3 Investigating the Effect of Elevated CO 2 in the Growth Environment ... 51
to pump water. Thus, the physiology of high-salinity-stressed plants resembles the
physiology of drought-stressed plants (Mahajan and Tuteja 2005 ). To avoid loss of
water through osmosis, plants have to increase the osmotic pressure of their cells.
In light of this need, under salinity stress plant cells tend to accumulate metabolites
that act as osmolytes, e.g., proline (Delauney and Verma 1993 ; Ford 1984 ). The
net effect of this metabolic “deviation” is that the plants have to use part of their
resources towards the production of osmolytes, thus decreasing carbon flux towards
their growth (Kanani et al. 2010 ). Moreover, under high-salinity stress plants tend
to close their stomata to reduce water loss by transpiration. Carbon dioxide fixa-
tion through the Calvin cycle is then reduced and the photosynthesis rate declines
(Chaves et al. 2009 ). Furthermore, reduced CO 2 in the chloroplasts combined with
intense light enhances the photoproduction of ROS (Asada 2006 ). High levels of
sodium may also have deleterious effect on the functioning of some of the enzymes
(Niu et al. 1995 ). Lower photosynthesis and transpiration levels in combination
with a lower flux towards the plant growth cause a decrease in the development and
productivity of the salinity-stressed plants (Cuartero and Fernandez-Munoz 1999 ;
Shannon and Grieve 1999 ). It is thus apparent that the salinity stress is a substantial
constraint to crop production especially in the arid and semiarid climates (Wang
et al. 2003 ). In greenhouses, the problem of high-salinity stress may be even more
intense, especially in the case of poor quality water in combination with high tem-
peratures (Ayers and Westcot 1985 ). In addition, in hydroponics, the salinity of the
small volume nutrient solution can increase rapidly, especially in closed systems
with nutrient solution recycling (Magan et al. 2008 ).
3.2.2 Elevated CO 2 in the Growth Environment of the Plants
Short-term enrichment of the CO 2 in the growth environment of the plants up to
three times the current ambient (375 ppm) level has a positive impact in the plants
as it stimulates photosynthesis and reduces stomatal conductance (Ainsworth and
Rogers 2007 ). However, the long-term exposure of plants to elevated CO 2 leads to
photosynthetic acclimation and reduced CO 2 uptake (Rogers and Ellsworth 2002 ).
Positive responses to elevated CO 2 are mainly attributed to the competitive inhibi-
tion of the photorespiration by the carbon dioxide. Increase in the CO 2 levels in
the growth environment of the plants increases carbon fixation. The elevated CO 2
conditions can also enhance growth through improved plant water relations, since
the increased CO 2 slows down the transpiration by inducing the partial closure of
stomatal guard cells of the leaves (Prior et al. 2011 ).
3.2.3 Combined Application of High Salinity and Elevated CO 2
Based on the observed physiological characteristics of the plants under short-term
elevated CO 2 and high-salinity treatment, the former perturbation can be beneficial
for the plant growth, while the latter initiates a series of negative physiological con-
sequences on plants upon its application. Thus, it is of interest to investigate how