Ecologia Mediterranea

T. S. KSIKSI, N. O. ALSHAYGI
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2005). It also inhibits plants’ access to soil
water by increasing the osmotic strength of
soil solution (The Regional Institute Ltd.
2008). High salinity has a substantial effect
on plants’ major processes such as photosynthesis,
protein synthesis and energy and lipid
metabolism (Parida & Das 2005). In addition,
growth and gas exchange properties such as
net photosynthesis and water use efficiency,
which are related to stomatal conductance, are
also affected by high salinity (Koyro 2006).
Salinity stress reduces plants growth because
of the low photosynthesis rate in plants that
are exposed to high salinity, which can
decrease the amount of carbohydrates that are
supplied mainly by photosynthesis and are
needed for cell growth (Parida & Das 2005).
This low photosynthetic rate comes as a result
of salt concentration which has an osmotic
and toxic effect, leading to an inhibition of
plant uptake of some minerals such as N, K,
and Ca (Majeed et al. 2010; Koyro 2006).
Additionally, salinity stress decreases blade
water and osmotic potential (Koyro 2006).
Blades growth is more sensitive to salinity
than root growth (Munns & Termaat 1986).
Whereas for fruits, high salinity shortened the
time of fruit development in some plants and
shortened the fruit life span in others (Mizrahi
1982). Seed germination and dry biomass
production are also negatively affected by
high salinity, but seeds seem to be less inhibited
by salinity than growing plant parts
(Ungar 1996). Salinity did not affect seed
recovery after stress (Ungar 1996). But the
extent of plant response to such stress under
enriched CO2 conditions requires more investigations.
Scientists believe that plants’ response to the
enrichment of CO2 is mainly manifested
through increasing photosynthesis and
decreasing stomatal density (Ainsworth &
Rogers 2007). While all other effects of CO2 elevation on plants derived from those two
fundamental responses (Ainsworth & Rogers
2007). It is supposed that 95% of plant
species didn’t reach their maximum photosynthetic
efficiency under ambient CO2 , that’s
why the current increase in atmospheric CO2 could stimulate plants’ growth and it is
expected to increase more by the current raise
in atmospheric CO2 (Ksiksi & Youssef 2010;
Ziska et al. 2009). Others believe that the
response of plants to elevated CO2 will stabilize
in the long run because of the photosynthesis
acclimation, where plants can’t use
more atmospheric CO2 (Barrett & Gifford
1995). Carbon dioxide absorption, however,
depends on photosynthetic pathways (ie: C3
vs. C4).
The difference in responses between C3 and
C4 plants to elevated CO2 has been a topic
debated by various scientists. Environmental
stresses that are caused by the elevation of
carbon dioxide worldwide are expected to
ameliorate C4 plants’ response to the elevated
CO2 (Leakey 2009). Some studies suggest that
the response of C4 plants growing under elevated
CO2 is smaller than the response of C3
plants, especially in non-stressed conditions
(Barrett & Gifford 1995; Curtis et al. 1989).
The reason behind the difference is that C4
photosynthesis rate does not increase with the
increase in CO2 levels (Wang et al. 2003),
caused by specialized C4 plant mechanisms
(Bowes 1993). The majority of studies on
atmospheric CO2 enrichment dealt with C3
plant species, and few studies were on C4
species (Wang et al. 2003). Most of those
studies have focused on weeds and dicotyledonous
species (Wang et al. 2003). This bias
in studies seems surprising since half of the
world’s grass species possess C4 photosynthesis
(Brutnell et al. 2010). Any change in
C4 species productivity carried out by the
atmospheric CO2 enrichment could have a
huge impact on the gross primary productivity,
since the C4 plant contribution is about
21% worldwide (Wand et al. 1999). However
some of those limited studies on photosynthetic
activity of C4 species showed that there
is a direct enhancement of atmospheric CO2 increase on C4 species (Wang et al. 2003).
The objectives of the current study are to:
(1) assess the extent of Cenchrus ciliaris L.
responses to CO2 enrichment, (2) study the
appropriate environmental conditions for optimal
growth, (3) test the morphological
responses to CO2 enrichment, and (4) evaluate
the role of salt stress in affecting C. ciliaris
responses to CO2 enrichment.
Materials and Methods
The experiment was carried out between
December 2009 and May 2010 within the
United Arab Emirates (UAE) University campus
in Al-Ain city (N 24.19, E 55.62). The
trial was conducted in a greenhouse with controlled
temperature, within two plastic chambers
(336 x 244 x 22 cm) erected for this trial.
ecologia mediterranea – Vol. 38 (2) – 2012