Investigating physiological and biochemical

2.3.1.4 Photosynthesis 
Salt stress adversely affects the carbon metabolism of plant and carbon 
assimilation is severely affected in terms of photosynthesis. To cope with the 
conditions of osmotic stress, plants conserve their water and hence close stomata. This 
stomatal closure reduces the intracellular concentration of CO
2
and hence inhibits the 
carboxylation process of photosynthesis. This disturbance reduces the photosynthetic 
rate and carbon assimilation in the plants and ultimately plant growth is sharply 
reduced. Some other factors are also involved in the reduction of photosynthesis in 
salt stressed plants. These factors include disruption of photosynthetic pigments and 
enzymes (Stepien and Klobus, 2006). High salt concentration causes thylakoid 
shrinkage and stacking of grana membranes. Ion imbalance during salt stress causes 
K
+
deficiency in plants and hence K
+
is reduced in chloroplasts which disintegrate the 
photo system.
One of the most notable effects of salt stress is the alteration of photosynthetic 
pigment biosynthesis (Maxwell and Johnson, 2000). However, contradictory reports 
are available in literature for increase or decrease of chlorophyll contents under salt 
stress. Although the reduction in chlorophyll contents during salt stress is a frequently 
observed phenomenon, but many studies show an increase in chlorophyll contents in 
salt stressed plants. For instance, increase in chlorophyll contents were observed in 
plants such as mustard (Jamil and Rha, 2013), cotton (Higbie et al., 2010), sugar beet 
and cabbage (Jamil et al., 2007b) under salt stress conditions while a decline in 
chlorophyll contents due to salinity was observed in Oryza sativa (Amirjani, 2011, 
Chutipaijit et al., 2011), Vigna radiata (Saha et al., 2010), tomato (Dogan et al., 
2010), radish (Jamil et al., 2007a) and pea (Hamada and El-Enany, 1994). In different 
studies, the chlorophyll contents were used as a sensitive indicator of the cellular 
metabolic state (Chutipaijit et al., 2011).

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Based on the integration and correlation between salt tolerance and 
photosynthetic rate, it can be suggested that photosynthetic rate is useful selection 
criterion only for those species in which this correlation exists under saline conditions. 
Thus selection, breeding or genetic engineering of the species having high rate of 
photosynthesis and improved performance can be fruitful on salt-affected soils.
To develop practical strategies for screening salt tolerant genotypes of any crop, it 
is very important to investigate that whether physiological or biochemical changes 
during salt stress are the part of salt tolerance mechanism or these are the 
consequences of detrimental effects of salt stress. Keeping in view the above 
discussion, it is obvious that none of the above traits or attributes can be suggested as 
universal criteria for the determination of plant tolerance during salt stress. So, it 
would be better and valuable if different attributes are specified for different 
individual crop species.