Sugarcane (Saccharum X officinarum): A Reference Study for the Regulation of Genetically Modified Cultivars in Brazil
1CanaVialis/Alellyx S.A., Rua James Clerk Maxwell, 320, 13069-380 Campinas, São Paulo Brasil
2Centro de Biologia Molecular e Engenharia Genética, Universidade Estadual de Campinas, 13083-875 Campinas, São Paulo Brasil
3Universidade Federal do Paraná, Rua dos Funcionários, 1540, Cabral, 80035-050 Curitiba, Paraná Brasil
4Centro de Tecnologia Canavieira, CP 162, 13400-970 Piracicaba, São Paulo Brasil
5IAC/APTA - Centro de Cana, Instituto Agronômico de Campinas, Rodovia Antonio Duarte Nogueira, Km 321, CP 206, 14032-800 Ribeirão Preto, São Paulo Brazil
6Amyris Crystalsev Biocombustíveis Ltda., Rua James Clerk Maxwell, 315, 13069-380 Campinas, São Paulo Brasil
7Amyris Crystalsev Biocombustíveis Ltda., Amyris Inc, 5885 Hollis St, Ste 100, Emeryville, CA 94608 USA
8Campus de Jaboticabal, Departamento de Tecnologia, Universidade Estadual Paulista, 14884-900 Jaboticabal, SP Brasil
9CENA/USP - Laboratório de Melhoramento de Plantas, CP 96, 13400-970 Piracicaba, São Paulo Brasil
10Reserva Ecológica do IBGE, CP 08770, 70312-970 Brasília, Distrito Federal Brazil
11Embrapa Recursos Genéticos e Biotecnologia, Parque Estação Biológica, Av. W5 Norte, 70770-900, CP 02372 Brasília, Distrito Federal Brasil
12Embrapa Recursos Genéticos e Biotecnologia, Empresa Brasileira de Agropecuária, Av. W5 Norte, 70770-900, CP 02372 Brasília, Distrito Federal Brasil
13Embrapa Tabuleiros Costeiros, Unidade de Execucao de Pesquisa, BR 104 Norte, Km 85, 57061-970, CP 2013 Maceió, Alagoas Brasil
14Campus de Araras, Universidade Federal de São Carlos, Rodovia Anhanguera, Km 174, CP 153, 13600-970 Araras, São Paulo Brasil
15Araujo & Macedo Ltda., Rua Oswaldo Cruz, 205, Jardim Santa Cruz, 13601-252 Araras, São Paulo Brasil
16Dow Agroscience, Rodovia Anhanguera, Km 344, Jardinópolis, 14680-000 São Paulo, Brasil
17Departamento de Genética, Escola Superior de Agricultura Luiz de Queiroz, Universidade de São Paulo, Avenida Pádua Dias, 11, CP 83, 13400-970 Piracicaba, São Paulo Brasil
Adriana Cheavegatti-Gianotto, Email: email@example.com.
Communicated by: Marcelo C. Dornelas
Author information ►Article notes ►Copyright and License information ►
Received 2010 Oct 6; Accepted 2011 Jan 13.
Copyright © The Author(s) 2011
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Global interest in sugarcane has increased significantly in recent years due to its economic impact on sustainable energy production. Sugarcane breeding and better agronomic practices have contributed to a huge increase in sugarcane yield in the last 30 years. Additional increases in sugarcane yield are expected to result from the use of biotechnology tools in the near future. Genetically modified (GM) sugarcane that incorporates genes to increase resistance to biotic and abiotic stresses could play a major role in achieving this goal. However, to bring GM sugarcane to the market, it is necessary to follow a regulatory process that will evaluate the environmental and health impacts of this crop. The regulatory review process is usually accomplished through a comparison of the biology and composition of the GM cultivar and a non-GM counterpart. This review intends to provide information on non-GM sugarcane biology, genetics, breeding, agronomic management, processing, products and byproducts, as well as the current technologies used to develop GM sugarcane, with the aim of assisting regulators in the decision-making process regarding the commercial release of GM sugarcane cultivars.
Keywords: Biosafety, Ethanol, Biofuel, Saccharum, Sugarcane
Economic interest in sugarcane has increased significantly in recent years due to the increased worldwide demand for sustainable energy production. The Brazilian experience in sugarcane ethanol production has paved the way for the establishment of a consolidated world supply to meet the demand of a proposed ethanol addition of approximately 10% to gasoline worldwide. It is estimated that the Brazilian production of sugarcane must double in the next decade to meet this goal. In the future, biotechnological advances might help to reduce the environmental impacts of increased sugarcane production by developing solutions that would produce more sugarcane with decreased requirements for fertilizers and water. Among these solutions, GM sugarcane will play a key role in providing growers with more productive and resistant varieties. In Brazil, the National Biosafety Technical Commission (CTNBio) has already approved field trials with genetically modified sugarcane, incorporating traits such as increased yield, drought tolerance, insect resistance and herbicide tolerance. It is expected that the enormous research and development efforts being conducted by government and private institutions will in the medium-term result in commercial release in Brazil of genetically modified sugarcane.
This review was produced to serve as a source of baseline information of conventional sugarcane to assist in the decision-making process related to possible future commercial release of GM sugarcane cultivars. Here, we will cover the origin of the species that have contributed to the genetic makeup of modern sugarcane cultivars, the agronomic behavior and growth habit of sugarcane, the botanical aspects of its reproductive biology, its genetic composition, potential for lateral gene transfer by pollen, its model of seed dispersion, and allergenicity of its derived products. This review also addresses the utilization of sugarcane and the byproducts generated by the sugarcane agribusiness.
The sugarcane crop has been relevant to the Brazilian economy since the beginning of the 16th century. The first sugarcane plants were brought from Madeira Island and established in Brazil around 1515; the first sugar mill was established in 1532. Currently, Brazil is the world’s largest sugarcane producer, with approximately 7.5 million cultivated hectares, which produced approximately 612 million tons in the 2009/2010 crop season. Approximately half of the sugarcane was used to produce sugar, and the remainder was used to produce 25 billion liters of ethanol (CONAB 2009). Brazil also produces sugarcane for animal feed, cachaça (sugarcane spirit), and sugarcane syrup, among other products.
In 2009, Brazil’s sugar and ethanol exports generated approximately US$ 9.9 billion in revenue, ranking sugarcane third among its exports (CONAB 2009).
Currently, the Brazilian sugar-ethanol agribusiness is experiencing a period of excitement due to the combination of extremely favorable factors, including prospects for the growth of internal and external markets, a long-term trend of rising international oil prices, possession of the world’s lowest ethanol production costs, growth of the flex fuel fleet, and a universal concern for sustainable energy alternatives. These conditions have attracted global attention to the Brazilian fuel ethanol program and the country’s potential to supply a significant part of the world’s demand for renewable fuel. The result has been a substantial increase in sugarcane production over the last few years. In 2007, the planted area grew by 12% over the previous year and the same level of growth is expected for the next several decades (FNP 2009).
Sugarcane is grown in all tropical and subtropical regions of the world, on both sides of the equator, up to approximately 35° N and 35° S (van Dillewijn 1952; Gomes and Lima 1964). In 2007, the main sugarcane-producing countries were Brazil (33% of the world’s production), India (23%), China (7%), Thailand (4%), Pakistan (4%), Mexico (3%), Colombia (3%), Australia (2%), the United States (2%) and the Philippines (2%) (FNP 2009).
In Brazil, sugarcane cultivation is concentrated in the southeastern region, which is responsible for approximately 70% of the national sugarcane production (Fig. 1). Northeastern Brazil, another traditional producing area, is responsible for 14% of sugarcane production, and midwestern Brazil, where the crop is rapidly advancing, represents 13% of the national production (CONAB 2009). It should be noted that, despite the concerns about the expansion of sugarcane cultivation toward the Amazon region, the actual cultivation in this region is minimal and decreasing.
Sugarcane production in regions of Brazil (Source: Conab 2009)
Classification and Nomenclature
The genus Saccharum was first described by Linnaeus (1753) in his book Species Plantarum. The generic name is derived from the Greek word sakcharon, which means sugar and was duly Latinized by the author. The book described two species: Saccharum officinarum L. and S. spicatum L., which is currently classified under the genus Perotis (P. spicata (L.) T. Durand and H. Durand) (Dillon et al. 2007).
The taxonomy and nomenclature of the genus has always been challenging (Bentham 1883; Hackel 1883; Pilger 1940; Hitchcock 1951; Bor 1960; Almaraj and Balasundaram 2006). The genus has two known synonymous names, Saccharophorum and Saccharifera. When it was initially described, the genus consisted only of five to ten species from the Old World, including S. officinarum, S. spontaneum, S. sinense, S. edule and S. barberi. Later, several species that were allocated in other genera, including Andropogon, Anthoxanthum, Eriochrysis, and Erianthus, were transferred to Saccharum. The case of Erianthus is particularly interesting.
Currently, the genus Erianthus comprises species from the Old World and New World that were previously separated into two different genera since the Old World species were firstly placed under the genus Ripidium (Grassl 1971). Later, authors classified the Old World species under Erianthus, section Ripidium (Almaraj and Balasundaram 2006). This distinction is supported by the fact that the Old World species contain a flavonoid (di-C-glycoside) that is absent in New World species (Williams et al. 1974). Cordeiro et al. (2003), analyzing these two groups by microsatellites markers also found that Erianthus accessions clustered separated in accordance to their geographical origin.
Erianthus is considered to be closely related to Saccharum and many species have been assigned to either of these genera, depending on the criteria used. Several botanists, however, have considered that they are distinct genera (Hooker 1896; Haines 1921, Jeswiet 1925; Grassl 1946; Dutt and Rao 1950; Mukherjee 1958). This distinction has been reinforced by studies using the presence of root tip tannin (Rao et al. 1957), leaf lipoid (Vijayalakshmi and Rao 1963), esterase isozyme alleles (Waldron and Glasziou 1972), and flavonoid composition (Williams et al. 1974) and microsatellites markers (Cordeiro et al. 2003).
Despite these data, the genera and species are more often separated and identified primarily on the basis of floral characteristics, which are considered by botanists to be more stable than vegetative morphological characters and the basic criterion used to differentiate both genera is the presence (in Erianthus) or absence (in Saccharum) of a floret structure called an awn, which is an extension of the mid-rib of the floral bract, on the top of lemma II or the upper lemma. The lemma in which the awn is absent is referred to as awnless. The current trend regards Erianthus as synonymous with Saccharum because this commonly used criterion to differentiate both genera (presence/absence of awn on the lemma) is variable and is not a consistent characteristic in the complex (Bor 1960; Renvoize 1984; Clayton and Renvoize 1986). Therefore, the genus Saccharum currently comprises all species that were previously described under Erianthus. The current sugarcane botanical classification is shown in Table 1.
The tribe Andropogoneae consists of tropical and subtropical grass species that are grown in the Old World and New World. The most important cultivated members of the tribe are corn (Zea mays) and sorghum (Sorghum bicolor) (Daniels and Roach 1987). Saccharum and Sorghum share many similarities in their genetic composition, as they originated from a common ancestral lineage that diverged approximately 5–8 million years ago (Al-Janabi et al. 1994; Guimarães et al. 1997; Figueira et al. 2008).
The currently cultivated sugarcane plants are hybrids derived from crossings mainly between plants of S. officinarum and S. spontaneum (Dillon et al. 2007). The plants are perennial grasses that form stools of stalks or culms that can be several meters in length and are juicy, with high concentrations of sucrose.
The sugarcane root system consists of adventitious and permanent shoot root types. Adventitious roots emerge from the culm root zone and are responsible for water uptake during bud sprouting and plant support until the permanent roots develop. Permanent roots are fasciculated at the base of growing shoots and are classified into support or anchor roots and the more network like absorption roots. (Mongelard 1968; Thompson 1964; Moore and Nuss 1987). The ratio between one root type and another are somewhat species specific. Saccharum officinarum generally contains fewer support roots than does S. spontaneum (Moore 1987a), which could explain the increased vigor and resistance to environmental stresses characteristic of S. spontaneum.
The stalk or culm consists of alternating nodes and internodes. On the node, there is a leaf scar, an axillary bud and a circumferential band of axillary root primordia. Stalk morphology is highly variable from one genotype to another and represents an important element for varietal characterization (Martin 1961). Sugarcane leaves are alternate and are attached to the stalk, with one leaf per internode. Sheathes consist of the sheath proper and the much smaller acropetal blade joint consisting of a leaf collar, dewlap, ligule and auricles. The shape, size and distribution of trichomes and the shape of the ligule and auricles are traits of taxonomic importance for varietal identification. Sugarcane leaves are numbered from top to bottom starting with the uppermost leaf showing a visible dewlap designated as leaf +1 (Fig. 2) (Moore 1987a).
The inflorescence of sugarcane is a ramified, conoidal panicle with a main stem, called the rachis, which is the continuation of the last stalk internode. The rachis holds secondary branches which in turn hold tertiary branches. The spikelets are located at the base of the tertiary branches and on the top of the secondary branches. Each spikelet has one flower, which is disposed alternately along the inflorescence secondary and tertiary branches. At the base of the spikelet, there is a ring of silky, colorless trichomes (‘coma’) that covers the spikelet (Fig. 3a) and help with spikelet dispersion. Next, there is a series of bracts called glumes (‘glume I’ and ‘glume II’), both glabrous (Fig. 3b and c); upper lemma (or fertile lemma); and palea, which is hyaline and without veins and may be either rudimentary or absent (Fig. 3d). When the inflorescence matures, an anemochoric (by wind) dispersion of the propagules begins. The propagules consist of the coma, some floral clusters and the spikelet (Fig. 3a). The flowers (Fig. 3e) consist of two lodicules, the androecium and the gynoecium. The pollen grains are spherical when fertile and prismatic when sterile. Sugarcane fruit, called the caryopsis (Fig. 4), is dry, indehiscent and one-seeded, and it cannot be separated from the seed. The fruit can only be distinguished from the seed when viewed with scanning electronic microscopy.
Diagram of a sugarcane propagule: a evident propagule, cupulate coma, empty pedicel and rachilla plus sessile spikelet. The other figures depict the different parts of the spikelet. b Glume I. c Glume II. d Palea. e Flower with two lodicules, three stamens,...
Sugarcane caryopses. Note the presence of the remains of styles at the tip of the caryopsis and the differentiated embryo region (at the opposite extremity of the style remains) (Photograph: Alellyx)
Hybridization and Introgression
Modern sugarcane cultivars are the products of crosses between species of the genus Saccharum that were made by breeders in the late 19th century (Matsuoka et al. 1999). The most important species contributing to modern sugarcane varieties were S. officinarum, which was widely cultivated for its ability to accumulate sucrose in its stalks, and S. spontaneum, which is a vigorous, widely adapted wild species which contributed genes for disease and stress resistance. The species S. sinense, S. barberi and S. robustum also provided minor contributions toward the development of some modern sugarcane varieties.
S. officinarum L. is generally known as the noble cane because it is stout and produces abundant sweet juice. Culms are thick (normally over 3.5 cm in diameter) and soft due to low fiber content. Assessions of this species display long, wide leaf blades (1 m long × 5 cm wide) and a relatively small, shallow root system (Scarpari and Beauclair 2008). S. officinarum is highly demanding in specific climate conditions, high soil fertility and water supply. S. officinarum accessions are generally susceptible to diseases such as mosaic, gummosis, leaf scorch, root rot and Fiji disease (Martin 1961; Ricaud and Autrey 1989; Ricaud and Ryan 1989), but they tend to be resistant to sugarcane smut (Segalla 1964). S. officinarum includes all old traditional sugarcane varieties that were cultivated throughout the world prior to the introduction of hybrid varieties (Segalla 1964).
Known as wild sugarcane, S. spontaneum L. is highly polymorphic, with plant stature ranging from small grass-like plants without stalks to plants over 5 m high with long stalks. Leaf blades vary in width from very narrow mostly restricted to the mid-rib or up to a width of 4 cm (Matsuoka et al. 1999). Plants show highly adaptive plasticity and are found in different environments. S. spontaneum is the species that has contributed to the improvement in sugarcane vigor, hardness, tillering, ratooning ability and resistance to biotic stresses (Mohan Naidu and Sreenivasan 1987). These plants tend to be immune to most diseases, including ‘Sereh’ and mosaic, but they are susceptible to sugarcane smut (Segalla 1964). In some regions of the world, such as the United States, S. spontaneum is considered a harmful invasive species (USDA 2008). In Brazil, it is considered an exotic and non-invasive plant (Instituto Horus 2008; Global Invasive Species Database 2008).
Saccharum sinense Roxb. and Saccharum barberi Jesw. are known as Chinese or Indian canes, as they were initially grown in China and India before the spread of modern varieties (Mohan Naidu and Sreenivasan 1987). Some taxonomists consider them a single species (Matsuoka et al. 1999), and according to more modern studies, they are natural hybrids between S. officinarum and S. spontaneum (Irvine 1999; D’Hont et al. 2002). Stems from both species are long (up to 5 m), thin (approximately 2 cm in diameter) and fibrous, presenting long, fusiform internodes. The plants have a vigorous, well-developed root system and good tillering, which enables adaptation to poor and dry soil and allows the production of large volumes of biomass. However, both species have medium sugar content and early maturation. S. sinense and S. barberi tend to exhibit resistance to root diseases; some individuals are resistant to mosaic, immune to ‘Sereh’ disease, resistant to sugarcane borers and susceptible to sugarcane smut (Segalla 1964). Due to poor flowering and sterility of most genotypes of these species, they are rarely used in sugarcane breeding programs (Roach and Daniels 1987), although they may have contributed to the development of some modern varieties (Mohan Naidu and Sreenivasan 1987; Dillon et al. 2007).
S. robustum Brandes and Jeswiet ex Grassl represents wild sugarcanes adapted to broad environmental conditions. It possesses a high fiber content and vigorous stalks that are 2.0–4.4 cm in diameter and up to 10 m high, but like S. officinarum, it does not have rhizomes. The culms are hard and have little juice, are poor in sugar content and have a hard rind, a characteristic that is exploited to build hedges (Matsuoka et al. 2005; Mozambani et al. 2006). S. robustum tends to be highly susceptible to mosaic (Segalla 1964). Few current commercial cultivars have received a contribution from this species. However, there are reports of a successful program to broaden the genetic basis of the current varieties by introgressing S. robustum into Hawaiian sugarcane varieties (Mohan Naidu and Sreenivasan 1987).
Centers of Origin and Diversity
The genus Saccharum probably originated before the continents assumed their current shapes and locations. The genus consists of 35–40 species and has two centers of diversity: the Old World (Asia and Africa) and the New World (North, Central and South America). Asia has approximately 25 native species, North America six native species and four or five introduced species, and Central America has three or four native and some introduced species (Webster and Shaw 1995). Africa has two native and Australia have one naturalized species (Darke 1999; Bonnett et al. 2008).
The Brazilian Saccharum species have not been well characterized. Only regional floristic surveys have reported the presence of these species. One study described the native species S. asperum, S.angustifolium, S. purpureum, S. biaristatum, S. glabrinodis, S. clandestinus and S. villosum, but the authors commented that these species were poorly defined so that it is possible that they all might be variations of a single species (Smith et al. 1982). In fact, from the species listed on this work, only S. asperum, S. angustifolium and S. villosum are currently accepted scientific names (The Plant List 2010). In another study, the native species were identified as S. villosum, S. asperum and S. baldwinii (Filgueiras and Lerina 2001).
The Saccharum species involved in the development of modern sugarcane cultivars originated from Southeast Asia (Roach and Daniels 1987). Because S. officinarum and S. spontaneum are the major contributors to the genomes of modern varieties, the geographical origins of these species will be described in more detail.
S. officinarum has been cultivated since prehistoric times (Sreenivasan et al. 1987). It is believed that its center of origin is Polynesia and that the species was disseminated throughout Southeast Asia, where a modern center of diversity was created in Papua New Guinea and Java (Indonesia); this is the region where the majority of specimens were collected in the late 19th century (Roach and Daniels 1987).
The center of origin and diversity of S. spontaneum is the more temperate regions of subtropical India. However, because S. spontaneum can be grown in a wide range of habitats and altitudes (in both tropical and temperate regions), it is currently spread over latitudes ranging from 8°S to 40°N in three geographic zones: a) east, in the South Pacific Islands, Philippines, Taiwan, Japan, China, Vietnam, Thailand, Malaysia and Myanmar; b) central, in India, Nepal, Bangladesh, Sri Lanka, Pakistan, Afghanistan, Iran and the Middle East; and c) west, in Egypt, Kenya, Sudan, Uganda, Tanzania, and other Mediterranean countries. These zones roughly represent natural cytogeographical clusters because S. spontaneum tends to present a different number of chromosomes in each of these locations (Daniels and Roach 1987).
Saccharum species present high ploidy levels. S. officinarum is octoploid (2n = 80) having x = 10 chromosomes, which is the basic chromosome number of members of the Andropogoneae tribe (D’Hont et al. 1995; Cesnik and Miocque 2004; Nobrega and Dornelas 2006). S. spontaneum has x = 8 chromosomes (D’Hont et al. 1996) but presents great variation in chromosome numbers with five main cytotypes: 2n = 62, 80, 96, 112 or 128 (Daniels and Roach 1987; Sreenivasan et al. 1987).
Modern sugarcane cultivars, which were derived from the hybridization between these two species, are considered allopolyploid hybrids (Daniels and Roach 1987), with most exhibiting a 2n + n constitution, representing two copies of the S. officinarum genome plus one copy of the S. spontaneum genome (Cesnik and Miocque, 2004). The S. officinarum genome usually duplicates when it is hybridized with S. spontaneum. This phenomenon facilitated the work of the first breeders because nobilization consisted of increasing the ratio of the S. officinarum to that of the S. spontaneum genome (Bremer, 1961). In situ hybridization studies have shown that the genomes of modern hybrids are composed of 10–20% of S. spontaneum chromosomes, 5–17% of recombinant chromosomes containing part of S. officinarum and part of S. spontaneum chromosomes and the remainder composed of S. officinarum chromosomes (Piperidis and D’Hont, 2001; D’Hont 2005).
The hybrids are usually aneuploid, with a prevalence of bivalents, a significant proportion of univalents and rare multivalent associations during meiosis (Daniels and Roach, 1987). Despite this genome complexity, evidence suggests a diploid-like mode of inheritance (Hogarth, 1987).
The “Saccharum Complex” Theory
It has been hypothesized that an intercrossing group named the “Saccharum complex” consisting of the genera Saccharum (including species previously classified under Erianthus sect. Ripidium), Sclerostachya, Narenga and Miscanthus sect. Diandra provided the basis for modern sugarcane varieties (Mukherjee, 1957; Roach and Daniels 1987; Daniels and Daniels, 1975). Saccharum officinarum was likely derived from crosses involving S. spontaneum, Miscanthus, S. arundinaceus (Syn: Erianthus arundinaceus) and S. robustum (Roach and Daniels, 1987). The presence of whole S. officinarum chromosomes, homologous to chromosomes from Miscanthus and from some Saccharum species previously classified as belonging to Erianthus sect. Ripidium, supports the hypothesis of hybridization among these species giving rise to Saccharum officinarum (Daniels and Roach, 1987; Besse et al., 1997).
Despite the fact that the aforementioned Saccharum complex hypothesis is currently broadly accepted, particularly by sugarcane breeders, who consider species within the “Saccharum Complex” as the primary gene pool for sugarcane breeding, recent molecular data do not support this theory (D’Hont et al. 2008). Thus, there is a suggestion that Saccharum is a well-defined lineage that diverged over a long evolutionary period from the lineages leading to the Erianthus and Miscanthus genera (Grivet et al. 2004).
Sugarcane breeding is based on the selection and cloning of superior genotypes from segregating populations that was obtained by crossing contrasting individuals. To maximize the efficiency of this rather long process, it is divided into various phases, including choosing suitable parentals and quantifying environmental effects on the expression of the selection characters.
The first step of a sugarcane breeding program consists of establishing a germplasm collection in a heavy flowering area where the flowering time of the parental lines can be synchronized. To meet sugarcane flowering requirements, the breeding stations operating in Brazil are all located in the heavy flowering northeastern region and include the breeding programs of CanaVialis at Maceió (AL), Rede Interuniversitária para o Desenvolvimento do Setor Sucroalcooleiro - RIDESA at Murici (AL), Centro de Tecnologia Canavieira – CTC at Camamú (BA) and Instituto Agronômico de Campinas - IAC at Uruca (BA). Other sugarcane breeding programs have been active in Brazil in the past and the codes for identifying varieties developed in those programs are listed in Table 2.
A typical sugarcane variety development program (Fig. 5) begins by making a large number of crosses among selected parental genotypes. The resulting seeds give rise to a large number of progeny (seedlings), which are needed to increase the chance of obtaining improved cultivars from superior genetic combinations. The selection process is conducted in distinct locations to identify superior genotypes with improved agronomical performance and tolerance to biotic and abiotic stresses. On average, one commercial variety can be obtained for every 250,000 seedlings evaluated in the first stage of the breeding program (T1). The selection process continues in the second and third phases, which are evaluated under different environmental conditions (Fig. 5).
Flowchart of a sugarcane breeding program. T1: seedling selection. T2: clone selection. T3: local trial. T4: regional trial. Source: CanaVialis
Sugarcane flowering is regulated by day length known as the photoperiod. Flowering induction and development occur when the hours of light decreases from 12.5 h to 11.5 h. Panicle emergence occurs with an additional decrease to approximately 11 h. Under these conditions in the southern hemisphere, flowering occurs close to the autumnal equinox (March 21st) and is delayed by approximately 2 days for each additional degree of latitude (Brett, 1951; Moore and Nuss, 1987). Flowering induction is only effective after the juvenile period has been completed, i.e., when at least two to four internodes have matured (Clements and Awada, 1967; Coleman, 1969, Julien, 1973). Adequate water and temperatures above 18°C are also necessary (Barbieri et al., 1984; Coleman, 1969).
S. officinarum is refractory to flowering, which effectively occurs only at low latitudes. The low flowering percentage of S. officinarum is exploited as a beneficial agronomic trait of hybrid cultivars once flowering is undesirable since sugar yield decreases during flower development as flowering culms stop to grow, become diseased and, ultimately, senesce. (Moore and Osgood, 1989).
The optimal temperatures for panicle development and pollen fertility are 28°C during the day and 23°C at night. Temperatures below 23°C delay panicle development and reduce pollen fertility (Brett and Harding, 1974; Berding, 1981). Daytime temperatures above 31°C and nighttime temperatures below 18°C are detrimental (Clements and Awada, 1967; Moore and Nuss, 1987).
Sugarcane pollen is small, ca. 50 μM, with a honeycombed exine and is dispersed primarily by wind. Sugarcane pollen grains dry rapidly after dehiscence with an estimated half-life of approximately 12 min. After 35 min at 26.5°C and 67% relative air humidity, the pollen has lost viability (Moore 1976; Venkatraman, 1922). Therefore, pollen dispersed over large distances is not expected to be viable. In addition, even under ideal conditions, some sugarcane varieties show poor pollen fertility or even male sterility due to the cytogenetic abnormalities that occur during meiosis that are associated with sugarcane’s high polyploid multispecies genetic complex (Ethirajan, 1987).
The conditions required for sugarcane flowering can be summarized as follows:
Latitudes between 5° and 15°: a region with a gradual reduction of photoperiod is an essential factor for flowering induction and panicle development (Midmore, 1980).
High temperatures, mainly during the night. Temperatures below 18.2°C are considered non-inductive. Sugarcane requires at least 10 inductive nights for flowering, but 15 nights are ideal. Non-inductive nights delay panicle development and reduce pollen fertility (Berding, 1981).
High relative humidity is critical not only for the induction and development of the panicle and pollen fertility, but also for anthesis and seed formation. Flower opening and anthesis, which are both affected by relative air humidity (RAH), only occur a few hours before sunrise, when the plant is totally hydrated and RAH is high (Moore and Nuss, 1987).
Because sugarcane is predominantly an outcrossing large stature plant, the first breeding programs used seeds from the open pollination of field grown plants, with no control over the male parent. Currently, crosses are conducted in a controlled manner under plastic domes or lanterns, where parentage can be guarenteed (Fig. 6). For specific crosses, the flowering dates of desired parents are synchronized by manipulating the photoperiod (Moore, 1987b). Stalks with inflorescences from selected parents are harvested and kept in an acidic solution to keep the stalks fresh which facilitates successful fertilization and seed maturation (Heinz and Tew, 1987). Panicles from both parents are placed under a dome for 12–15 days, with the male parent positioned slightly above the female parent. Normally, crosses are conducted in protected locations, such as in the middle of the forest or in covered sheds containing isolation cells, to avoid undesired cross-contamination. After 3–4 days for hybridization, fertilized panicles are kept in a shed for 1 week to promote seed maturation. Panicles are then harvested and placed in a heated chamber to dry the seeds (Ethirajan, 1987).
Sugarcane crossing conducted under domes (lanterns). Source: CanaVialis
In a variety development program, sugarcane cultivars are selected for low flowering. However, because flowering is influenced by environmental conditions, flowering in cultivation fields may still occur in a given location or in a given year. If seeds are produced and fall onto the soil, germination only occurs under conditions of high temperature and humidity. Therefore, sexual reproduction is strongly compromised in locations that have a dry, cold autumn, such as southern Brazil and the southern parts of the southeastern and midwestern regions of Brazil. In the northern parts of the southeastern and midwestern regions, relative humidity but not nighttime temperature is normally restrictive. In general, some flowering may occur in the southern, southeastern and midwestern regions of Brazil, and in some years, it may even be intense. Nevertheless, because RAH is low heavy flowering does not mean that seedings are produced; even if seeds are formed, field conditions are very unfavorable for germination because of low soil humidity, since it is well knwn that sugarcane seeds lose approximately 90% of their viability after 80 days at 28°C (Rao, 1980). However, in the northeastern region, conditions are favorable for both flowering and seed formation. Seed dehiscence occurs during the wet season, which favors seed germination under field conditions.
Potential for Lateral Gene Transfer
Commercial sugarcane production is performed exclusively using vegetatively propagated material of comercial hybrids. Under ideal flowering conditions, sugarcane pollen is dispersed by wind, with no participation of animal or insect vectors (McIntyre and Jackson, 2001). Because sugarcane pollen has low viability, natural hybridization can only occur close to the pollen-supplying plant (Moore, 1976; Venkatraman, 1922). Thus, little seed set is expected since pollen rapidly loses its viability.
Although crossings between species of the genus Saccharum with other closely related species have been suggested to occur in the wild (Grassl 1980; Daniels and Roach, 1987), wild hybridization has not been reported with current sugarcane varieties.
Species belonging to the “Saccharum Complex” exhibit different levels of sexual compatibility with S. officinarum and S. spontaneum under artificial controlled crossings (Bonnett et al., 2008). Hybridization among sugarcane species and Erianthus sect. Ripidium and Miscanthus species are more probable than with Narenga and Sclerostachya under breeder’s intervention. However, genetic transfer among commercial hybrids and these ancestral species, if existent, are much lower under natural conditions (Bonnett et al., 2008). It is important to note that there are no members of the “Saccharum Complex” species native in Brazil. In addition, there is no data on the biology of the wild Brazilian Saccharum species such as S. villosum, S. asperum, S. angustifolius and S. baldwinii (Filgueiras and Lerina, 2001; Kameyama, 2006; Carporal and Eggers, 2005) nor on the possibility of gene flow occurring between them and commercial sugarcane hybrids.
The Saccharum species that gave rise to commercial sugarcane varieties (S. officinarum and S. spontaneum, with minor contributions of S. robustum, S. barberi and S. sinense) are not native to Brazil. In Brazil, these species exist only in germplasm collections which are used in sugarcane breeding programs. Under breeding station conditions, they can flower synchronously and successfully hybridize with modern varieties. However, lateral transfer of genes among modern sugarcane hybrids and those species is not expected to occur under natural Brazilian environmental conditions.
Commercial Sugarcane Cultivation
The first step towards establishing a commercial sugarcane field is the production of vegetative planting material from the desired commercial variety under approved sanitary conditions at nurseries. To assure the starting material is disease-free, it is common practice that the stalks to be used as planting material are either exposed to thermotherapy (a hot water treatment to control systemic bacterial infections such as ratoon stunting disease), or they are obtained aseptically through meristem culture (free of bacteria and viruses), or from a combination of methods. Essentially, there are three types of nurseries differing primarily in size and generations removed from initial asepsis:
Basic Nursery or Pre-Primary Nursery originates from buds of aforementioned treated stalks or meristem propagated plants.
Primary Nursery originates from the Basic Nursery but is roughly ten times larger than that source. The first ratoon of the Basic Nursery is also considered a Primary Nursery.
Secondary Nursery originates from the Primary Nursery and is 10–15 times larger than the previous nursery. The second ratoon of the Basic Nursery and the first ratoon of the Primary Nursery can also be considered Secondary Nurseries (Xavier et al., 2008).
Commercial plantations are generally established using time proven conventional methods. Plowing is 30 cm deep, and the furrows cut to a depth of 25–30 cm. Rows are spaced at distances varying from 0.8 to 1.5 m and are planted with 8–12 tons of planting material per hectare. Stalks are distributed in furrows in pairs with the base of one stalk paired against the upper part of the other, i.e., two stalks are laid in opposite directions because the buds from the upper part of the stalk tend to germinate better than those of the base. After the stalks are distributed in the furrow, they are sectioned into 2 to 3-node seed pieces to interrupt apical dominance that exists in the intact stalk. In soils known to be infested by insect pests or nematodes, pesticides are applied over the cuttings in the furrows. The last step of the planting operation is to cover the cuttings in the furrows with 10–15 cm of soil (dos Anjos and Figueiredo, 2008).
In Brazil, the use of irrigation in commercial sugarcane fields is generally not necessary, contributing to low production costs. Currently, as marginal production areas, primarily drier areas with inadequate rainfall, are added to the sugarcane industry through crop expansion, drought tolerance is seen as an increasingly important trait for sugarcane varieties (Pires et al., 2008).
When the crop begins to grow, the most important agronomic practice is weed control. Once an optimal plant stand is established, the major concern is to employ practices to insure good crop development to achieve good maturation, i.e. accumulation of sugar, in an optimal time span. Proper practices assure optimization of the harvesting-milling operations and, consequently, overall economic return. This aim is achieved by mills cultivating a range of varieties having different soil nutrient requirements, rates of maturation and reliable disease resistance.
In Brazil, sugarcane harvesting is either semi-mechanized or completely mechanized. In the first case, the cane is harvested manually, but it is loaded onto trucks mechanically; in the second case, the cane is harvested by machines that load it directly onto trucks. Although the fully mechanized harvest system has the advantage of not requiring a prior burning step, it cannot be adopted everywhere because current harvesting machines cannot operate in areas where the slope exceeds 15–17% (Ripoli and Ripoli, 2008).
Commercial Sugarcane Crop Cycle
Sugarcane is a semiperennial crop in commercial fields. It has to be replanted approximately every three to six harvests when grown under the rainfed conditions of Brazil. Replanting is required because of declining yields due to crop and soil damage caused by the heavy traffic of machines and trucks over the stumps during harvesting. In addition, there could be a progressive accumulation over time of pathogens in the sugarcane crop, some of which reduce stand population while others impair plant growth. There may also be a genetic component contributing to yield decline because most of the commercial cultivars have been selected to produce well only for the first three to four cultivation cycles. The overall result is a decrease in year-over-year productivity, which can reach economically unfeasible levels and the need to replant the field (Matsuoka et al., 1999).
There are two basic sugarcane production cycles. The plant-cane cycle starts with planting and ends after the first harvest. The ratoon, or ratoon-cane, cycle starts after the harvest of the plant cane and continues with successive ratoon crops until field renewal (Fig. 7). The complete cycle of a sugarcane field lasts either four or five seasons, after which time the crop is renewed. Eradication of the crop after it has become economically unfeasible is performed by ploughing it under and harrowing the soil, which is often preceded by the application of a systemic herbicide.
Sugarcane phenological cycle. a Stalk pieces used in planting; b Beginning of bud sprouting and rooting; c Tillering initiation; d Intense tillering; e Beginning of maturation; f Manufacturable stalks in optimal sucrose concentration; g Harvesting; h...
The most important sugarcane insect pests in Brazil are the sugarcane borers (Diatraea saccharalis and Diatraea flavipennella), the giant sugarcane borer (Telchin licus), spittlebugs (Mahanarva fimbriolata and Mahanarva posticata), termites (different genera), the migdolus beetle (Migdolus fryanus), the sugarcane weevil (Sphenophorus levis) and herbivorous ants (Atta spp. and Acromyrmex spp.).
D. saccharalis is widely distributed in Brazil, while D. flavipenella is restricted to the northeastern region of the country. Both species construct galleries in the stalks, leading to less biomass and sugar production, and an increase in fungal infections and juice contamination. These sugarcane borers are mainly controlled by massive release of the parasitoid Cotesia flavipes, which is normally reared in labs at the various mills. Sugarcane borers are also controlled, although by a lesser extent, by the release of the parasitic wasp Trichogramma galloi. Currently there is strong evidence that the sugarcane borer population has been increasing due to the expansion of cane into new areas, the cultivation of susceptible varieties, and the failure to use biological control. The increase in borer populations has been causing an increase in the use of pesticides to control these insects (Dinardo-Miranda, 2008a). The giant sugarcane borer (T. licus) is another lepidopteran pest that attacks the crop. T. licus was considered to be restricted to northeastern Brazil, but there have been recent reports of its occurrence in São Paulo State, Brazil’s main sugarcane producing state. T. licus also construct galleries in the stalks, but they more easily outright kill the ratoons due to the extensive damage caused by their large size (Dinardo-Miranda, 2008a). Biological and chemical control mechanisms against T. licus are not effective, and the economic impact of the pest, if it spreads throughout Brazilian sugarcane-producing regions, has yet to be assessed.
The root froghopper (M. fimbriolata) has become an important sugarcane pest since Brazil started to abolish crop burning. Crop damage is caused by the young insect (nymph), which sucks water and nutrients from plant roots and injects toxins into them, leading to a decrease in root function and, consequently, a loss of productivity. Release of the fungus Metarhizium anisopliae results in good biological control, which can be complemented or replaced by pesticide spraying (Dinardo-Miranda, 2008a). The leaf spittlebug (M. posticata) is more predominant in northeastern Brazil; this insect sucks leaf sap, causing leaf drying; it can be controlled in the same way as M. fimbriolata.
There are many species of termites that attack sugarcane in the the country. Among these species, H. tenuis is the most harmful. These underground insects attack the stalks used for planting, leading to low bud germination and the need for replanting (Dinardo-Miranda, 2008a). Farmers control termites by spraying pesticides over the stalks in the furrow during planting.
The migdolus beetle Migdolus fryanus is a native Brazilian insect that attacks the roots of many crops, including sugarcane, coffee, eucalyptus and beans (Bento et al. 1985). The insect can destroy the root system, leading to an early need for field replanting. Control of M. fryanus is difficult because the larvae live deep within the soil so that pesticide application during planting is not very effective. Recently, the use of pheromone traps has been shown to be very promising for controlling this pest (Nakano et al., 2002). The sugarcane weevil (Sphenophorus levis) is another beetle that attacks the sugarcane root system, leading to damage similar to that caused by the migdolus beetle. S. levis has low dissemination ability so its spread is linked to human activities (Dinardo-Miranda, 2008a). Consequently, one of the more effective control practices for this pest is to avoid planting cane that is harvested from infected areas.
Ants that behave as pests in the sugarcane crop belong to the genera Atta and Acromyrmex. The species Atta bisphaerica and Atta capiguara cause most of the losses, but Atta sexdens and Atta laevigata also cause damage. Studies have shown that the ants of one anthill can reduce sugarcane productivity by 3.2 ton ha−1 (Dinardo-Miranda, 2008a). Control of these pests is accomplished using pesticides that must be applied very carefully because the pesticides could kill predator ants that are beneficial to the crop.
Many species of nematodes are found in association with sugarcane, but in Brazil, most of the damage is caused by five species: Meloidogyne incognita, Meloidogyne javanica, Pratylenchus zeae, Pratylenchus brachyurus and Helycotylenchus dihystera. These nematodes are mainly controlled with chemical pesticides because nematode-resistant varieties have not been developed in the Brazilian sugarcane breeding programs (Dinardo-Miranda, 2008b). Nematicide application during sugarcane planting can increase productivity up to 30% in some infested areas (Copersucar, 1982). It is also possible to use nematocides in ratoon cycles, but the control of nematodes on ratoons is not as effective (Dinardo-Miranda, 2008b). The pesticides that are registered in Brazil are displayed in Table 3.
Registered products to control insects and nematodes in sugarcane fields in Brazil. Source: AGROFIT (2010)
In addition to insects that act as pests, sugarcane fields contain other associated insects with different biological functions, which comprise the sugarcane insect fauna (entomofauna). There have been several sugarcane entomofauna studies aimed at identifying the impact of sugarcane field burning on the associated insect crop population. These studies revealed a species rich system (Macedo and Araújo, 2000a; Macedo and Araújo, 2000b; Araújo et al., 2004; Araújo et al., 2005). However, identifying the risks to the insect fauna of adopting specific technologies, such as new agrochemicals and genetically modified cultivars for crop improvement requires detailed analyses of the individual species that are most important and meaningful for monitoring (Romeis et al., 2008). Species selection must be based on the biological functions of the insects, their abundance and their economic importance. Other, less objective criteria such aesthetic value, cultural value and species at risk, may also be used (Romeis et al., 2008).
Table 4 displays a no exhaustive list of insects and nematodes that are most relevant to the sugarcane agroecosystem.
Insects and nematodes there are most relevant to sugarcane agrosystem in Brazil
Sugarcane Weed Control
Although sugarcane is a vigorous plant, the crop suffers from weed competition in its initial development stages. The most detrimental weed species are in the Poaceae and Cyperaceae families, but morning glory species may also interfere by coiling around the sugarcane plants, reducing leaf unfurling to decrease the photosynthetic area and slowing mechanical harvesting. In most sugarcane production areas of the world, herbicide use (chemical control) is the most common way to control sugarcane weeds. Table 5 displays a non exhaustive list of the most common sugarcane weeds occurring in sugarcane fields and Table 6 displays the registered products used to control them.
List of common weeds occurring in Brazilian sugarcane fields. Source: Azania et al. (2008)
Registered products to control weeds in sugarcane fields in Brazil. Source: AGROFIT (2010)
Most commercial sugarcane varieties are genetically resistant to most sugarcane diseases. In addition to genetic resistance, use of pathogen-free planting material is commonly used to avoid spreading of diseases. The Brazilian sugarcane industry does not usually control sugarcane diseases in commercial fields, but recently, coinciding with the first detection of the Orange rust (Puccinia kuehnii) in the country, the Agriculture Department has registered a product (azoxystrobin and ketoconazole) to control fungal disesases at sugarcane fields (Santos, 2008; Agrofit, 2010). Additionally, the Triazole fungicides triadimefon and triadimenol are registered to treat sugarcane stalks before planting to prevent smut contamination caused by Ustilago scitaminea (AGROFIT 2010). Sugarcane smut disease is also contolled by destroying contaminated plants in the field (roguing) when varieties having intermediate resistance to the pathogen are planted.
Table 7 displays a list of the most common sugarcane diseases in Brazil.
List of common diseases occurring at Brazilian sugarcane fields. Source: Dinardo-Miranda (2008a); Dinardo-Miranda (2008b); Almeida (2008)
Sugarcane’s high efficiency in fixing CO2 into carbohydrates for conversion into fuel has awakened the world’s interest in the crop. Emerging data indicates that sugarcane could be the best crop for the production of renewable energy, which could reduce some effects of global warming caused by the use of fossil fuels (Buckeridge, 2007). The impact of sugarcane on the environment might be reduced by adopting environmentally friendly agricultural practices such as the elimination of burning before harvest, modifying other practices for a reduction in diesel-driven transportation and a reduction in the use of oil-based fertilizers (Ometto et al., 2005).
Brazil’s land area currently occupied by sugarcane is mainly the result of the large expansion of the sugarcane industry that occurred in the 1970s, when the Pró-Álcool (pro-ethanol) program was created. During this period, sugarcane expansion occurred in areas that were originally covered with Atlantic Rain Forest, but which were already being used for pastures and annual crops. The current rapid expansion of sugarcane into new areas that have never been used to grow the crop raises questions about sustainability, particularly in those areas of the cerrado biome that are already threatened by the expansion of other crops (Rodrigues and Ortiz, 2006). This recent concern about the potential environmental impacts of crops has encouraged government agencies to promote studies to establish zones for planting sugarcane and to regulate how expansion will take place to avoid expansion into protected areas. This zoning proposal was recently approved and will allow the Brazilian Government to use Climatic Risk Zoning as a tool for the establishment of a sustainable sugarcane agribusiness in the country (Embrapa, 2009).
Hitorically, the sugarcane has been burned prior to harvest as a means to facilitate and thus reduce the costs for harvest and hauling of cane, whether harvested by hand or by machine. In addition, burning of the crop generally increased recovery of the sucrose contained in the plant. However, in the process of burning, carbon dioxide and other greenhouse gases, and soot or ash are released (Cançado et al., 2006). Brazilian law No. 11.241 (Sept. 19, 2002) in São Paulo State established a goal for the gradual elimination of sugarcane burning and associated measures by 2021, with all harvesting being entirely mechanized on non-burned cane by 2031. On June 4, 2007 in anticipation of this law, UNICA (União da Indústria de Cana-de-açúcar) which is the Brazilian Sugarcane Industry Association representing the sugar, ethanol and bioelectricity producing industry of the State of São Paulo, and the state government signed the Protocolo Agroambiental do Setor Sucroalcooleiro [Sugar and Alcohol Industry Agro-Environmental Protocol]. This protocol established several environmental principles and technical guidelines to be observed by the sugarcane industry. Perhaps the most important guidelines are those addressing the anticipation of the legal deadline for the end of sugarcane harvesting within 14 years.
Weediness of Commercial Hybrids and Related Species
As a result of continuous breeding and selection for agronomic traits of value, sugarcane has lost the competitiveness or invasiveness of the original species; modern cultivars have largely lost the ability to persist in non-agricultural habitats and only poorly perpetuate without human assistance (Holm et al., 1997; OGTR, 2008). Sugarcane hybrid cultivars do not possess true rhizomes or produce vigorous seedlings. It is possible to find leftover stools in cultivated areas, but there is no indication that these stools will perpetuate, and there is even less evidence that they have any invasive capacity. It is possible that if ratoons are not properly eradicated at the end of a cultivation cycle, they can regrow and become volunteer plants in the next crop. However, since the ratoons do not possess the ability to spread, they remain as isolated stools in the new crop.
Some species from which modern sugarcane hybrids are derived are classified as weeds. S. spontaneum has an invasive potential because it produces rhizomes that contribute to natural vegetative propagation and enable the species to adapt to a great range of environmental conditions. S. spontaneum is classified as a noxious weed in the United States (USDA, 2008) and is considered a weed in many other countries (GCW, 2009). It is important to emphasize that the rhizomatous nature of S. spontaneum was not incorporated into modern sugarcane hybrids. S. arundinaceum (Syn: Erianthus arundinaceum) is also recorded as a weed in the United States and in some Asian countries (GCW, 2009). The other species of the Saccharum complex (S. robustum, S. sinensis and S. edule) that may have been involved in the evolution of sugarcane varieties have not been recorded as exhibiting weed potential (GCW, 2009). Among other genera of the Saccharum complex (Miscanthus, Narenga and Sclerostachia), only Miscanthus and Narenga represent species classified as having weed potential (GCW, 2009). Narenga porphyrochoma (Syn = Saccharum narenga) is cited as a weed in Vietnam (Koo et al., 2000 in GCW, 2009). The genus Miscanthus contains five species that have been reported as having weed potential in some parts of the world: M. floridulus (Syn = M. japonicus), M. nepalensis, M. purpurascens (Syn = Miscanthus sinensis Subsp. purpurascens), M. sacchariflorus and M. sinensis (GCW, 2009).
There are other Saccharum species, which are not involved in the origin of sugarcane hybrids, that have also been recorded as having weed potential in some parts of the world, including: S. angustifoulius, S. bengalense, S. florindum, S. procerum, S. ravenae and S. villosum (Syn = S. trinii) (GCW, 2009).
Weediness of Commercial Sugarcane Varieties and Their Related Species in Brazil
After more than five centuries of sugarcane cultivation in Brazil, there is no evidence that this crop presents any traits that favor persistence and invasibility and there is no evidence of dispersion outside of agricultural environments. Commercial propagation is vegetative, and seedlings from breeding programs lack vigor.
As stated previously, no species that were involved in the origin of sugarcane hybrids are native to Brazil, but some species of Saccharum did originate in this country. Among those species, only S. angustifolium and S villosum are recorded as having some weed potential. S. angustifolium
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