SHORT COMMUNICATION NODULATION AND GROWTH OF COMMON BEAN UNDER NaCl-STRESS M. E. AOUANI*, R. MHAMDI, M. MARS and R. GHRIR Institut National de Recherche Scienti®que et Technique, BP 95, 2050 Hammam Lif, Tunisia (Accepted 20 January 1998) Phaseolus vulgaris is the most important legume for human nutrition in the world (Velazquez et al., 1988). It can grow by assimilation of mineral nitro- gen or molecular N ®xation. In the ®eld, N nutri- tion of legumes is aected by many environmental stresses. Among these factors, soil salinity has been reported to cause considerable damage to growth of NO 3 -fed or nodulated plants (Lauter et al., 1981; Singleton and Bohlool, 1984; Zahran and Sprent, 1986; Bekki et al., 1987). However, these reports are somewhat contradictory concerning the most tolerant N nutrition mode to salt stress. On the other hand, interactions between cultivar, strain and environmental factors have been established for many legumes (Rai et al., 1985; Balatti and Pueppke, 1990; Wol et al., 1991) and are con- sidered as a necessary fact in the selection process. We have compared the eect of NaCl on the growth of inoculated or NO 3 -fed two common bean cultivars. Then, the eects and interactions between the two cultivars, singly- or doubly-inoculated strains, and NaCl-treatments were analysed. Two local common bean Rhizobium isolates, 12a3 and 31c3, selected for their symbiotic eectiveness and the reference strain of R. tropici, CIAT899 (obtained from the Centro International de Agriculture Tropicale, CIAT, Cali, Colombia) were used to inoculate two cultivars of Phaseolus vulgaris L. grown by farmers in Tunisia: Belna cv. for green beans and Coco cv. for grain production. NO 3 -fed or nodulated common bean plants were grown using an aseptic hydroponic culture system in the glass house (Beck et al., 1993). Surface steri- lized seeds were germinated in agar 0.9%, sown in cups (one plant per cup), and irrigated by an auto- matic syringe. Nodulated plants were irrigated with N-free Broughton and Dilworth nutrient solution (Somasegaran and Hoben, 1985). Plants were inocu- lated at seedling emergence with 100 ml of rhizobial culture containing 10 9 colony-forming units (cfu). In the case of double strain inoculation, 0.5 Â 10 9 bacteria from each strain were mixed in 100 ml inoculum. However for NO 3 -fed plants, plants were irrigated with Broughton and Dilworth nutrient sol- ution supplemented with 70 mg NO 3 -N l À1 (KNO 3 form). For NaCl-treatments, nutrient solution was supplemented with NaCl (0 mM, 20 mM or 50 mM). NO 3 -fed or nodulated plants were irrigated alter- nately with nutrient solution or with sterile distilled water. Twelve plants of each treatment were har- vested 7 weeks after sowing. Nodules were enumer- ated and detached from roots. Shoots of inoculated or NO 3 -grown plants were oven dried, weighed and analysed for total N (Bremner, 1965). All plants survived and grew at the two salinity levels (20 mM and 50 mM) in the ®rst trial. Inoculated plants were all nodulated. Shoot dry matter data were calculated as a percentage of shoot dry matter of the controls (without salt) and treatments were compared. It appeared that for the two cultivars shoot dry matter decreased for NO 3 - fed or inoculated plants under NaCl stress (Table 1). However, the Belna cultivar was more severely aected than the Coco cultivar. On the other hand the inoculated plants (Table 1) appeared to grow better with 50 mM NaCl than the NO 3 -fed plants. This result may suggest that N 2 -®xation is a better source of nutrition under salt-stress condition. The fact that mineral N appeared to be a bad source under NaCl-stress could be explained by the fact that NO 3 -uptake was balanced by cation (Na + ) uptake (Aouani et al., 1992). In fact under salt stress, NO 3 -fed plants had larger amounts of shoot N than the inoculated plants (data not shown). Similar results have been shown on Rhizobium- chickpea symbiosis by Lauter et al. (1981) who found that NO 3 -fed plants subjected to salinity had higher relative yields for shoot N than those of similarly treated symbiotic plants. Soil Biol. Biochem. Vol. 30, No. 10/11, pp. 1473±1475, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-0717/98 $19.00 + 0.00 PII: S0038-0717(98)00034-0 *Author for correspondence. 1473