Comparative study of nitrogen fixation and carbon metabolism in two chick-pea (Cicer arietinum L.) cultivars under salt stress

salinity is one of the major environmental constraints on agriculture in many regions of the world (Boyer, 1982; Two cultivars of Cicer arietinum with differential tolerSerrano and Gaxiola, 1994). Chick-pea (Cicer arietinum ance to salinity have been compared by analysing L.) nodules are indeterminate (Hirsch, 1992) and export growth, photosynthesis, nodulation, nitrogenase activfixed N as amides and ureides (Schubert, 1986). This ity, and carbon metabolism in the nodule cytosol. The legume is considered to be sensitive to salt stress, which aim was to help elucidate the relationships between, affects its growth (Dua, 1992; Rao and Sharma, 1995), on the one hand, sucrose and malate metabolism in nodulation and nitrogenase activity (Elsheikh and Wood, nodules and, on the other, the inhibition of nitrogen 1990; Sheokand et al., 1995), and photosynthesis and fixation under salt stress. Chick-pea cultivars Pedrocarbon metabolism in root nodules (Soussi et al., 1998). sillano (sensitive) and ILC1919 (tolerant) inoculated Nevertheless, some authors have reported that nitrogen with Mesorhizobium ciceri strain Ch-191 were grown fixation in indeterminate nodules is more tolerant of salt in a controlled environmental chamber and were stress that in determinate ones (Bordeleau and Prevost, treated with salt (0, 50, 75, and 100 mM NaCl) from 1994; Sinclair and Serraj, 1995). sowing to harvest time (28 d). Plant growth and photoThe microsymbiont, i.e. the bacteroid, depends upon synthesis were more affected by salt in Pedrosillano than in ILC1919. Also the effect of salt on nodulation oxygen and photoassimilates supplied by the host plant. and nitrogen fixation was much more pronounced in The photosynthate translocated to the root nodules, Pedrosillano. The increase in nodular mass in ILC1919 mainly sucrose (Gordon et al., 1992), is metabolized to can partially counteract the inhibition of nitrogenase produce dicarboxylic acid, mainly malate, which is the activity. The enzymes of sucrose breakdown were preferred respiratory substrate for bacteroids ( Kim and inhibited by NaCl, but in ILC1919 a rise in alkaline Copeland, 1996). Although photosynthesis is inhibited invertase was observed with salinity, which could comby salt, limitation in the photosynthate supply to the pensate for the lack of the sucrose synthase hydrolytic nodules does not appear to inhibit nitrogenase, given that activity. The activity of PEPC was stimulated by salt in stress promotes an accumulation of soluble sugar in ILC1919. Also, this cultivar showed higher malate connodules (Fougère et al., 1991; James et al., 1993). In centrations in root nodules. addition, it has been observed that the reduction of nitrogenase activity under salt stress was associated with