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  Morphology

Morphology > Growth > Taxonomy >


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Morphology

Plant Roots

  • The root system of sorghum is extensive, and there are many root hairs. An embryonic or primary root first appears upon germination.
  • Several such roots develop, these are not branched or are sparsely branched. Secondary roots develop from the first node.
  • It is these roots that develop into the extensive root system of the plant.
  • The primary roots subsequently die. Brace roots may appear later on the lowermost nodes and may be numerous if the plant is unadapted. These roots are not effective in uptake of water and nutrients.

Culms (Stem)

  • The culm, or stem, is made up of a series of alternating nodes and internodes.
  • The stem is slender to very stout, measuring 0.5 to 5 cm in diameter near the base, becoming narrower at the upper end, and varying in length from 0.5 to 4m.
  • It is solid, with a hard cortex or rind and a softer pith. Vascular bundles are scattered throughout the stem, but there are more near the peripheral area, where they are so closely associated that they form almost a solid ring.
  • The vascular bundles in the central portion of the stem are larger than those at the periphery. The central bundles branch into leaf midribs, while the peripheral bundles branch to form the smaller veins in the leaf blade.
  • The node appears as a ring at the base of the leaf sheath. This is the point at which the leaf is attached to the stem.
  • A bud forms at each node except at the node to which the flag leaf is attached.
  • These buds, at successive nodes, arise on alternating sides of the stem. At times these buds will develop to form axillary tillers. Basal tillers if any, form at the first node.

Leaves

  • Leaves are variously distributed along the stem in sorghum; in some types they may be concentrated near the base, while in others they are more or less uniformly distributed.
  • Leaves are borne at different angles to the stem, varying from almost vertical to near-horizontal. Leaves may be as long as 1 m and may vary in width from 10 to 15 cm.
  • The number of leaves vary greatly. In well-adapted plants there are usually 14 to 17 leaves, but less adapted plants may have as many as 30 leaves.
  • The leaves are borne alternately in two ranks along the stem, and consist primarily of a sheath and a blade.
  • The sheath is attached to a node and surrounds the internode, and frequently the node above it, before the blade extends outwards
  • Frequently, the sheaths attached to lower nodes will cover the nodes above, but those higher on the plant will not extend as far as the node above.
  • The sheath is frequently covered with a waxy bloom; at times the bloom is quite pronounced.
  • The blades are broad at the base and taper upward to a fine point; they are glabrous, except on the inside just above the ligule and on the outside near the junction with the sheath.
  • The margins of the leaf are smooth or scabrid, especially on the upper half. The midrib is prominent, greenish or white, flattened or slightly concave on the upper surface and convex on the lower one.
  • The blades are thicker at the base than at the tip and along the midrib than along the margins.
  • There is a short (1 to 3 mm) membranous ligule at the junction of the leaf blade with the sheath.
  • Leaves of the wild species are frequently long (30 to 75 cm) and slender (0.5 to 7 cm in width).

Inflorescence Panicle

  • The panicle may be short and compact or loose and open; 4 to 25 cm or more long, and 2 to 20 cm or more wide.
  • The central axis of the panicle, the rachis, may be completely hidden by the density of the panicle branches or completely exposed.
  • The rachis differs greatly in its shape and length-from long and thin to short and stubby. The rachis may be striated hairy or glabrous and divided into nodes and internodes.
  • Several branches are borne at each node. Primary branches or rays arise at each node.
  • They are arranged in whorls. Rays branch and rebranch, final branches bear spikelets.

Spikelet

  • Spikelet is unit of inflorescence. Spikelets occur in pairs.. One spikelet is always fertile, sessile and the other is sterile and pedicellate except the terminal sessile spikelet which is accompanied by two pediceled spikelets.
  • The racemes vary in length according to the number of nodes and the length of the internodes.
  • There are 1 to 4 nodes in some species, and 5 to 8 nodes in others; internodes vary in length, thickness, and hairiness depending on the species.
  • Sessile Spikelets : The sessile spikelet varies in shape from lanceolate to almost rotund and ovate and is sometimes depressed in the middle and bears the grain.
  • The colour is green at flowering, changing to shades of straw, cream, buff, yellow, red, brown, purple, or almost black at grain maturity.
  • Glumes vary from quite hairy to almost hairless. The lower glume is usually somewhat flattened and conforms more or less to the shape of the spikelet, while the upper one is more convex or boat shaped.
  • The seed may be enclosed by the glume or may protrude from it, being just visible to almost completely exposed. There are two lemmas, each a delicate white tissue.
  • The lower lemma is elliptic or oblong, about equal in length to the glume. The upper lemma is shorter, more ovate, and may be awned. There are also two lodicules and a palea, but these are much reduced.
  • Sorghum has two pistils and three stamens. Each fluffy stigma is attached to a short stout style extending to the ovary. The anthers are attached to long threadlike filaments.

Seeds

  • Seed or Caryopsis: Seeds are more or less spherical in shape, varying to somewhat flattened on one side (turtle-backed). They range tremendously in pericarp color (red, brown, white, yellow, cream) and have either a dull or pearly luster.
  • The testa may also be coloured, usually a dark red to dark brown. The endosperm is usually white, though it may be yellow.
  • Pediceled Spikelets : These are much narrower than the sessile spikelets, usually lanceolate in shape.
  • They may be smaller, the same size, or longer than the sessile spikelets. They are male or neuter sex, but (very rarely) may have a rudimentary ovary.
  • The lemmas are much reduced in size and only rarely does the upper lemma have an awn.

 
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Growth

Growth phase Duration (Days)
Vegetative 1-35
Reproductive 35- 50
Maturation 50 -100

Vegetative Phase

  • When a seed is placed in moist soil, it takes up water and swells. Germination occurs quickly, the coleoptile first appears above the ground after 3 or 4 days.
  • As the seed swells the seed coat breaks, and a small coleoptile and primary root (radicle) emerge. The coleoptile grows longer and several more primary roots appear.
  • The coleoptile begins to emerge from the ground, and the first leaf breaks through the tip. The young plant begins to grow, adding more leaves, and the coleoptile remains as a sheath at the base of the plant.
  • The mesocotyl grows during this period, and a node is formed at the base of the coleoptile just below the ground line. Secondary roots begin to develop from this node when the plant is 3 to 7 days from emergence.
  • The young seedling is using food stored in the endosperm during this period.
  • About the time the secondary roots have begun to develop, the mesocotyl begins to die and the major root system develops from secondary or adventitious roots. Some sorghums tiller profusely, especially the sudangrasses and forage sorghums.
  • The grain sorghums vary in their capacity to tiller, but usually do so only if there is adequate moisture or a poor stand.
  • In normally tillering varieties, tillers develop from adventitious buds at the basal node soon after the secondary roots develop.
  • The plant remains in a vegetative phase for about 30 to 40 days, during which all leaves are formed. After this period, growth occurs by cell elongation.

Reproductive Phase

  • The floral initial forms 30 to 40 days after germination when the plants are 50 to 75 cm tall. Floral initiation marks the end of the vegetative growth due to meristematic activity.
  • Sorghum usually flowers in 55 to 70 days in warm climates, but flowering may range from 30 to more than 100 days.
  • The sorghum head begins to flower at its tip and flowers successively downward over a 4 or 5 day period.
  • Because all heads in a field do not flower at the same time, pollen is usually available for a period of 10 to 15 days.
  • At the time of flowering, the glumes open and the three anthers fall free, while the two stigmas protrude, each on a stiff style.
  • Flowering frequently occurs just before or just after sunrise, but may be delayed on cloudy damp mornings.
  • The anthers dehisce when they are dry (but not in heavy dew or rain) and pollen blows into the air.
  • Sorghum is primarily self pollinated (about 2 to 10% or more cross-pollination). The pollen drifts to the stigma, where it germinates; the pollen tube, with two nuclei, grows down the style, to fertilize the egg.
  • The glumes close shortly after pollination, though the empty anthers and stigmas still protrude.

Maturation Phase

  • The ovule begins to develop as a light green, almost cream-coloured sphere; after about 10 days it begins to increase in size and becomes a darker green.
  • It takes about 30 days for the seeds to reach maximum dry weight (physiological maturity).
  • During this development, the seed passes through three stages : "milk", "early dough" and "late dough".
  • The seeds contain about 30% moisture at physiological maturity; they dry to about 10 to 15% moisture during the following 20 to 25 days.
  • The seed is ready for harvest at any time from physiological maturity to seed dryness; however, seed with more than 12% moisture must be dried before storage.
  • Lower leaves begin to die and dry up during this period. By the time the grain begins to dry, four or five of the lower leaves may dry up and drop from the plant.
  • There is a distinct varietal difference in the rate of senescence of remaining leaves.
  • All leaves may be dried, or almost dried, at grain maturity, or the plant may remain green.

 
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Taxonomy

  • The genus Sorghum is divided in two sections :

Section I

  • Eusorghum, which includes cultivated grain, syrup, fodder and broomcorn varieties. The cultivated and wild grass sorghums are grouped under subsection Arundinacea and johnsongrass and related 20 haploid chromosome perennials, under subsection Halepensia.

Section II

  • Para-sorghum, includes the wild, grassy, 5-chromosome species in which the upper sheath nodes are bearded. The Arundinaceae subsection has two series.
  • Series a., Spontanea, includes the cultivated sudangrass and tunisgrass together with some wild species of grass sorghum. The grain and sweet sorghums are grouped into Series b., Sativa.
  • This series includes six sub series viz., Drummondii, Guineensia, Nervosa, Bicoloria, Caffra, and Durra. Thirty-one species were described within the six subseries.
  • Subseries I, Drummondii, includes the so-called "chicken corn" and similar cultivated or wild types with long, slender, pointed, nearly hairless glumes and with spikelets that often are deciduous, of the species, S. aterrimum, S. drummondii and S. nitens.
  • Subseries II, Guineensia, includes the shallus and other types characterized by glumes that spread open at maturity and with margins that usually become involute. The kernels often change positions by turning, after the glumes open. Many of the varieties grown in western and central Africa show these characters, but the panicles are usually more compact than the shallus and usually lack the yellow plant pigment that characterized most shallus. The species are S. conspicuum, S. exertum, S. gambicum, S. guineense, S. margaritiferum, S. mellitum, and S. roxburghii. The roxburghii group includes the shallus and similar lax-panicled varieties.
  • Subseries III, Nervosa, includes S. nervosum the kaoliangs and the "broom kaoliangs" S. membranaceum, S. basutorum, S. ankolib, S. melaleucum, and S. spendidum are other types also having outer (lower) glumes that are conspicuously striately nerved in the upper half.
  • Subseries IV, Bicoloria, varieties usually have obovate spickelets with obscure nerves. Included are the broomcorns, many of the sorgos and numerous grain sorghums. Important types are S. bicolor with stiff panicle branches; S. dochna, with lax panicles and long adherent glumes, which includes the broomcorns; and S. elegans with oblong or umbelliform panicles. Other species are S. miliiforme, S. simulans, and S. motabile.
  • Subseries V, Caffra, includes varieties mostly with compact or mid-compact panicles, and often with glumes almost as wide as long and that open to expose 1/3 or more of the grain at maturity. This subseries includes most of the kafirs and the hegaris (S. caffrorum); some sorgos; the feteritas, and other types (S. caudatum) with large, dense panicles and large rounded grains, S. nigricans with grains rounded at the tip as in Suma sorgos. Other species are S. coriaceum and S. dulcicaule.
  • Subseries VI, Durra, includes types with broad spikelets bearing kernels as long as, or longer than, the glumes. Most of the varieties have dense, compact panicles. The types within this series are the durras S. durra and S. subglabrescens, which include milos and other varieties with a pronounced transverse wrinkle on the glume, S. cernuum which is somewhat intermediate between the above types with a less pronounced transverse wrinkle, and S. rigidum.

 
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