The Biology and Ecology of Sugarcane (Saccharum spp hybrids) in Australia December 2004

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Section 1.5 Vegetative Morphology and Anatomy

The morphology and anatomy of sugarcane has been extensively reviewed and so will not be explored in detail here. See Moore (1987) for a comprehensive treatment of the morphology and anatomy of sugarcane.

Sugarcane is a large tropical grass that produces multiple stems or culms each of which consist of a series of nodes separated by internodes. Following germination the terminal vegetative bud of each shoot lays down a series of nodes, each with a dormant bud and one or more rows of root primordia, and a growth ring (intercalary meristem). The internodes consisting of sucrose storing parenchyma cells and vascular tissue.

As the stem develops the leaves emerge, one leaf per node, attached at the base of the node. The leaves eventually form two alternate ranks on either side of the stem. At the top of the stem is an apical meristem set on top of a number of very short internodes. Mature stems consist of seven leaves still enclosed in the leaf spindle, a dozen or so green leaves and a number of senescent leaves, increasing in number with increasing age of the plant. New leaves emerge and expand over a period of between one and three weeks. Internode length can reach over 30 cm, depending on growth conditions, and stems normally reach two to three metres in the normal growing season (Bull 2000).

The leaf blade is pubescent (hairy) on the abaxial (under) side of the leaf and glabrous (without hairs) on the adaxial (top) side (Moore 1987). Sugarcane uses a C4 mechanism of photosynthesis similar to other tropical grasses and consequently the anatomy of the leaves reflects this underlying physiology. The base of the leaf attaches to the stem at the node but then wraps the stem to form a sheath that loosely encloses the internode to which the node subtends.

The stem of sugarcane is the major sink for photosynthate (sucrose) within the sugarcane plant, rather than fruit or seed structures. Transverse cross section through an internode reveals vascular bundles surrounded by parenchyma cells with a thick outer epidermis covered in an external layer of wax (Moore 1987). Developing leaves and internodes develop in a basipetal direction in that the leaf blade expands from the tip to the base then the internode elongates.

The node consists of a growth ring or intercalary meristem, the root band (containing root primordia) and a bud above the leaf scar where the leaf sheath attaches, which delimits the node from the internode below.

Like most grasses the sugarcane root system is fibrous and shallow. However, the plant develops buttress roots that serve to anchor the plant and some deeply penetrating roots that grow downwards for 5-7 metres allowing for water absorption under water stress (Moore 1987).

Section 1.6 Reproduction

Sugarcane flowers

Sugarcane forms an open panicle type of inflorescence, whose shape, degree of branching and size are highly variety specific. The inflorescence or arrow consists of a main axis and first, second and third order branches. Attached to the branches are spikelets arranged in pairs that contain individual flowers. The sugarcane flower consists of three stamens (male) and a single carpel with a feathery stigma (female) typical of wind pollinated flowers. Frequently the male stamens may be abortive resulting in reduced or absent pollen production (Moore 1987).
Flower initiation and seed propagation

The occurrence of flowering under field conditions is variable, influenced by variety as well as by environmental conditions. Flowering is most reliable and occurs earliest between latitudes 7º and 12º and later northward and southward in the tropical region (Fauconnier 1993; Moore & Nuss 1987). Flowering in northern Australia (latitude 15º-20º) begins at the start of May. Flower initiation causes the apical meristem to switch from vegetative to floral development, causing stalk elongation to cease, consequently flowering of the crop can affect yields. The older and more vigorous stems in a stool are the most likely to initiate flowering (Moore & Nuss 1987).

The ability of sugarcane to reproduce sexually was not recognised until 1888. Floral development is initiated by shortening day length to an intermediate value, and occurs in Australia from mid February to mid March (Cox et al. 2000). Cool night temperatures, high day temperatures and lack of moisture interferes with flower initiation. Flowers take two to three months to mature after initiation.

Sugarcane is a cross-pollinating species although selfing occurs at low levels (McIntyre & Jackson 1995; Moore & Nuss 1987). Although sugarcane flowers often have reduced male fertility they are rarely male sterile. Sugarcane pollen is very small, hairy and wind dispersed. It is rapidly desiccated after dehiscence, having a half-life of only 12 minutes, and is no longer viable beyond 35 minutes, under unmodified environmental conditions (26.5º C and 67% relative humidity) (Moore 1976; Venkatraman 1922). As a result, viable pollen is not expected to disperse far in the field. Sugarcane pollen stored at 4°C under 90 -100 % relative humidity retains some viability for up to 14 days (Moore & Nuss 1987).

As intensity of Saccharum flowering is dependent on interaction of cultivars and environmental factors such as daylength and temperature, some varieties can flower profusely in their natural environment but flower sparingly when introduced to other regions (Bull & Glasziou 1979). Sugarcane breeding programs are severely limited by the nature of flowering of each sugarcane variety (Bull & Glasziou 1979), particularly decrease in flowering and pollen viability at high latitudes (Dunckelman & Legendre 1982). However, controlled breeding is also possible when cultivars of low pollen viability are used as a female parent while low seed viability cultivars are used as a male parent (Bull & Glasziou 1979). Crosses can often be made but only between varieties which have overlapping flowering period. Various techniques have been developed including alteration of photoperiod which can induce flowering so that flowers could be available for crossing when required (Bull & Glasziou 1979).

Sugarcane seed or ‘fuzz’ is the entire flower panicle without the main flower axis and larger lateral axes. Mature fuzz consists of the mature dry fruit (caryopsis), glumes, callus hairs, anthers and stigma. The superfluous parts of the inflorescence are generally handled, stored and sown with the seed because it is not practical to separate them. Although many commercial varieties of sugarcane can produce seed, fuzz is only used in breeding programs, as the proportion of sugarcane seedlings with agronomic qualities near to those of the parental commercial clones is extremely low. Sugarcane fuzz is short lived, loosing 90% of its viability in 80 days at 28ºC if not desiccated (Rao 1980).

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