*3.1. Phenotypic Variation of Mucilage and Hull Contents*

Flaxseed mucilage and seed hull possess valuable nutritional and rheological attributes [33,34] but are also known to affect animal performance [7]. The presence of mucilage and fiber components (i.e., acid detergent lignin) in flaxseed meal reduces the energy uptake in both monogastric and ruminant animals [35]. Therefore, knowledge about the phenotypic variation and genetic control of seed mucilage content (MC) and hull content (HC) is pivotal to better design breeding strategies aiming to improve the overall food and feed value of flaxseed. The broad phenotypic variation of MC and HC in the association panel and the degree of additivity of the genetic components hint at the potential for improving flaxseed for either high or low MC and reduced HC through marker-assisted selection.

Kaewmanne et al. [4] reported MC ranging from 1.8 to 2.9% in seven Italian flaxseed cultivars, while Oomah et al. [16] found that MC ranged from 3.6 to 8.0% in 109 flaxseed accessions. We found a slightly wider range from 2 to 10% in our diversity panel. Little information exists for HC variation in large collections of flaxseed. In general, HC ranges from 22–27% to 36–48% were reported in mechanically treated and hand-dissected seeds, respectively [7,36], which is much higher than canola at 18.6% and soybean at 16.1% [6]. Reduction of HC can be achieved through the use of yellow-seeded cultivars, known to contain higher oil content and less HC than their brown-seeded counterparts [7,37]. Indeed, the yellow-seeded accessions displayed a lower HC compared to the brown-seeded genotypes. Nevertheless, caution should be exercised in adopting yellow-seeded flaxseed cultivars for reduced HC flaxseed because their susceptibility to natural splitting and mechanical cracking of the seed coat can negatively affect seed quality [38]. Consequently, breeding and seed tests to mechanical damage during harvesting should be conducted together in order to identify the ideal HC that would ensure seed mechanical resistance. All considered, our association panel harbored abundant phenotypic variation for dissecting the genetic landscape of MC and HC.
