Selection and evaluation of nitrate-tolerant strains of Rhizobium leguminosarum biovar viceae specific to the lentil

Abstract

Seventeen strains of Rhizobium leguminosarum biovar viceae specific to the lentil (Lens culinaris L.) were screened, using the high-yielding lentil cultivar L 4076, for their tolerance to three levels of NO ^sup-_inf3 : 0, 4, and 8 mM NO ^sup-_inf3 . Preliminary screening of this symbiosis for nodulation and N fixation in the presence of NO ^sup-_inf3 showed significant variations among the strains. The number of nodules decreased and nitrogenase activity was depressed in all strains in the presence of NO ^sup-_inf3 . Strains L-1-87, L-27-89, L-33-89, and L-40-89 tolerated 8 mM NO ^sup-_inf3 . Four strains, three tolerant of NO ^sup-_inf3 (L-1-87, L-27-89, and L-33-89), and one sensitive (L-11-89) to NO ^sup-_inf3 , were selected from preliminary screening and used in a pot experiment to assess the symbiosis in the presence of 6 mM NO ^sup-_inf3 at three stages of plant growth, viz., 40 days, 60 days, and at the final harvest. In general, the weight of nodules and C₂H₂ reduction activity was significantly higher after 60 days than after 40 days. Inoculation with strain L-1-87 produced the maximum number of nodules, and root and shoot biomass both in the presence and the absence of NO ^sup-_inf3 . Nitrate reductase activity in the tops and nodules was assayed only after 60 days and did not show significant variations among strains and NO ^sup-_inf3 treatments. The grain yields for all strains except L-11-89 were significantly higher in the presence of NO ^sup-_inf3 than in the absence of NO ^sup-_inf3 , indicating that tolerant strains contributed symbiotically fixed N to the plant's N pool, resulting in an additive effect on yield. Inoculation with strain L-1-87 produced the maximum grain yield and this strain appears to have potential use as an inoculant in the presence of high levels of soil N.