**6. Conclusion**

According to our investigations, Ibrahim & Bafeel, 2008 concluded that dark chilling imposes metabolic limitation on photosynthesis and ROS are involved, to some degree, in

Over-expression of ROS scavenging enzymes like isoforms of SOD (Mn-SOD, Cu/Zn-SOD, Fe-SOD), CAT, APX, GR, DHAR, GST and GPX resulted in abiotic stress tolerance in various plants due to efficient ROS scavenging capacity. Pyramiding of ROS scavenging enzymes may also be used to obtain abiotic stress tolerance plants. Therefore, plants with the ability to scavenge and/or control the level of cellular ROS may be useful in future to withstand

Osmotic response and their adjustment was considered as a biochemical marker in plants subjected to abiotic stress such as salinity can occur by the accumulation of high concentrations of either inorganic ions or low molecular weight organic solutes. Although both of these play a crucial role in higher plants grown under saline conditions, their relative contribution varies among species, among cultivars and even between different compartments within the same plant (Greenway & Munns, 1980). The compatible osmolytes generally found in higher plants are low molecular weight sugars, organic acids, polyols, and nitrogen containing compounds such as amino acids, amides, imino acids, ectoine (1,4,5,6-tetrahydro-2-methyl-4-carboxylpyrimidine), proteins and quaternary ammonium compounds. According to Murakeozy et al.(2003), of the various organic osmotica, sugars contribute up to 50% of the total osmotic potential in glycophytes subject to saline conditions. The accumulation of soluble carbohydrates in plants has been widely reported as a response to salinity or drought, despite a significant decrease in net CO2 assimilation

**5.4 Role of amino acids and amides on the avoidance of abiotic stress** 

Amino acids have been reported to accumulate in higher plants under salinity stress (Bielski, 1983; Moller, 2001; Mahajan and Tuteja, 2005). The important amino acids include alanine, arginine, glycine, serine, leucine, and valine, together with the imino acid, proline, and the non-protein amino acids, citrulline and ornithine (Mahajan and Tuteja, 2005, Hu, 2007) .Proline, which occurs widely in higher plants, accumulates in larger amounts than other amino acids in salt stressed plants (Bielski et al., 1983; McDowell and Dangl, 2000; Navrot et al., 2007; Pastore et al., 2002; Reumann et al., 2004). Proline accumulation is one of the common characteristics in many monocotyledons under saline conditions (Dybing et al., 1978; Grant and Loake, 2000), although in barley seedlings, NaCl stress did not affect proline accumulation (Bolwell & Woftastek, 1997). However, proline accumulation occurs in response to water deficit as well as to salt. Thus, synthesis of proline is a non-specific response to low growth medium water potential (Navrot et al., 2007). Proline regulates the accumulation of useable N, is osmotically very active (Bielski et al., 1983; Moller, 2001), contributes to membrane stability (Heath, & Packer, 1968; Garg and Manchanda, 2009; Montillet et al., 2005) and mitigates the effect of NaCl on cell membrane disruption (Fam and Morrow, 2003). Even at supra-optimal levels, proline does not suppress enzyme activity

According to our investigations, Ibrahim & Bafeel, 2008 concluded that dark chilling imposes metabolic limitation on photosynthesis and ROS are involved, to some degree, in

harsh environmental conditions.

**5.3 Osmotic adjustment in stressed plants**

rate (Carm, 1976; Popp & Smirnoff, 1995).

(Hayashi and Nishimura, 2003; Moller et al., 2007).

**6. Conclusion** 

the limiting photosynthetic capacity of alfalfa leaves. After recovery period the alfalfa plants showed physiological and biochemical changes that contribute to its superior dark chilling resistance and prevent the leaves from undergoing photooxidation damage and eventual death. Also our results showed that high cellular levels of H2O2 accumulated during the dark chilling treatment can induce the activation of a defense mechanism against chilling stress or programmed cell death. The accumulation of H2O2 can be induced by the increase in SOD activity. Therefore, during the recovery treatment the accumulated H2O2, in turn, may activate a protective mechanisms that increase the activities of several antioxidant enzymes such as APX, CAT and GR .Also induce alterations in the relative concentration of several non-enzymatic antioxidant compounds such as phenolics and tocopherols. (Bafeel & Ibrahim, 2008).

Results reported by Ibrahim & Alaraidh, 2010 demonstrated that changes in gene expression do occur in the two cultivars of Triticum aestivum in response to drought, and these differentially expressed genes, though functionally not known yet, may play important roles for cultivars to exhibit its response to drought stress before and after rehydration. Moreover, Ibrahim & Bafeel, 2009 concluded that prolonged stress induced by Pb2+ concentrations, can result into the activation of antioxidative enzymes and also enhance the gene expression of these antioxidant enzymes.

Although oxidative stress is potentially a lethal situation, it is also clear that plant systems exploit the interaction with oxygen. The production and destruction of active oxygen species is intimately involved with processes such as the hypersensitive responses and the regulation of photosynthetic electron flow. There are numerous sites of oxygen activation in the plant cell, which are highly controlled and tightly coupled to prevent release of intermediate products. Under stress situations, it is likely that this control or coupling breaks down and the process "dysfunctions" leaking activated oxygen. This is probably a common occurrence in plants especially when we consider that a plant has minimal mobility and control of its environment. Activated forms of oxygen are important in the biosynthesis of "complex" organic molecules, in the polymerization of cell wall constituents, in the detoxification of xenobiotic chemicals and in the defense against pathogens. Thus, the plant's dilemma is not how to eliminate the activation of oxygen, but how to control and manage the potential reactions of activated oxygen.

Genetic engineering also offer advantages in terms of the study of the physiological roles of enzymes where a classical genetic approach, such as selection of enzyme-deficient mutants, is difficult or almost impossible to carry out. In plant systems, the situation is often considerably complicated by the presence of a large number of isoenzyme forms, for example, the large GR and SOD families of isoenzymes, encoded by different genes. In the future, however, the use of antisense technology combined with selection of specific cDNA clones for isoenzymes may facilitate investigation of such enzyme-deficient mutants. Current observations suggest that increasing the level of stress tolerance by reinforcing the plant's defense system with new genes is an attainable goal.
