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Climatic changes can cause serious reductions in yield and crop quality. Under the threat of climatic changes, one of the precautions to cope is selection and development of resistant vegetable genotypes to abiotic stresses. Several physiological and biochemical reactions and different tolerance levels can occur according to plant species. When plants are subjected to environmental stresses such as salinity, drought, temperature extremes, herbicide treatment and mineral deficiency, the balance between the production of reactive oxygen species ROS and the quenching activity of antioxidants is upset, often resulting in oxidative damage. Since activated oxygen species can disrupt normal metabolism through oxidative damage to lipids, protein and nucleic acids, plants possess a number of antioxidant enzymes that protect them from these cytotoxic effects. To control the level of ROS and to protect cells under stress conditions, plant tissues contain several enzymes for scavenging ROS. The high levels of antioxidative enzyme activities were determined in the tolerant genotypes of tomatoes, eggplant, peppers, cucumbers, melons, squash, beans, okra, etc. to several abiotic stress factors. "oth the whole plant and in vitro callus culture experiments gave similar results. "ntioxidant enzymes can be useful for screening to determine the tolerant and sensitive plant genotypes against abiotic stresses.

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The present study investigated different levels (S1: 100% of field capacity-Control S2: 50% of field capacity-moderate stress; S3: 0% of field capacity-severe stress) of drought stress on oxidative damages and variations in antioxidants in the two bean varieties Bn-16 (drought-sensitive), to elucidate the antioxidative protective mechanism governing differential drought tolerance. The shoot fresh weight, shoot height, leaf number and area, RWC were reduced with different level of drought stress. However this reduction clearly occurred in Bn-16 (sensitive). Antioxidative enzyme activities, such as superoxide dismutase, catalase, ascorbate peroxidase and glutation reductase, had a greater increase in tolerant genotypes (Bn-150) than in sensitive ones (Bn-16). The level of lipid peroxidation was measured by estimating malondialdehyde content. Lipid peroxidation increased with increasing drought conditions in all genotypes, although Bn-150 was the least affected when compared with the other genotype. Total phenolic and flavonoid content increased in bean genotypes under S2 and S3 conditions. The highest total phenolic and flavonoid contents were attained in Bn-150 subjected to S3 treatment. These results indicated that an antioxidant defence system, osmolytes (such as proline), and secondary metabolites play important roles in common bean (Phaseolus vulgaris L) during drought stress and recovery.

Citrulline has been recently shown to behave as a novel compatible solute in the <i>Citrullus lanatus</i> (<i>Cucurbitaceae</i>) growing under desert conditions. In the present study we have investigated some aspects of the relationship which might occur in leaves of melon seedlings, also known to produce citrulline, between the capacity to accumulate this ureido amino acid and salt tolerance. With this end in view, salt-induced changes at the citrulline level have been compared in two melon genotypes exhibiting contrasted abilities to withstand the damaging effects of high salinity. Progressive salinization of the growing solution occurred at 23 days after sowing. The final 250 mmol/l external NaCl concentration was reached within 5 days and further maintained for 16 days. In response to this treatment, it was found that the citrulline amount increased in fully expanded leaves of both genotypes according to different kinetics. The salt tolerant genotype Midyat was induced to accumulate citrulline 4 days before the salt sensitive Yuva and as a consequence the final amount of this amino acid was twice higher in the former than in the latter. Compared with citrulline, the free proline level was found to be relatively low and the changes induced in response to the salt treatment exhibited different trends according to the genotypes under study. Thus at the end of the treatment mature leaves of the salt sensitive Yuva contained higher amount of proline than those of Midyat. The changes in the calculated molar ratio between citrulline and free proline suggested that salt tolerance might be associated with high values for this ratio and vice et versa for sensitivity. The interest of citrulline as a biochemical marker for salt tolerance of melon genotypes is discussed.

Salt- and drought-induced alterations in citrulline were assessed in 4 local melon genotypes, 2 sensitive (CU-52, CU-94) and 2 tolerant (CU-196, CU-280), grown in vermiculite in a growth chamber. Salt and drought stresses were started using 30-day-old plants, with 250 mM NaCI and 45 mM PEG (−1.0 MPa) and continued for 12 days. After 12 days under salt and drought conditions, the citrulline contents were increased in the tolerant CU 196 to 25.10 μmol gDW−1 and 24.10 μmol gDW−1 for salt and drought stresses, respectively. However, the citrulline contents of the sensitive CU-52 were 11.68 μmol gDW−1 and 11.76 μmol gDW−1 for salt and drought, respectively. The striking alteration was obtained in the citrulline accumulation. The tolerant melons accumulated 2 times more citrulline than the sensitive melons. For assessing or screening melon genotypes in a large number of accessions or breeding lines for their tolerance to salinity and drought during their young plant stage, the amount of citrulline accumulation in response to the given treatments might be considered as a novel biochemical indicator of interest in early selection studies.