Journal of Applied Microbiology 1999, 86, 889–898 Effect of high salts concentrations on the growth of rhizobia and responses to added osmotica H. Abdelmoumen 1 , A. Filali-Maltouf 2 , M. Neyra 3 , A. Belabed 4 and M. Missbah El Idrissi 1 1 Laboratoire de Microbiologie Applique ´ e, De ´ partement de Biologie, Faculte ´ des Sciences, Universite ´ Mohamed I, Oujda, 2 Laboratoire de Microbiologie et de Biologie Mole ´ culaire, Faculte ´ des Sciences, Rabat, Morocco, 3 Laboratoire de Microbiologie, ORSTOM, Dakar, Senegal, and 4 Laboratoire d’Ecophysiologie et de production ve ´ ge ´ tales, De ´ partement de Biologie, Faculte ´ des Sciences, Oujda, Morocco 6925/98: received 14 October 1998, revised 6 January 1999 and accepted 11 January 1999 H. ABDELMOUMEN, A. FILALI-MALTOUF, M. NEYRA, A. BELABED AND M. MISSBAH EL IDRISSI. 1999. Twenty-eight reference strains, 79 rhizobia isolated from Trigonella foenum graecum (fenugreek), 26 strains isolated from root nodules of Ceratonia siliqua (carob tree), 30 strains isolated from nodules of Adenocarpus decorticans and five isolated from Cytisus arboreus root nodules, were screened for their tolerance to increased concentrations of NaCl. Nine isolates of fenugreek were able to grow on medium containing 10% NaCl and one strain still grew at 14%. The effect of a range of salts at 2·5 and 5% (w/v) on the growth of rhizobia was assessed and it was shown that this effect depended on the ion form and the strains used. In general, NaCl appeared to be a good indicator of the tolerance of the strains to salts. The concentrations of the salts used were high and even at 5%, growth was not completely arrested in the less tolerant strains. Some substances, such as glutamate, proline, glycine betaine and CaCl 2 , were tested as osmotica. The effect of the two amino acids and betaine was confirmed with all the strains used whereas the alleviating effect of CaCl 2 was not observed in all strains. This salt had different effects on two isolates of fenugreek. These results revealed a great diversity in salt tolerance, correlated with different responses to other stress conditions, which may be due to diversity in microbial ecology. INTRODUCTION Saline soils are common in regions of arid or semi-arid climate where transport of soluble salts to the ocean does not occur because of low rainfall (Hayward and Wadleigh 1949; Pillai and Sen 1973; Webster and Wilson 1980). They are char- acterized by the presence of high levels of neutral salts in the surface layers resulting from the capillary rise of water when evaporation exceeds precipitation. In the flood plains of rivers, low-lying lake margins and coastal plains, saline groundwater within a few meters of the soil surface can be a major contributory factor (Eaglesham and Ayanaba 1984). The predominant salts are usually sulphates and chlorides Correspondence to: Dr M. Missbah El Idrissi, Laboratoire de Microbiologie Applique ´e, De ´partement de Biologie, Faculte ´ des Sciences, Universite ´ Mohamed I, Oujda, Morocco (e-mail: missbah49@hotmail.com). © 1999 The Society for Applied Microbiology of sodium, calcium and sometimes magnesium, and small quantities of carbonates and bicarbonates are often present. Many legumes appear to offer potential for planting on saline sites where secondary salinity is causing loss of agri- cultural land. In the developing countries, fast-growing leg- umes are required for both fodder and sustained fuelwood production (Craig et al. 1991). Leguminous plants growing in highly saline environments require both the free-living rhizobia and the host to be tol- erant to salt. Some workers (Bhardwaj 1975; Lauter et al. 1981; Singleton et al. 1982; Kassem et al. 1985) have found that the upper limits for salinity tolerance of rhizobia appear to be higher than those of their host legumes. Salt may affect symbiosis by its effects on the growth and survival of rhizobia in the soil, restrictions on root colon- ization, inhibition of processes of infection and nodule devel-