The Biology of the Barley Plant
The scientific name of barley is Hordeum vulgare L. Barley is a flowering plant belonging to the family Poaceae (the grasses). In addition to barley, the grass family includes the crops most important to human existence, including rice, wheat, and maize, and other species such as sorghum, oats, rye, millet, and sugarcane. In European contexts, barley grain is often referred to as “corn.” The genus Hordeum includes approximately thirty species that are indigenous to at least four continents. Barley is the only domesticated species to have emerged from the genus Hordeum, in contrast to other crop genera such as Triticum (the wheats) and Phaseolus(the dry beans), which each contain several domesticated species. The other members of the genus Hordeum exist as wild plants.
Growth and development. The life cycle of the barley plant first begins with seed germination underground. The first visible sign of germination is root emergence, followed by the emergence of the cylindrically shaped coleoptile, which is the first structure to appear above ground. Interestingly, most above-ground tissues of the barley plant initially develop from the crown, a structure located below ground. The first leaf grows upward within the cylinder of the coleoptile, and emerges above the soil. This and other leaves do not expand along their entire length; rather the outer section (the blade) does so, while the base (the sheath) remains formed in a hollow cylinder. New leaves emerge in succession up through the sheaths of the older leaves.
The barley plant is not restricted to the development of one main stem, as observed in maize. Rather, like most other small-grain cereals such as wheat and rice, barley produces several additional secondary stems termed tillers that emerge up from the crown beginning a few weeks after the emergence of the main stem. The number of tillers produced varies depending upon the barley genotype and the environment. For instance, under highly fertile conditions, plants will produce more tillers than if nutrient starved.
Reproduction. Barley varieties are classified as spring or winter types, depending on whether they need a cold treatment, ranging from two to several weeks before making the transition to the reproductive phase of growth. When barley switches to its reproductive phase, the true stems of some of the tillers, called culms, elongate upward. The flowering structure, known variously as the spike, ear, or head, is borne upward, ultimately emerging from the “boot,” which is the sheath of the uppermost leaf on the culm (the flag leaf). The height of a barley plant when the spike has emerged varies greatly, but averages approximately eighty centimeters. The spike consists of a large number of individual flowers called florets, which are present in individual spikelets that are attached to a central stemlike structure called the rachis. Spikelets are attached in groups of three on opposite sides of the rachis. The total number of spikelets on a spike varies, but averages approximately sixty. The awns are a notable feature of the spike. They are hairlike extensions that emerge upward from the lemma, one of two thin sheets of cells that surround the floret (the other is the palea). Awn length varies considerably among barley varieties.
Barley is also classified based upon the fertility of the florets on the spike. In six-rowed barleys, all of the florets are fertile, leading to six vertical rows of seeds on the spike. In contrast, in two-rowed types only the central floret of the three at each node is fertile, and thus just two rows of seeds develop on opposite sides of the rachis. The fertile florets consist of both male and female reproductive structures, and fertilization occurs as the spikes are emerging from the boot. Barley is thus predominantly self-pollinated. Between twenty-five and sixty seeds per spike are produced, and for spring barley the seed matures three to five months after planting.
Characteristics of the barley grain. Barley seeds are approximately eight millimeters in length and weigh approximately fifty milligrams when mature, though there is a considerable range in these values between varieties. In most cases, the harvested barley grain includes the seed, a small structure called the rachilla, and both the palea and lemma, all of which adhere firmly to the seed. Barley grain in which these structures remain attached is referred to as covered barley, with the palea and lemma, collectively termed the hull. However, barleys in which the palea and lemma do not adhere to the seed are also well known. These hulless barleys share harvesting features similar to wheat, because when harvested the seed is cleanly separated from all other components of the spike.
The barley seed consists of the embryo, a series of outer layers of cells called the pericarp, and the endosperm. The endosperm contains different nutrients that the embryo draws upon as it grows into a plant. The principal compound found in the endosperm is starch, which represents about two-thirds of the mass of the seed. This starch serves as an energy source for the seedling. Another significant carbohydrate, the -glucans, are components of the endosperm cell walls. The second largest component of the barley endosperm is protein. The amount of protein present is generally inversely proportional to the amount of starch. This protein provides a source of amino acids that can be used for protein synthesis by the seedling. The amount of protein present in a seed is positively correlated with the amount of nitrogen fertilizer applied when the parent plant is being grown. Additionally, the barley grain contains a large variety of other compounds present in minor amounts, including mineral nutrients and different organic compounds, including various vitamins.