100 I. M. Ahmed et al.
the stomata and the mesophyll (Flexas et al. 2007 ) or the alterations of photosyn-
thetic metabolism (Lawlor and Cornic 2002 ) or they can arise as secondary effects,
namely oxidative stress. Anjum et al. ( 2011 ) indicated that drought stress in maize
led to considerable decline in net photosynthesis, transpiration rate, stomatal con-
ductance, water-use efficiency, intrinsic water-use efficiency, and intercellular CO 2
as compared to well-watered control.
Suppression of the photosynthetic capacity by salinity stress has been reported
in a number of plant species (Robinson et al. 1983 ; Ball and Farquhar 1984 ; Perez-
Lopez et al. 2012 ) and might be due to lower stomatal conductance, depression in
specific metabolic processes in carbon uptake, inhibition in photochemical capaci-
ty, or a combination of these (Dubey 1997 ). Tavakkoli et al. ( 2011 ) reported specific
ion toxicities of Na+ and Cl− reducing the growth of four barley genotypes grown in
varying salinity treatments. High Na+, Cl−, and NaCl separately reduced the growth
of barley; however, the reductions in growth and photosynthesis were greatest un-
der NaCl stress and were mainly additive of the effects of Na+ and Cl− stress. High
concentrations of Na+ reduced photosynthesis mainly by reducing stomatal con-
ductance. Salt-tolerant species, Barque73, had significantly greater photosynthetic
rate and water-use efficiency than those of Sahara, Clipper, and Tadmor. It was
concluded that high salt tolerance of the Barque73 was associated with a high CO 2
assimilation rate, and water-use efficiency.
5.7.3 Chlorophyll Contents
Chlorophyll is one of the major components of photosynthesis, and decrease in
chlorophyll content under drought stress has been considered as a peculiar symptom
of oxidative stress and may be the result of pigment photooxidation and chloro-
phyll degradation. Drought stress caused a large decline in chlorophyll a content,
chlorophyll b content, and total chlorophyll content in different sunflower varieties
(Manivannan et al. 2007 ). Barley plants grown under drought showed inhibition of
chlorophyll synthesis as demonstrated by reduced SPAD (soil-plant analyses devel-
opment analyses, based on chlorophyll meter readings) values (Zhao et al. 2010 ).
Guo et al. ( 2009 ) reported that, after 13 days of drought stress, Martin and HS41-1
(drought tolerant) had much higher chlorophyll contents than Moroc9-75 (drought
sensitive).
The chlorophyll contents of leaves decrease in general under salt stress. The
oldest leaves start to develop chlorosis and drop-off with prolonged period of salt
stress (Hernandez et al. 1995 ; Gadallah 1999 ; Agastian et al. 2000 ). However, Wang
and Nil ( 2000 ) have reported that chlorophyll content increases under conditions
of salinity in Amaranthus. Salinity causes significant decreases in Chl-a, Chl-b,
and carotenoid in leaves of barley (Vysotskaya et al. 2010 ). Ahmed et al. (2013b)
reported that barley plants grown under combined drought and salinity treatment
showed a marked reduction in chlorophyll content (Chl-a, Chl-b, and carotenoids),
accompanied by a sharp decrease in net photosynthesis (Pn), stomatal conductance
(gs), and transpiration rate (Tr). These results indicate that photosynthetic inhibition
was caused by stomatal factors and by chlorophyll synthesis inhibition.