit significant morphological differences (for example, HM20 versus HM25). Moreover, the high sensitivity of the molecular marker technique, the number of individuals needed for consistency testing, and the method used to distinguish the allowable errors of different individuals need further exploration and research. As a result, molecular markers can help to identify the distinctness of candidate varieties but not to determine uniformity and stability.