The Sorghum Plant
The Root System
The root system of a sorghum plant can be divided into a primary and a secondary root system. The primary system develops first in the germinated seedling and supplies the seedling with water and nutrients. However, primary roots have a limited growth and their function is taken over by the secondary root system. The secondary roots develop from the root crowns which are located just under or just above the soil surface. The roots of an adult sorghum plant are all secondary adventitious roots. Lateral roots develop from the secondary adventitious roots and penetrate the soil in all directions. The root system of the sorghum plant spreads to at least 1,5 metres around the plant and is most dense in the top 90 centimetres of the soil.
The basal circumference of the stem of cultivated sorghum plants is about 2 to 3 centimetres. The stem grows erectly and is solid, although the core may be spongy and have cavities in the centre. Directly above the attachment of the leaf sheath at each node there is a meristematic layer which contains primordia which is arranged in 1 to 3 concentric circles around the nodes.
The lowest node develops roots, and in the case of some of the long varieties, buttress roots develop from nodes which are above the ground. When the plant falls over, roots may develop from nodes which come into contact with the soil. Growth rings are situated at each node above the meristematic layer. These can give rise to differential growth so that the top portion of the stem can again grow erectly when the plant has fallen over.
A single internode is found by every node which does not necessarily develop. Tillers may develop from the lower nodes, while lateral branching may occur higher up on the stem. If, for example, the main stem is damaged or destroyed by stalk-borer, the plant is capable of forming tillers which can produce a crop. In the case of the adult sorghum plant lateral branches usually develop when a great deal of moisture is available in the soil.
The ability of the plant to develop tillers is determined by genotype, ambient temperature and the density of the plant population. Cool temperatures of less than 18°C encourage tillering in plants which are at the 4 to 6 leaf growth stage. Cultivated sorghums are usually annuals, but some perennial types can survive for many years by producing tillers from the bases of the old plants. Perennial types produce 2 to 3 times more basal tillers than the annuals. In the banana producing regions of Uganda, where two rainy seasons occur per annum, it is possible to have two crops from the same stand by just cutting the plants off after the harvest.
A second crop (ratoon crop) then develops and if fertilizer is applied, consecutive crops can be obtained.
The leaves of sorghum plants are well adapted to the hot and dry African climate. The relatively small leaf surface area and the fact that leaves fold closed under conditions of drought stress, reduces transpiration effectively.
Cultivated sorghums have 4 embryonic leaves. The number of leaves on the main stem may vary from 7 to 24, as determined by genotype. Fully developed leaves may attain lengths of 30 to 135cm and widths of 1,5 to 13cm at the widest point. Leaves are lanceolate. When the leaf margins are longer than the midrib, the leaves develop a wavy appearance. They are light green and in some cases, cultivars are glabrous. A waxy sediment is found near the base of the midrib where it joins the leaf sheath. Stomata occur in single or double layers on both surfaces of the leaf and motor cells are also found. These cause the leaf to fold closed in times of drought stress.
The Leaf Sheath
The leaf sheaths surround the stem and overlap each other right over left and left over right alternatively in the consecutive nodes. A powdery waxy deposit is sometimes found, particularly at the upper leaf sheaths. Where the waxy layer is thickly deposited, the leaf sheath has a bluish – white appearance. Short, white hairs can be found at the base of the leaf sheath where it joins the node.
The Leaf Junction
The ligule by the leaf junction is about 2mm long and is initially translucent, but later dries completely. The upper free-standing half of the ligule is hairy. In some genotypes the ligule doesn’t occur and the leaves have more upright appearance. The ligule lies close to the stem and prohibits intrusion of moisture, dust & insects.
The inflorescence of a sorghum plant is a determinate panicle which is usually compact. The shape and colour of the panicle varies from cultivar to cultivar. The primary axis of the panicle is angular and the secondary axes are arranged in whorls around it. The peduncle is usually straight. The length of the panicle varies from 75 to 500 millimetres and the width from 40 to 200 millimetres. Each panicle contains between 800 and 3 000 seeds, which are usually partly covered by the glumes. The glumes are black, red, brown or tan in colour.
The spikelets are attached to the raceme branchlets and are borne in pairs, except at the end of the branchlets where groups of 3 usually occur. When the spikelets are borne in pairs, one member is sessile and the other is pedicilate. Where groups of 3 occur, 1 is sessile and 2 are pedicilate. The sessile spikelet is complete and contains two flowers of which the upper spikelet is fruitful, while the pedicilate spikelets are sterile or only contain male florets.
Sorghum flowers open at night or in the early morning. Those at the tip of the panicle open first and it takes 6 to 9 days before the entire panicle has flowered. Owing to the composition of its flower, sorghum tends to self-pollinate. However, some cross-pollination (about 6%) does occur.
The seed consists of 3 main components. The coat constitutes about 8%, the embryo or germ 10% and the endosperm 80% of the mature sorghum seed.
The coat consists of outer, waxy pericarp which is fused to the testa. The central pericarp contains up to 3,7% of total starch content. The testa is located directly under the pericarp and its presence and pigmentation thereof are genetically determined. Polyphenols, when present, are responsible for the pigmentation in both pericarp and testa.
The embryo contains the scutellum, the embryonic axis, the plumule and the radicle (little root) and is rich in structural proteins, lipids and minerals. The protein in the peripheral layer of the endosperm is gradually replaced by starch. In the case of sorghum the greatest quantity of protein is located in the endosperm, where a very limited quantity of the amino acid, lysine, occur. The embryo and pericarp have a 3 to 4 times higher lysine content than the endosperm.