Salt Response of Chickpea as Influenced by N Supply

Production of chickpea (Cicer arietinum L.) in semiarid and coastal areas may be limited by the salt sensitivity of the chickpea symbiosis. Accordingly, this study was done to analyze effects of salt on the symbiosis by comparing the NaCl tolerance of chickpea-specific strains of Rhizobium with tolerance of the chickpea plant grown either with combined N or dependent on symbiotic N₂-fixation. Effects of supplemental N on the salt response of symbiotic chickpea were also tested. In greenhouse sand cultures, growth of chickpea was depressed by NaCl at only 20 mM concentration unless mineral N was provided. With no added NaCl 22 strains of rhizobia were all effective, producing plant yields comparable with NH₄NO₃ control treatments; but with NaCl added at 75 mM only 1 strain did significantly better than controls with no N or inoculum. Poor symbiotic performance was not due to salt limitation of growth of rhizobia. Rhizobial growth rates determined by viable counts in yeast mannitol medium were unaffected by NaCl at 120 mM and only moderately depressed by 250 mM. Effects of supplemental N were studied in a solution culture experiment with four levels of NaCl (0, 15, 23, and 31 mM) combined factorially with four different N treatments: NH₄NO₃ absent, present continuously, present only during the first 31 days, or present only after onset of vigorous fixation at 31 days. The growth of both N-fertilized and symbiotic plants was inhibited by salt. Ammonium nitrate moderated salt stress most strongly during the later period of growth (after 31 days). Growth inhibition of N-fertilized plants was associated with excess Cl accumulation in shoots, which occurred regardless of N treatment. Additional effects could account for the greater inhibition in symbiotic plants. Salt delayed nodulation, and N-treated plants showed greater retention of Na in roots and lower concentrations of Na in shoots than completely symbiotic plants The data indicate a clear need for greater salt tolerance in chickpea. And since the nature of the salt response changed markedly with N-source, selection of cultivars and testing of management procedures should be done with both N-fertilized and symbiotic plants.