Barley for Food and Health: Science, Technology, and Products

GENETICS AND NUTRIENT COMPOSITION 67
It is important to note that although the general expectation is an increased
level of β-glucan in waxy barley, experimental waxy lines have been developed
that contain a genetic factor responsible for reduced β-glucan content
(Swanston 1997). These barleys were developed by crossing Chalky Glenn, a
barley cultivar having thin endosperm cell walls (a characteristic associated with
low β-glucan levels), with a waxy cultivar. In an earlier study, Aastrup (1983)
obtained two low-β-glucan barley lines by treating Minerva with a mutagenic
chemical. Endosperm cell walls in the mutant lines were much thinner than
those of the parent barley, 3.0 μm versus6.5 μm. β-Glucan levels in the two
mutant barleys ranged from 2.0 to 2.7% compared to 4.9 to 5.9% in the parent
barley, which had much thicker cell walls. The reports by Aastrup (1983) and
Swanston (1997) suggest that there are distinctive genes controlling cell wall
thickness that can be manipulated to achieve low β-glucan and potentially higher
β-glucan levels.
The single most important genetic factor affecting the NSP content of barley
grain is the Nud (hulled)/nud (hulless) gene, located on chromosome 1(7H). This
characteristic is simply inherited and is probably one of the most easily discernable
genetically controlled traits in barley. This factor controls the presence or
absence of the hull on the caryopsis (Nilan 1964). In the presence of the Nud
gene, the lemma and palea adher tightly to the caryopsis, remaining intact during
harvesting, whereas in the presence of the nud trait, they are loosely attached
and most are removed at harvest. The cementing substance causing hull adherence
in hulled barley is secreted by the caryopsis and appears about 16 days
after pollination. Contact between the hulls and the pericarp is not established
completely until grain filling is almost complete, which is about 10 days before
a kernel reaches its maximum size (Harlan 1920). The hulls then adhere to the
pericarp epidermis except over the embryo (Gaines et al. 1985). Major differences
in the extent of looseness of the hull attachment in hulless types have been
noted in certain genotype backgrounds (R. T. Ramage, personal communication).
Barley breeding techniques have been used to produce nearly isogenic isolines of
hulled and hulless barley in several genotype backgrounds (Hockett 1981). With
the removal of the hull, components of the aleurone, endosperm, and embryo
increase in relative proportions due to the removal of cellulose, lignin, and silica
in the hull (McGuire and Hockett 1981). For example, starch and β-glucan
contents were increased by 9.7 and 6.8%, respectively, while TDF was reduced
by 28.8% in nonwaxy hulless isotypes of Compana and Betzes barleys (Xue et
al. 1997). Low- and high-β-glucan barleys were studied by Zheng et al. (2000).
Low-β-glucan hulless barleys had a greater concentration of β-glucans in the subaleurone
layer, with slightly declining amounts toward the center of the kernel. In
high-β-glucan hulless cultivars, the β-glucans were evenly distributed throughout
the entire endosperm.
The major effects of the nud and wax genes on carbohydrate composition are
shown in Table 4.2. The data shown are typical of most hulled, hulless, waxy
and nonwaxy barley, but large variations from these values have been reported
(Hofer 1985; Danielson et al. 1996; Zheng et al. 2000).