An environmental factor that limits crop productivity or destroys biomass is referred to as stress or disturbance. Salinity in soil or water is one of the major stresses and especailly in arid and semi-arid regions, can severely limit crop production. Salt stress comprises two components - ionic stress and osmotic stress (Zörb et al. 2005).<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
Total sugars, free amino acids and osmotic potential in the leaves of maize at the three-leaf stage were not significantly different in transgenic and control plants (Quan et al. 2004).
In salt-resistant species such as celery and ice plant stress has been found to induce synthesis of polyols (linear polyhydric sugar alcohols) at the expense of more common storage carbohydrates such as starch and sucrose (Loescher and Everard, 1996). These observations have led to the suggestion that naturally occurring changes in carbohydrate metabolism may have an adaptive role in allowing plants to survive under saline conditions (Bohnert and Jensen, 1996). Support for this idea comes from experiments using transgenic plants in which significant resistance to salinity stress has been imparted to salt-sensitive plants by expression of gene coding for the synthesis of novel and specific carbohydrates for example polyol mannitol in tobacco (Tarczynski et al. 1993). What these transgenic plant studies suggest is that subtle changes in carbohydrate biochemistry can result in significant increase in salinity resistance. Cyclitols such as myo-inositol and more importantly its methylated derivatives pinitol and ononitol are now known to play some role in salinity and drought. Those polyols that are non-reducing sugars may also store excess carbon under environmental stress conditions.
It has long been recognised that sugar levels in plant tissues play an important role in source- sink relationships. However, it is now clear that sugar levels also affect carbohydrate-metabolising enzymes, changing both gene expression and enzyme activity. In source tissues,high carbohydrate levels have been shown to negatively affect key photosynthetic enzymes (Sheen, 1994) whereas in sink tissues genetic expression of catabolic enzymes such as Suc synthase appear to be regulated by levels of imported carbohydrates (Koch et al. 1992). It has been shown in Coleus (Coleus blumei Benth) (Gilbert et al. 1997) that stress can alter at least transiently, not only carbohydrate levels but also the types of carbohydrates that are synthesised and exported by the plant. Future research should therefore be aimed at investigation of the genetic effects of these transients, for it may prove that even subtle changes of specific carbohydrate types in source and sink tissues may serve as signals for new gene expression, allowing a redirection of growth responses for the long-term survival of the plant. Besides causing accumulation of specialized compounds water stress is accompanied by a shift in the partitioning of photosynthate. Under stress, increased levels of sucrose and/ or reducing sugars have frequently been reported (Drennan et al. 1993, Iyer and Caplan, 1998) and have often been proposed to contribute towards the maintenance of turgor. Plastidic starch represents a reserve of sugars and is often rapidly converted to sucrose under stress conditions, which may also be associated with inhibition of starch synthesis (Geigenberger et al. 1997). Sugars not only sustain the growth of sink tissues but they also affect sugar-sensing systems that regulate the expression either positively or negatively of a variety of genes involved in photosynthesis, respiration, starch and sucrose synthesis and degradation and nitrogen metabolism.