337 Improvement of Cellular Salt Tolerance of ‘Troyer’ Citrange In Vitro H. El. Yacoubi and A. Rochdi a Ecophysiologie & Plant Biotechnologies Faculty of Sciences University Ibn Tofail BP133, Kénitra 14000 Morocco Acta Hort. 911, ISHS 2011 Keywords: Citrus tissue culture, callogenesis, sequestration, halophytic behavior Abstract Selected cell lines of ‘Troyer’ citrange were developed by exposing calli to increasing concentrations of NaCl. At 8 g L -1 of NaCl the morphology and growth of the salt tolerant calli were similar to the control callus maintained in salt-free medium, whereas the sensitive wild-type calli expressed salt injury symptoms resulting in tissue browning and drastically inhibited growth. The selected cell lines maintained their growth after transfer to salt-free medium and after retransfer to salt-containing medium, respectively indicating the independence and the stability of the salt tolerance. The accumulation of Na + , K + , Proline and soluble sugars solutes in the citrus cells were also quantified. K + content of the selected tolerant cell lines was close to that of the control and higher than that of the sensitive calli. Na + contents in both tolerant and sensitive calli were relatively higher than in the control. Subsequently, we concluded that sodium was accumulated in two distinct cellular compartments according to the callus types: in the vacuole for the tolerant calli (halophytic behavior) and in the cytosol for the sensitive calli. Increased vacuolar Na + concentrations were supported through increased accumulation of Proline and soluble sugars which were compatible solutes in the selected tolerant but not in the sensitive calli. INTRODUCTION Soil salinity causes increasingly agricultural and environmental problems on a worldwide scale, especially in arid areas. Searching for or improving salt tolerance in plant species is an alternative. The maintenance of low salt concentration in the cytosol of plant cells has been recommended for salt adaptation of plant species (Munns and Tester, 2008). For Citrus species, sensitive glycophyte plants (Ben Hayyim and Moore, 2007), the tolerance is associated with salt transport restriction from the root to aerial part (Syvertsen and Yelenosky, 1988). In forest species, the strategy to tolerate stress is made by excluding the salt ions at the point of uptake and reducing the translocation of ions to the shoot (Niknam and McComb, 2000). Growth reduction seems to be closely related to the Na + and Cl - ion contents in plant leaves. Munns and Tester (2008) claimed that Cl - toxicity was more limiting than Na + toxicity in some woody species, e.g., Citrus. The differences in Cl - tolerance exhibited by plants are usually related to their ability to restrict Cl - transport to the aerial part (White and Broadley, 2001). Some Citrus rootstocks tolerate salt stress by combining the selective exclusion of Cl - (relatively slow absorption of Cl - ions) and the compartmentalization of excess Na + (Rochdi et al., 2005). Transmission of salt tolerance has been already demonstrated (Sykes, 1992). However, conventional breeding by hybridization is often inefficient towards salinity tolerance due to the pre-existing variability (quantitative nature of tolerance, high degree of heterozygotie, etc.) (Rains, 1982; Gorham and Wyn Jones, 2002). These observations have guided researchers to use techniques of tissue culture, that contribute to plant improvement programs for acquiring salinity tolerance and to help elucidate the cellular mechanisms contributing to this tolerance. Thus, in vitro tissue culture was commonly a AtmaneRochdi@Gmail.com Proc. First All African Horticultural Congress Eds.: J. Wesonga and R. Kahane Acta Hort. 911, ISHS 2011