Wednesday, July 27, 2011

Growth Morphology of Sugarcane

Propagation :
Stem cuttings or sections of the stalks called "setts" or seed pieces propagate sugarcane. Each sett contains one or more buds. The buds, located in the root band of the node, are embryonic shoots consisting of a miniature stalk with small leaves.

The outer small leaves are in the form of scales. The outermost bud scale has the form of
a hood. Normally, one bud is present on each node and they alternate between one side of the stalk to the other.

Variations in size, shape and ther characteristics of the bud provide a means of distinguishing between varieties. Each sett also contains a circle of small dots above the node, which are the root primordia. Each primordium exhibits a dark center, which is a root cap, and a light colored "halo".

The bud sprouts under favourable conditions and gives rise to a primary stalk, whereas from the primordial the sett roots originate.

During nearly one month after germination, that is, sprouting of the buds, the young plant lives at the expense of the reserves present in the seed piece, and partially using water and nutrients provided for by the first roots.

Stalk is also known as "millable cane". It develops from the bud of seed-cane. When seed-cane is planted, each bud may form a primary shoot.

From this shoot, secondary shoots called "tillers" may form from the underground buds on the primary shoot. In turn, additional tillers may form from the underground secondary shoot buds. The stalk consists of segments called joints.

Each joint is made up of a node and an internode. The node is where the leaf attaches to the stalk and where the buds and root primordia are found. A leaf scar can be found at the node when the leaf drops off the plant. The length and diameter of the joints vary widely with different varieties and growing conditions. The colors of the stalk seen at the internodes depend on the cane variety and environmental conditions.

For example, exposure of the internodes to the sun may result in a complete change of color. The same variety grown in different climates may exhibit different colors. All colors of the stalk derive from two basic pigments: the red color of anthocynin and the green of chlorophyll.

The ratio of the concentration of these two pigments produce colors from green to purple-red to red to almost black. Yellow stalks indicate a relative lack of these pigments. The surface of the internode, with the exception of the growth ring, is more or less covered by wax. The amount of wax is variety dependent.

The top of the stalk is relatively low in sucrose and therefore is of little value to the mill. The top 1/3 contains, however, many buds and a good supply of nutrients, which makes it valuable as seed cane for planting.

A cross section of an internode shows, from the outside to the center, the following tissues: epidermis, cortex or rind, and ground tissue with embedded vascular bundles. The cells of the rind are thick-walled and lignified. These cells help strengthen the stalk. More toward the center, the ground tissue contains the vascular bundles with the xylem and phloem.

Xylem tissue conducts water and its dissolved minerals upward from the roots, and phloem conductive tissue transports plant- manufactured nutrients and products, for the most part, downward toward the roots.

Two types of cracks are sometimes found on the surface of the stalk; harmless, small corky cracks, which are restricted to the epidermis, and growth cracks which may be deep and run the whole length of the internode.

Growth cracks are harmful since they allow increased water loss and expose the stalk to disease organisms and insects. Growth cracks are dependent on variety and growing conditions.

The Leaf
The leaf of the sugarcane plant is divided into two parts: sheath and blade, separated by a blade joint. The sheath, as its name implies, completely sheaths the stalk, extending over at least one complete internode.

The leaves are usually attached alternately to the nodes, thus forming two ranks on opposite sides. The mature sugarcane plant has an average total upper leaf surface of about 0.5 square meter and the number of green leaves per stalk is around ten, depending on variety and growing conditions.

The blade joint is where two wedge shaped areas called "dewlaps" are found. The leaves are numbered by Kuijper's system, as quoted by Casagrande (1991). The first leaf from top to bottom of the stalk with clearly visible dewlap is designated as +1. Downwards they receive, in succession, the numbers +2, and +3. The "top visible dewlap" leaf (+3) is a diagnostic tissue that is frequently used in the evaluation of the nutritional status.

The Inflorescence
When a sugarcane plant has reached a relatively mature stage of development, its growing point may, under certain photoperiod and soil moisture conditions, change from the vegetative to reproductive stage.

This means the growing point ceases forming leaf primordia and starts the production of an inflorescence. It is a short day plant. Its photoperiodic conditions can thus be attained largely in the tropics.

The inflorescence, or tassel, of sugarcane is an open-branched panicle .It is also known as arrow. Therefore flowering is also known as "arrowing". Each tassel consists of several thousand tiny flowers, each capable of producing one seed. The seeds are extremely small and weigh approximately 250 per gram or 113,500 per pound.

For commercial sugarcane production,inflorescence             
development is of little economic importance. Flowering is important for crossing and producing hybrid varieties.

Generally, a day length close to 12.5 hours and night temperatures between 20° to 25°c will induce floral initiation. Optimum growth conditions in the vegetative phase (fertile soil, abundant supply of nitrogen and moisture) restrict inflorescence while stress conditions induce formation of blossoms.
The Root System
In the commercial sugarcane crop, which is asexually propagated, development of the root system is initiated soon after planting a portion of stem (sett) with atleast one lateral bud.

The first roots formed are sett roots, which emerge from a band of root primordia above the leaf scar on the nodes of the sett.Sett roots can emerge within 24 hours of planting, although differences in the time required for root emergence occur among varieties. Sett roots are fine and highly branched roots, which sustain the growing plant in the first weeks after germination.     

Shoot roots are second type of root, which emerge from the base of the new shoot 5-7 days after planting .The shoot roots are thicker and fleshier than sett roots and develop in to the main root system of the plant. Sett roots continue to grow for a period of 6-15 days after planting, mostly senescing and disappearing by 60-90 days as the shoot root system develops and takes over supply of water and nutrients to the growing shoot. By the age of 3 months, sett roots comprise less than 2% of root dry mass.

Sett roots initially have an elongation rate of a few mm/day, reaching 20 mm/day within a few days of germination under favourable conditions. Shoot roots grow more rapidly, with maximum rates of elongation of up to 80 mm/day observed, though only for short periods. Mean growth rates for shoot roots over 10 days were 40 mm/day in sandy soils and 28 mm/day in heavy clay.

Mean rates of root penetration, or the rate of descent of the root system, of 20-30 mm/day were also reported. Root penetration in another trial was 20 mm/day down to a depth of 1.6 m for rainfed crops, but slowed in irrigated crops to 17 mm/day in the first 1.0 m and 6 mm/day between 1.0 and 1.6 m.

Genotypic variation in sugarcane root systems is well documented and those producing many tillers normally produce many roots because each new tiller is a source of shoot roots. Likewise cultivars with more horizontal (weak gravitropic) root penetration are more resistant to lodging than those with strongly gravitropic root system.

A longitudinal section of a root tip consists mainly of four parts: the root cap, the growing point, the region of elongation, and the region of root hairs. The root cap protects the tender tissues of the growing point as the root pushes through the soil. The growing point consists mainly of an apical meristem, where cell division takes place.

In the region of elongation, the cells increase in size and diameter until they reach their ultimate size. The region of root hairs is characterized by epidermal cells forming outgrowths (hairs), which dramatically increases the root-absorbing surface.

Information taken from : Thank you NETAFIM.

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