Identifying genotypic risk factors for grain skinning in malting barley
Grant, Kathryn Rosemary
Barley grains have a protective outer husk which is firmly adhered to the caryopsis at maturity. Mechanical processing can cause husk loss, leaving the caryopsis exposed. This is known as grain skinning. The malting industry considers skinning to be a serious flaw in grain quality. Malting skinned grains is highly inefficient due to increased embryo damage and uneven modification (Okoro et al., 2017; Meredith, 1959). Grain skinning is affected by both genotype and the environment (Brennan et al., 2017b, Brennan et al., 2017a). This thesis aims to understand what causes the variation in grain skinning between genotypes. A panel of 16 spring malting barley varieties were used to understand what causes grain skinning. Previous research has assumed that grain skinning is caused by adhesion failure. Here, adhesion failure was insufficient to explain grain skinning. Instead, material failure, through husk fragmentation, was sufficient to explain grain skinning. The area affected was strongly influenced by genotype and husk organ. Barley varieties were also screened for risk factors. During grain filling, the husk developed cracks in some genotypes. Husk cracks significantly increased the severity of the grain skinning phenotype, by accelerating husk fragmentation. The relationship between them was not causal, but the correlation between them could be explained by husk material properties. Husk material properties was influenced by structure and composition. Structure was assessed using husk thickness and tissue organisation. Husk thickness was associated with differences in husk fragmentation and grain skinning between genotypes and husk organs. Composition was assessed using a mutant screen. Reduced cellulose, hemicellulose or lignin content was associated with increased husk fragmentation and grain skinning. Increasing hemicellulose content had a protective effect and was associated with reduced husk fragmentation and grain skinning. Using a mutant screen approach demonstrated that genetic changes in the cell wall and starch pathways were sufficient to change grain skinning risk. In conclusion, as modern malting barley varieties are selected to have thin husks and low levels of the cell wall hemicelluloses, (1→3, 1→4)–β–D–glucan and arabinoxylan, continued breeding for malting performance may increase grain skinning risk in future genotypes.