The Biology and Ecology of Sugarcane (Saccharum spp. hybrids) in Australia
Table of Contents
Section 1.1 Origins and Distribution 3
Section 1.2 Uses of Sugarcane 4
1.2.1 Sugar production 4
1.2.2 By-products 5
Section 1.3 Taxonomy and Genetics 6
1.3.1 Tribe Andropogoneae 6
1.3.2 Genus Saccharum 6
1.3.3 Commercial hybrid cultivars 7
Section 1.4 Cultivation and Growth 8
1.4.1 Germination and establishment 8
1.4.2 Early growth 9
1.4.3 Maturation and ripening 9
1.4.4 Ratoon crops 9
Section 1.5 Vegetative Morphology and Anatomy 10
Section 1.6 Reproduction 11
Sugarcane flowers 11
Section 1.7 Diseases and Pests 12
1.7.1 Diseases 12
1.7.2 Pests 15
Section 2.1 Toxicity 18
Section 2.2 Allergenicity 18
Section 3.1 Weediness of Saccharum spp. hybrids 19
Section 3.2 Weediness of Saccharum Species 19
3.2.1 Saccharum spontaneum 19
3.2.2 Saccharum officinarum 20
3.2.3 Other Saccharum weed species 21
Section 3.3 Weediness of Species in the Saccharum Complex in Australia 21
Section 4.1 Gene Transfer to Cultivated Sugarcane and naturalised sugarcane 21
Section 4.2 Gene transfer to other related Saccharum species 22
Section 4.3 Gene transfer to other genera in Saccharum complex 23
Section 4.4 gene transfer to other genera in tribe andropogoneae 23
Section 4.5 Gene Transfer to other Organisms 24
This document is designed to be a regularly revised resource document for dealings involving the intentional release (DIR) into the Australian environment of genetically modified (GM) sugarcane either for the purpose of field trials or commercial release. This document addresses the biology and ecology of conventional (non-genetically modified) sugarcane. Included is the origin of sugarcane, general descriptions of its growth and agronomy, its reproductive biology, toxicity and allergenicity and its general ecology. This document also addresses the potential for sugarcane to transfer genes via pollen and seed movement and for weediness. This document will support the detailed risk assessment and risk management plan (RARMP) prepared for each DIR application involving GM sugarcane. However the Regulator will assesses the risks associated with each particular genetic modification on a case by case basis which will be then detailed in the RARMP prepared for each application.
Sugarcane is a tall growing monocotyledonous crop plant that is cultivated in the tropical and subtropical regions of the world primarily for its ability to store high concentrations of sucrose, or sugar, in the internodes of the stem. Modern sugarcane varieties that are cultivated for sugar production are complex interspecific hybrids (Saccharum spp.) that have arisen through intensive selective breeding of species within the Saccharum genus primarily involving crosses between the species Saccharum officinarum L. and S. spontaneum L. (Cox et al. 2000) (see more details in Section 1.3.3).
S. officinarum or the ‘noble canes’ accumulate very high levels of sucrose in the stem but have poor disease resistance. S. officinarum itself is thought to be the product of complex introgression between S. spontaneum, Eriathusarundinaceus and Miscanthussinensis (Daniels & Roach 1987). A possible intermediate form in the development of S. officinarum is S. robustum, a diverse riparian species that grows in the wet tropics with many distinct populations. The origins of S. officinarum are intimately associated with the activities of humans as S. officinarum is a purely cultivated or garden species with no members found in the wild (Sreenivasan et al. 1987). The centre of origin of S. officinarum is thought to be in Polynesia. The species was probably transported throughout south east Asia by humans, leading to a modern centre of diversity in Papua New Guinea and Irian Jaya (Indonesia) where the majority of specimens were collected in the late 1800s (Daniels & Roach 1987).
Hypotheses for the origin of S. officinarum involve selection of sweet forms of S. robustum for use as food, possibly with the aid of animals such as pigs or rats that were attracted to sweeter individual plants (Daniels & Roach 1987).
S. spontaneum is believed to have evolved in southern Asia. S. spontaneum is a much more adaptable species and grows in a wide range of habitats and at various altitudes in the tropics through to temperate regions from latitude 8˚S to 40˚N extending across three geographical zones: a) the East zone which is South Pacific islands, Philippines, Taiwan, Japan, China, Vietnam, Thailand, Malaysia and Burma; b) the Central zone, which includes India, Nepal, Bangladesh, Sri Lanka, Pakistan, Afganistan, Iran and Middle east and c) the West zone which includes Egypt, Sudan, Kenya, Uganda, Tanzania and other countries in the Mediterranean (Tai & Miller 2001; Daniels & Roach 1987; Pursglove 1972).
Section 1.2 Uses of Sugarcane
1.2.1 Sugar production
Sugarcane is an established agricultural field crop with a long history of safe use. It is believed to have become established as a domestic garden crop possibly as early as 2500 BC (Daniels & Roach 1987). Sugarcane has been cultivated in Australia for over 100 years (Canegrowers 2004).
Sugarcane is primarily grown as a source of sugar. Sugar is initially extracted from the raw cane at sugarcane mills distributed throughout the growing region. The cane is shredded and the juice extracted by crushing. The juice is then clarified by a combination of heating in the presence of lime (Ca(OH)2) which complexes with phosphorus in the juice to produce a precipitate of calcium phosphate which is allowed to settle out taking other impurities with it. Flocculants (substances added to solutions to produce woolly looking masses of particles which assists settling out suspensions) are added to speed up this process (Mackintosh 2000).
Clarified sugar juice is then concentrated by evaporation to produce ‘syrup’. The syrup then goes through multiple rounds of crystallisation to extract the sucrose. It is boiled and the sucrose crystallises from the remaining molasses fraction. The product of this step is known as massecuite. The massecuite is then centrifuged to separate the sucrose from the molasses. This process is repeated three times in Australian sugar mills. Thus clarified sugar juice is boiled and centrifuged the first time to produce ‘A’ sugar and ‘A’ molasses. ‘A’ molasses is then boiled again to produce ‘B’ sugar and ‘B’ molasses. The ‘B’ molasses is boiled a third time to produce ‘C’ sugar which is mixed with water and is used to seed the next round of crystallisation (Mackintosh 2000). The ‘C’ molasses is referred to as ‘final’ or ‘blackstrap’ molasses (Preston 1988). The ‘A’ and ‘B’ sugar are dried to produce raw sugar, which is shipped in bulk to sugar refineries worldwide for further purification resulting in a high quality, purified product.
Sugarcane quality is measured at the mill and partly determines the actual return the grower receives. The formula to determine payment to the grower is complex and outside the scope of this document however there are three measures of cane quality that are important, which will be briefly mentioned here. Brix is the percentage of dissolved solids on a weight per weight basis and is measured by refractometer or density meter. Pol is a measure of the passage of polarised light through the clarified juice. These two measures of juice quality (corrected for fibre content of the stem) allow determination of the level of impurities in the cane (ie. Brix minus Pol equals total impurities in the cane). Furthermore this allows estimation of the sugar content or commercial cane sugar (CCS) of a grower’s cane (Mackintosh 2000).
To calculate CCS it is assumed that three quarters of the impurities remain after the juice is clarified. These impurities end up in the final molasses, which in turn consists of ~40% non-recoverable sugar and 60% impurities. Therefore:
CCS = Pol of juice (corrected for fibre content of stem) – ¾ (impurities in cane x 40/60)
= Pol in cane - ½ (impurities in cane)
CCS is a measure of how much pure sucrose can be extracted from the cane. The final return that the grower receives is determined by additional factors (see Mackintosh 2000).
Garside et al. (1997) reported that the Australian sugar industry had reached a productivity plateau in the period 1970-1990. In that period, 50 new cultivars were released and plant breeders estimated productivity gains of 1% per year (ie 0.01tonne/ha/year). However, in the same period CCS decreased by ~1 unit.
Several by-products are produced from crushing sugarcane at the sugar mill. These primarily include bagasse (fibre) and molasses.
Bagasse is the fibrous portion of sugarcane that remains after the juice has been removed. It has several applications, including generation of power for the mill, papermaking and livestock feed. It consists of two types of fibre, which constitute 55% of bagasse dry weight. These are the cellulose fibre of rind, vascular tissue and the pith of the cane stem. Bagasse cellulose fibres are longer (1-1.5mm) than hardwood fibres (0.7-1mm), but shorter than softwood fibres (2.5-5mm) and are suitable for papermaking. The pith material of the stem is considered a contaminant for papermaking and production of high quality paper consequently requires it to be removed. Bagasse is used to make paper in many countries although not in Australia (Allen et al. 1997).
Bagasse is also used as an animal feed but is limited by the low digestibility, even for ruminants (~25%). Chemical, biological or thermo-mechanical treatment improves the digestibility to approximately 65% (Allen et al. 1997; Playne 1984; UN Industrial Development Organisation 2002; de Medeiros & Machado 1993; de la Cruz 1990; Pate 1982).
Further chemical treatment of bagasse can be used to produce other by-products, which may be useful sources of income if further developed in Australia. These include production of various fermented and chemical derivatives of cellulose and fermentation of bagasse to produce fuel ethanol (Allen et al. 1997).
Molasses is the thick syrupy residue left over after the sucrose has been removed from the clarified sugar juice (syrup). The ‘C’ molasses (final or blackstrap molasses) is used for alcohol fermentation, as a stock feed supplement, for fermentation processes and as a fertiliser for cane fields (Mackintosh 2000; Sansoucy et al. 1988).
The composition of various stages in the production of molasses are shown in Table 1.
Table 1: Sugarcane by-products from the processing of sugarcane (adapted from Preston 1988; Allen et al. 1997). Note that the balance of the fraction in each case is mostly water.
Approximate composition (% fresh weight basis)
* glucose and fructose produced by the action of the enzyme invertase.
† mainly inorganic salts.
1 clarified and concentrated cane juice (syrup) concentrated to the point where the sucrose is almost ready to crystallise.
2 clarified and concentrated cane juice (syrup) from which no sugar has been crystallised that has been filtered and partially inverted (converted to reducing sugars) to avoid crystallisation.
3 concentrated soluble residue following the second centrifugation to remove the ‘B’ sugar.
4 concentrated soluble residue after the last centrifugation to remove the ‘C’ sugar, also known as ‘C’ or blackstrap molasses.
Power is generated from burning bagasse to produce steam to run the mill. Excess energy is directed to the electricity grid. The ash produced is mixed with other impurities (mud) left over after the sugarcane juice is clarified and fine bagasse known as bagacillo to produce filter cake which is used as a fertiliser on cane farms (Mackintosh 2000).
Trash refers to the sugarcane plant material left over after harvesting of the sugarcane stalks. It is generally retained in the field as mulch in northern Queensland. Baled trash is gaining in use as garden mulch and as a low-grade cattle feed from the south east Queensland growing region (Dawson 2002).