Environmental Engineering Reference
In-Depth Information
appropriately represent the harmful effects of salt stress on plants and also can be
conveniently measured. We propose that plant biomass, crop yield and photosynthesis are the
most reasonable candidates, because plant growth and food production mainly depend on
them. It should be stressed that photosynthetic diagnosis becomes a very convenient way to
detect the plant healthy status, particularly after the exploitation of chlorophyll a fluorescence
transient. In addition, the adaptive mechanisms in responses to salinity-induced negative
effects were generalized and simplified, and some representational parameters were suggested
in this review.
Plant Biomass and Photosynthesis
Decrease of plant biomass was commonly induced by environmental stress including
salinity stress (D'Souza and Devaraj 2010; Gorai et al. 2011; Tarchoune et al. 2012). It is a
traditional and comprehensive index for the negative effects on plant in adverse environment.
However, plant biomass is not a sensitive parameter, because the decrease in biomass
accumulation results from long term effect of adverse environment. Photosynthesis is closely
related to plant growth and crop yields and highly sensitive to environmental stress as well.
Photosynthesis involves the parts of CO
2
fixation, photosynthetic electron transport for
reducing power, and proton transport for adenosine triphosphate generation. Photosynthetic
rate would decrease if any part of photosynthesis was inhibited under environmental stress.
Decrease in photosynthetic activity can be commonly found in plants under salt stress, which
is often induced by various reasons. Initial negative effect of salt stress on photosynthesis
may be the decreased CO
2
availability as a result of the diffusion limitations of stomata
(Flexas et al. 2004; Dasgupta et al. 2011; Benzarti et al. 2012; Tarchoune et al. 2012).
Stomatal conductance responds to soil salinity owing to the perturbed water relations or the
synthesis of abscisic acid (ABA) (Fricke et al. 2004). It has been reported that ABA increased
in the photosynthetic tissues transitorily within 10 minutes of the addition of 100 mM NaCl to
barley (Fricke et al. 2004; Fricke et al. 2006). The rapid increase suggested that ABA was
synthesized in situ rather than transport from the roots. In addition, salt stress also can reduce
the mesophyll conductance to CO
2
, and then result in the decrease of photosynthetic rate
(Loreto et al. 2003). The resistance of the chloroplast envelope and stroma is mainly
responsible for the total mesophyll resistance, and it is also influenced by respiratory and
photorespiratory CO
2
diffusing towards the chloroplasts from the mitochondria (Tholen et al.
2012). The dark enzymatic process of CO
2
fixation can be directly perturbed by salt stress, for
example, the decrease in Rubisco activity and content (Downton et al. 1985; Brugnoli and
Bjorkman 1992; Yang et al. 2008; Tarchoune et al. 2012). Photosystem II (PSII) plays an
important role in plant photosynthesis, and it often operates abnormally under environmental
stress. So far, great efforts have been made to figure out PSII responses to salt stress,
however, inconsistent results were obtained. PSII function was proved to be inhibited by salt
stress in some studies (Belkhodja et al. 1994; Everard et al. 1994; Netondo et al. 2004;
Benzarti et al. 2012), whereas high resistance of PSII to salt stress was demonstrated in other
studies (Morales et al. 1992; Lu et al. 2002; Lu et al. 2003a; Lu et al. 2003b; Chen et al. 2004;
Tarchoune et al. 2012). Different plant species used in these studies may be one of the reasons
leading to the inconsistent results. Up to now, the responses of photosystem I (PSI) have not
