An environmental factor that limits crop productivity or destroys biomass is referred to as a stress or disturbance
An environmental factor that limits crop productivity or destroys biomass is referred to as a stress or disturbance (Grime, 1979). Salinity in soil or water is one of the major stresses and, especially in arid and semi-arid regions, can severely limit crop production (Shannon,1998). Over 800 million ha of land throughout the world is salt affected either by salinity 397 million ha or associated conditions of sodicity 434 million ha (FAO, 2005). Munns ( 2005) reviewed growth response due to salinty. The deleterious effects of salinity on plant growth are associated with (1) low osmotic potential of soil solution (water stress), (2) nutritional imbalance, (3) specific ion effect (salt stress), or (4) a combination of these factors (Ashraf,1994, Marschner1995). All of these cause adverse pleiotropic effects on plant growth and development at physiological and biochemical levels (Munns, 2002,) and at the molecular level (Winicov,1998, Mansour ,2000,Tester and Davenport2003) In recent decades, considerable improvements in salinity tolerance have been made in crop species through conventional selection and breeding techniques (Zorb, 2005). There are two phases of soil salinity. The first phase of the growth response results from the effect of salt outside the plant. The salt in the soil solution reduces leaf growth and, to the lesser extant root growth (Munns, 1993). The limitation of leaf growth in maize in first phase of salinty has been reported by Zörb et al., (2005). The second phase of growth response results from the toxic effects of salt inside the plant. The salt taken up by the plant concentrates in the old leaves: continued transport into transpiring leaves eventually results in very high Na+ and Cl_ concentrations. The importance of altered metabolism under abiotic stress for example diversion of carbon to polyol biosynthesis is exemplified by the metabolic reactions originating from the Glucose-6-P pool. The levels of glycine betaine are diverse in different varieties of maize (Brunk et al., 1989). The salt in the soil solution reduces leaf growth and, to the lesser extant root growth (Munns, 1993). The limitation of leaf growth in maize in first phase of salinity has been reported by Zörb et al., (2005). In maize, salt stress may reduce the assimilate supply to the growing shoot by partially inhibiting photosynthesis (Yang and Lu, 2005). This reduction of assimilate supply could also be responsible for shoot growth inhibition during the first phase of salt stress. In addition to its role in controlling growth, assimilate supply influences the plant’s capacity to maintain turgor through osmotic adjustment (Munns, 1988). We observed a concentration of chloroplasts in leaves of salt treated plants in the first days after salt treatment, thus may result from the growth reduction in consequence of negative Na+ effects especially in the growing cone of the leaf (Zörb et al., 2005). The darker green colour of the leaves changed to chlorotic patches beginning in older leaves at their tips in the second face of plant growth as a consequence of Na+-toxicity. When rice plants were applied to NaCl stress a darker green colour of the leaves in comparison to control plants was observed in the first phase, the leaves were narrower, the cells were smaller, and so the chloroplast density was greater (Munns et al., 2006; Zörb et al., 2005). With time, photosynthesis per unit area decreased due to reductions in stomatal conductance, and later there were non-stomatal limitations associated with a build-up of Na+ in the whole tissue in rice above 250 mM (Munns et al., 2006).