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

28 BARLEY: TAXONOMY, MORPHOLOGY, AND ANATOMY
cellulose that lies between two cuticularized layers. The outer cuticle layer is
thicker than the inner layer. The testa varies in thickness, being thicker over
the sides of the furrow and at the apex of the kernel. It is thinner on the sides,
over the embryo, and at the micropyle. At the ventral furrow, the testa merges
with the pigment strand that runs the length of the kernel, providing a seal at
the edges. The remains of the nucellus tissue that surrounded the megaspore and
megagametophyte lie just under the testa.
The endosperm is the largest portion of the kernel, consisting of the aleurone,
the subaleurone, and starchy endosperm. The aleurone layer is a more
intricate structure than the outer integuments. Aleurone tissue is a protein-rich
layer of cells found immediately under the nucellus and testa. The cells are separated
by thick cell walls and are filled with dense cytoplasm with prominent
nuclei, organelles, endoplasmic reticulum, mitochondria, microbodies, proplastids,
lipid-containing spherosomes, and complex spherical aleurone grains. Aleurone
grains are storage granules containing protein, phytic acid, and hydrolytic
enzymes (Briggs 1978). Two large-molecular-weight polysaccharides, arabinoxylan
and β-glucan, are structural components of the thick cell walls of the aleurone.
The cellular structure immediately adjacent to the aleurone, designated the
subaleurone, is actually more consistent in composition with the starchy endosperm
than with the aleurone. The cells of the subaleurone are smaller than those
of the starchy endosperm and contain more protein and less starch, which is
particularly noticeable in high-protein barleys. The starchy endosperm, making up
the largest portion of the kernel, contains cells that are packed with starch grains
that are embedded in a protein matrix. Protein is also stored in protein bodies in
the protoplasm of the starchy endosperm. The cell walls of the starchy endosperm
contain the same two polysaccharides found in the cell walls of the aleurone
tissue, but there is a greater amount of β-glucan compared to arabinoxylan.
The embryo, a very complex section of the kernel, is located on the dorsal
side of the caryopsis at the end attached to the rachis. The embryo contains
the material necessary for initiating the growth of a new plant. It is comprised
of the embryonic axis, the plumule, and the radicle, which are surrounded by
the coleoptile and coleorhiza, which act as protective sheaths for these tissues.
A bud primordial is also located within the coleoptile. The scutellum is a flat
protective tissue positioned between the embryo and the endosperm. On the outer
side it is recessed so as to fit next to the embryonic axis, and on the inner side
it fits against the endosperm. The subcellular constituents of the embryo include
mitochondria, protein bodies, spherosomes that contain lipid, Golgi bodies, and
rough endoplasmic reticulum, large nuclei, and thin cell walls transversed by
plasmodesmata (Briggs 1978). The rachilla, a bristlelike hairy structure lies within
the crease on the ventral side of the kernel and is attached at the base where the
kernel is attached to the rachis.
The mature barley grain is an elongated oval structure, although more spherical,
globular kernels exist among the genotypes. In most cultivated barley, the
dorsal side and laterals are rounded and cumulate on the ventral side in a crease
sometimes referred to as the furrow or groove (Figure 2.7). The hull comprises