الفهرس 
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Abstract
Two cultivars of lettuce namely: Lactuca sativa L. cv. Balady and cv. Eskandrany contrasting in salt tolerance were used to test their differential response to salinity (3, 7 and 9 dS/m) at whole plant level. Comparative response studies could provide insights on the salt tolerance mechanism in lettuce. Both lettuce cultivars (balady and eskandrany), could still tolerate EC levels up to 9 dS/m with varying degrees although this was associated with a decrease in growth and photosynthetic efficiency. The higher salinity tolerance of Eskandrany cultivar can be interpreted on the basis of its higher capability of coping Na and Cl accumulation in roots by translocation to leaves, this was accompanied with a high rate of synthesizing micro- and macro- molecules including osmoprotectants, specific enzymes and antioxidant system leading to osmoregulation and radical scavenging in such cultivar. Molecular studies (DD-PCR) performed on both cultivars to identify both up-regulated and down-regulated partial cDNAs with respect to salinity stress. Two differentially expressed cDNAs were cloned and sequenced and submitted to GenBank with accession numbers EU349289 (Lactuca Sativa betaine aldehyde dehydrogenase gene; BADH) and EU349290 (Lactuca Sativa proline dehydrogenase gene; PDH). The identified two genes (one up-regulated; BADH and one down-regulated; PDH) were mainly related to the regulation of osmoprotectants (proline and betaine) synthesis.