Environmental Engineering Reference
In-Depth Information
of lignocellulose and sugar conversion into ethanol is not yet economical but its development is
highly desired because it could lead to an increase of 40-50% in ethanol production. Potentially,
with the industry of cellulosic ethanol it is expected that the ethanol output might increase from the
current 7,500 to 13,000 L/ha. Sugarcane second-generation ethanol has not yet been used commer-
cially but many R&D initiatives are underway.
To improve yield and other traits of interest that will allow for a sustained industry of sugar-
cane and for the development of an energy-cane, research groups in biotechnology, transgenics,
sugarcane genomics, statistical genetics for polyploid genomes, and gene discovery are gathering
efforts all over the world to devise the mechanisms involved in the regulation of sucrose content,
yield, drought resistance, biomass, and, more recently, cell wall recalcitrance. It is important to note
that the whole sugarcane genome sequence is unknown and that sugarcane has a highly polyploid
giant genome (~10 Gb). Recent works on expressed sequence tags (EST) added value to this crop's
genomics but whole genome sequencing efforts are underway that will make available chromo-
somal gene structures and allelic variations of sugarcane. The SUCEST (a project that has gen-
erated the largest collection of ESTs—http://sucest-fun.org) database consists of 33,620 putative
transcripts with a sequence mean size of 864 bp (Vettore et al. 2003) which represents about 30 Mb
of sugarcane genome sequence, a small fraction of the complete genome sequence. Only with the
recent developments of next-generation sequencing technologies has the identification of genes,
alleles, and promoters as well as the definition of the overall structure of the genome been made
possible. Also, if sugarcane is to be improved for bioenergy production, a significant number of cul-
tivars and genotypes need to be evaluated at the biochemical and physiological level and molecular
biologists have to join efforts with breeders bringing biotechnological tools to the game. Although a
lot is known about plant cultivation, the biochemical and genetic characterization of this crop are at
early stages. The remainder of this text will focus on the different aspects of bioethanol production
using this crop and research developments for the improvement of sugarcane.
21.2 suGarcane cultIvatIon
Optimum temperature for sugarcane cultivation is between 30 and 34°C. Plant growth is greatly
reduced below 21°C for most varieties. However, in the maturation stage, sucrose accumulation
is triggered by dry conditions or low temperature, usually with average temperatures below 20°C.
Death of leaves may occur below 2.5°C and apical and lateral buds die at -1 to -3.3°C and -6°C,
respectively (Alfonsi et al. 1987; Liu et al. 1999). Usually at least 900-1000 mm of rain are neces-
sary for rain fed production (Inman-Bamber and Smith 2005) but the need of irrigation depends
also on rain distribution along the season.
Sugarcane is a semi-perennial bushy plant, in which several long stalks germinate from rhi-
zomes or stools. Long alternated leaves are attached to the stalk nodes. The cylindrical stalks
may reach 2-5 m tall and accumulate sugars mainly in the internodes. The world average stalk
yields are around 70.9 t/ha but this varies with soil, climate, cycle length, and growing conditions
(Table 21.1). Under favorable conditions, yield may reach above 200 t/ha but the theoretical yield
according to different authors varies from 285-470 t/ha per year (Landell and Bressiani 2008;
Waclawovsky et al. 2010).
In commercial fields sugarcane is planted with stem cuttings (seed cane) instead of seeds.
Usually 8-12 t of 8- to 10-month-old stalks containing 12-18 buds per meter of row are planted in
furrows spaced at 0.8-2.0 m. Small farmers adopt a narrow spacing whereas wider spacing is used
in mechanized fields (Anjos and Figueiredo 2008).
The harvest takes place 10-24 months after planting. After harvest the plant sends up new
stalks or ratoons that will be cut again usually within one year. Normally, yields decline in
subsequent ratoons but two to ten cuttings can be performed before the crop needs to be planted
again, depending on the variety, climate, pest and disease incidence, soil type, and management
conditions.
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