Geoscience Reference
In-Depth Information
regulation is mediated by hexose, but little is known about the
downstream pathways of this signal (McCormick et al. 2008a).
The relationship between sink and source is a key step in the
identification of targets that can be changed in order to improve
sucrose accumulation. Sucrose production and storage is associ-
ated with the demand imposed by sink organs (McCormick et al.
2008b). For example, when the leaf growth is reduced, sucrose
content tends to increase in culm (Inman-Bamber and Smith
2005). Furthermore, transgenic varieties that express an enzyme
that converts sucrose into isomaltulose showed increased pho-
tosynthesis, probably due to the introduction of this new carbon
sink (Wu and Birch 2007). Finally, the reduction of leaf elonga-
tion induced by water deficit redirects the carbon partition and
provides an increase in sucrose content (Inman-Bamber et al.
2004). Experiments showed that water stress reduced the whole
plant photosynthesis by 18% and the plant extension rate by
41%, resulting in a 19% reduction in total biomass.
However, water stress increased the sucrose mass gain by 27%
and increased sucrose content of the dry mass by 37%, confirm-
ing that water deficit reduced the demand for photo assimilation
for producing fibre and tops so that excess assimilate was allowed
to accumulate in the form of sucrose (Inman-Bamber et al. 2008).
The impact of water deficit on the physiology or develop-
mental process and on gene expression is also under study
on six different sugarcane varieties in four regions of Brazil.
As expected, the preliminary physiological measures showed
that different cultivars utilise different mechanisms to sur-
vive water stress (Paros et al. 1989). For example, one cultivar
utilised leaf rolling to reduce water loss, whereas a different
variety increased root to shoot growth to reduce water loss and
to increase water uptake (L. Endres, personal communication).
Over the next decades, climate change and increased CO
2
lev-
els are projected to impact the productivity of all crops. The CO
2
levels are predicted to increase from about 379 ppm in 2005 to
730-1020 ppm by the end of the century (IPCC 2007). To design
sugarcane crops for maximum productivity in such a changing
environment, it is necessary to study how the increase of CO
2
levels affects sugarcane physiology. An increase in the levels
of CO
2
will reduce the rate of photorespiration in all plants, but
considerably more in C3 plants than C4 plants. Nevertheless, C4
plants do increase their biomass when CO
2
levels are increased
from 370 to 720 ppm. This increase in biomass of C4 plants is
associated more with the increase in water use efficiency than in
the reduction of photorespiration (Vu et al. 2006; de Souza et al.
2008). An efficient use of water leads to a lower rate of water
