Environmental Engineering Reference
In-Depth Information
incorporate CO 2 first into a four-carbon atom compound by a different enzyme (phosphoenol pyru-
vate carboxylate—PEPc). That is why sugarcane is referred to as a C 4 photosynthesis plant. This
four-carbon compound (phosphoenol pyruvate) is subsequently processed to produce CO 2 in another
cell type (the cells of the bundle sheath). The process occurs in the leaves in a much higher pressure
of CO 2 than in the outside air, and proceeds through the biochemical pathways of C 3 photosynthesis,
providing sugarcane with an extremely efficient system that processes light and CO 2 for production
of sugars. This photosynthetic system occurs especially in species that live in environments where
there is abundance of light and relatively higher temperatures. The C 4 photosynthesis system was
discovered in 1966 in Australia in leaves of sugarcane (Hatch and Slack 1966). Later on, other sci-
entists found that plants like maize also display this type of metabolism.
The control of sugar metabolism in sugarcane is associated with the plant hormone ethylene.
This hormone cross-talks with several other hormones and also affect the nitrogen metabolism. It is
likely that a complex system including multiple genes controlled by environmental factors (mainly
water stress), plant hormones that lead to changes in plant metabolism, and ecophysiology (e.g.,
photosynthesis) are related to the level of production of sucrose and subsequently to biomass and
bioethanol.
Physiologically, the accumulation of sucrose in stems of sugarcane appears to be directed for
flowering. In 1998 Carlucci et al. reported results of a cultivar (IAC 52-150) growing in Piracicaba,
SP, Brazil, that flowers if grown under long days (sugarcane flowers with a 12- to 12.5-h photo-
period). Induction of flowers occurred in March, when humidity was high and flowers started to
develop in May, reaching up to 70 cm in June. Flowering initiation is thought to have its optimum
between 18 and 31ºC. In fact, the difference between maximal and minimal temperatures is cru-
cial. Pereira et al. (1986) and Carlucci et al. 1988 found that flowering was intense when this dif-
ference was of 10ºC and that flowering did not happen when the maximal-minimal temperature
difference was on the order of 14ºC. According to these authors, the combination of extreme tem-
perature differences with water stress during the flower induction period negatively affects flower-
ing, retarding or preventing it completely. Thus, flowering seems to be controlled by a combination
of factors including temperature and water. Because of that, sugarcane plants hardly flower in the
Southeast of Brazil (ca. 24º of latitude), where most of the sugarcane crops are planted. On the
other hand, this is probably the reason why a period of water stress is desirable for high accumula-
tion of sugar. The stress is likely to delay flowering initiation, but not induction, which probably
leads to a change in the pattern of source-sink relationship, provoking an accumulation of sugar
to prepare the plant for flowering. Therefore, avoiding or delaying flowering is very important
from the agricultural viewpoint as the sucrose that is stored in the parenchyma cells of the stems
is the reserve of carbon that the plant will use to produce the flowers. Thus, by harvesting, farm-
ers intervene in the flowering process, preventing the plant from using the sucrose stored for that
purpose and extracting it for sugar or ethanol production. The lack of low temperature or water
stress becomes critical when plants are cultivated in the Amazon, in the North of Brazil because it
makes it difficult for most varieties to accumulate sucrose; therefore, that region has no favorable
conditions to grow sugarcane.
21.5 suGarcane Genome and GenomIcs InFrastructure
Genomics is increasingly recognized as a powerful approach to address scientific questions in biol-
ogy. It establishes the nucleotide sequence of an organism and as a result enables gene content
prediction. The adaptation of an organism to the environment, its performance and its phenotype
are a result of multiple gene products interactions. Moreover, knowledge transfer from one model
organism to another of yet less information is made possible with comparative analysis. Sugarcane
biology will greatly benefit from nucleotide sequence determination, as it will foster a systems biol-
ogy approach by understanding genome structure and regulatory networks. The challenge in deter-
mining the sugarcane genome sequence is the complexity of its genome structure as a polyploid and
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