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The perennial Medicago sativa cv. Gabes is widely grown on saline soils in Tunisian oases. Morphological and physiological analyses of two populations (Mareth and Gannouch) were conducted in order to study the effect of salinity and gamma radiation (350 Gy) interaction on two populations of this species. It has been shown that the two irradiated populations of Medicago sativa are fairly tolerant to salt at growth phase compared to the non-irradiated. Exposure to gamma irradiation (350 Gy), alone or in combination with salt stress, increased significantly (p < 0.001) shoot number, stem height and chlorophyll b pigment especially for the Gannouch population, while no change occurred for the Mareth population. The presence of salt to 9 g/l affected significantly the root biomass and induced a reduction of shoot development of both control and irradiated alfalfa populations. For all treatments, plants of two populations remained able to produce and to allocate dry matter to the different organs. The survey of Na(+)/K(+) ratio showed that the growth of the aerial organs of two non-irradiated populations was at least determined by a selectivity in favour of the K(+) ions (r(2) = 0.97 and r(2) = 0.59 for Mareth and Gannouch non-irradiated populations, respectively). However, the rather weak correlation detected for the irradiated populations, particularly for the irradiated Gannouch, seems to be the consequence of the effect of irradiation that improved potassium availability, essential element for growth and development. Results also showed that the two irradiated populations, especially the Mareth, accumulated Na(+) ions in its photosynthetic organs. This accumulation was associated with an improvement of foliar water content at a level of salinity around 5 g/l. Such a mechanism reflects probably an inclusive behaviour of the plants and a good aptitude to use the dominant ions (Na(+)) for the osmotic adjustment. However, the non-irradiated populations are unable to adjust their internal osmotic potential. Consequently, irradiated plants have probably adapted to the osmotic stress by either closing their stomata or increasing the osmotic pressure of the leaf cells. (C) 2008 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved.