63The alleviating role of the elevated CO 2 in the growth environment of salt-stressed
plants was also supported by the downregulation of transcripts related to the ethyl-
ene signaling, a pathway that is characteristically upregulated at the transcriptional
level in plants exposed to salt stress. These observations suggest that the controlled
use of the CO 2 in greenhouses could offer a pragmatic solution for counteracting
the negative effect of high soil or water salinity and lead to plant crops of consistent
quality and yield.
3.7 Experimental Design Specifications of Integrated
Omic Analyses of Combined Salinity and Elevated
CO 2 Stresses
A controlled study of the combined effect of high salinity and elevated CO 2 on the
plants should include at least four plant groups: plants grown in a control medium
or soil and the ambient CO 2 concentration (i.e., the control group), plants grown in
a high-salinity medium or soil and the ambient CO 2 concentration (i.e., the high-sa-
linity group), plants grown in a control medium or soil and an elevated CO 2 concen-
tration (i.e., the elevated CO 2 group), and plants grown in a high-salinity medium
or soil and an elevated CO 2 concentration (i.e., the combined perturbation group).
In this type of studies, hydroponic cultures provide a more controlled system over
the soil-grown plants, as the effect of any perturbations in other growth parameters,
e.g., the nutrient composition of the soil, are minimized. Comparison between the
four measured physiological states can provide information about changes in the
physiology of the plants due to stresses that are not directly measured. For example,
the comparison between the omic profile of the combined perturbation and the high-
salinity groups can provide information about the effect of the elevated CO 2 on the
salt-stressed plants, even when this experiment has not been carried out. This is how
the alleviating role of the elevated CO 2 on salt-stressed plants was identified in Ka-
nani et al. ( 2010 ). Depending on the investigated species (or cultivars or ecotypes),
the imposed salinity should be high enough to act as stressor for the plants, but
not too high to cause tissue necrosis. In the high-salinity experiments, the utilized
salt concentration usually ranges from 50 to 150 mM, reaching 250 mM in some
studies of halophile species. In most reported studies, elevated CO 2 conditions are
characterized by concentrations between 500 and 1500 ppm to simulate the plant
responses to the predicted increase in the ambient CO 2 due to the greenhouse effect
(Kaplan et al. 2012 ; Li et al. 2008 ; Miyagi et al. 2011 ). However, in some studies, a
much greater CO 2 concentration has been used to ensure changes in the physiology
of the plants due to this perturbation (Kaplan et al. 2012 ; Dutta et al. 2009 ). Apart
from growth chambers, FACE facilities have also been used to study the effect of
the elevated CO 2 on the plants (Li et al. 2008 ). At these facilities, horizontal or verti-
cal pipes are placed in a 1 m to 30 m diameter circle around the experimental plot,
and emit CO 2 -enriched air around the plants (Ainsworth and Long 2005 ).
3 Investigating the Effect of Elevated CO 2 in the Growth Environment ...