Vol. 11, Issue 3 (2022)

Author(s):
Diptimayee Dash, SB Gupta and RK Bajpai

Abstract:
Soil acidity is the one of the factors which restricts production of pulses by restricting nodulation, N fixation and limiting Rhizobium survival and persistence in soils. These constraints lead to sub optimal productivity of legume crops raised in acid soils; consequently it becomes inevitable to inoculate the crop with adequate effective Rhizobium. Therefore, the present study was undertaken to screen rhizobial isolates of black gram to find out most promising and low pH tolerant ones for enhancing black gram productivity in acid soil of Chhattisgarh. Out of a total of 106 black gram-Rhizobium isolates 12Screened promising, different acidity tolerant strains including one standard strain (IARI; Urd-10-B) were characterized under laboratory condition for their polysaccharide production and nitrate reduction. The correlation of these physiological properties with their nitrogen fixing ability was established through a field experiment conducted in 2016 with 14 treatments including 12 screened acid tolerant black gram rhizobial isolates, one national check (U-10-B) and one uninoculated –control in blackgram cv. KU-96-3. The rhizobial strains produced varying amounts of extracellular polysaccharides with different carbon sources ranging 177.44 mg l^‑1 to 286.19 mg l^‑1 in mannitol as C source. In general, all the strains produced maximum polysaccharide with mannitol as carbon source as compared to glucose. The strain RhiU3415 produced maximum amount of polysaccharide (286.19 mg l^‑1) in mannitol, whereas the lowest amount of polysaccharide (177.44 mg l^‑1) was formed by Rhi KU40. In glucose maximum polysaccharide i.e. 270.86 mg l^‑1 was produced by strain Rhi U3415 and the minimum amount was produced by Rhi U 3516 (141.63 mg l^‑1). All 12 promising AT isolates showed a positive nitrate reduction test that the accumulation of nitrite was continuous after growth of rhizobial isolates in nitrate broth leading to a color change from creamy to red colour. Amount of nitrite accumulation (8.08 mM) was observed as maximum with the strain Rhi KU34 and the minimum amount (5.12 mM) was accumulated by the strain Rhi KU13when growth was occurred for 120 hours. On the basis of efficiency ratio at harvest, the strains Rhi KU34, Rhi KU40 and Rhi U3516seemed to be most efficient in N₂ fixation in symbiosis with black gram (Comparable to standard strain U- 10-B)followed by Rhi U137, Rhi KU20, Rhi U1511 and Rhi UKh2. The efficiency ratios of the strains at harvest showed a significantly positive correlation with nitrate reduction by AT strains measured as nitrite accumulation in broth at the end of 120 hours of growth. Whereas production of polysaccharides in different carbon sources was not positively correlated with efficiency. Hence the ability of Blackgram rhizobia to reduce nitrate may be taken as a test to screen effective strains.