The impact of salt stress (NaCl 100 mM) on two lettuce varieties Romaine and Vista was conducted at germination and early seedling stages. The seeds of lettuce varieties were provided by the Seed Laboratory of Tunisian Ministry of Agriculture. The seeds were germinated in Petri dishes with double filter paper in distilled water (control) or NaCl solution (100 mM) for 5 days. The result showed that salinity significantly affected percentage and rate of germination in Vista variety but 100% of germination was found in Romaine. Length and fresh weight of root and shoot were reduced significantly with salt treatment in two lettuce varieties. Regarding biochemical analysis, acid phosphatase activity in root increased in Romaine and decreased in Vista. In shoot, this activity showed no difference with the control in the two varieties. However in cotyledons, and during 24 hours after germination, salinity decreased acid phosphatase activity in both varieties whereas in the later hours (48-96 h) this activity reached the value of the control in Romaine and Vista.

Seeds of four lettuce (Lactuca sativa L.) varieties (Romaine, Augusta, Vista and Verte) differing in their salt sensitivity were sown at 0 (Control), 50, 100 and 150 mM NaCl. The final germination percentage decreased with the increasing salinity and was annulated at the highest salt concentration in Vista and Verte, the most sensitive varieties. However, in the less sensitive ones, Romaine and Augusta, it was slightly modified at 50 and 100 mM NaCl and then decreased by 50% compared with the control, at 150 mM. The effects of NaCl 100 mM on seedling growth, phytase activities, phytate and inorganic phosphorus contents were studied in Romaine and Vista showing different behaviours towards salinity. Radicle and hypocotyl length and fresh and dry weights were reduced by salt treatment in both varieties. In addition, radicle phytase activity exhibited an increase in Romaine (less sensitive) and a decrease in Vista (more sensitive). In hypocotyl, this activity showed no difference with the control in the two varieties. However, in cotyledons, and during early hours after germination, salinity decreased phytase activity in both varieties whereas in the later hours (72-96 h) this activity reached the value of the control in Romaine. The enhancement of phytase activity was concomitant with an increase in orthophosphate content and a decrease in phytate reserve. These results suggest that salt presence in the medium delays Pi remobilization from phytate stock, but stimulates assimilation of phosphorus more than its accumulation in the organs of the two lettuce varieties.

Marjoram oil is a rich source of many compounds such as essential oils and fatty acids, but the distribution of these compounds differed significantly between the two varieties studied.

Potassium-sodium interaction was compared in two natural accessions of Arabidopsis thaliana, Columbia-0 and NOK2. Seedlings were grown in the presence of 0 or 50 mM NaCl and 0.1; 0.625 or 2.5 mM K + . At the lowest K + concentration, salt treatment inhibited both K + uptake and growth. Increasing the K + availability did not modified salt response in Columbia-0, but restored nearly normal net K + uptake in NaCl condition and alleviated NaCl growth reduction in NOK2. The effect of K + and NaCl on transcript level of several K + and Na + transporters in both shoots and roots was assessed using semi-quantitative RT-PCR. The mRNA abundance of the NHX1 and SOS1 Na + /H + antiporters was significantly increased by 50 mM NaCl in the two accessions. NHX1, which is responsible for Na + sequestration into vacuoles, was more up-regulated in NOK2 leaves than in Columbia-0's in NaCl stress condition. AKT1, which is the major channel involved in K + absorption, was down-regulated in salt stress condition, but was not responding to K + treatments. Only in NOK2, SKOR and AKT2, which respectively control xylem and phloem K + transport, were markedly up-regulated by 2.5 mM K + in both roots and shoots, independently of NaCl. Phenotypic and gene expression analyses suggest that the relative salt tolerance of NOK2 is mainly due to a high ability to sequester Na + in the vacuole and to take up and transport K + . Up-regulation of SKOR and AKT2 by K + , and of NHX1 by NaCl could participate in determining this phenotype.

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