Azospirillum Inoculation Alters Nitrate Reductase Activity and Nitrogen Uptake in Wheat Plant under Water Deficit Conditions

Water deficit stress usually diminishes nitrogen uptake by plants. There are evidences that some nitrogen fixing bacteria can alleviate this stress by supplying nitrogen and improving its metabolism in plants. Four Azospirillum strains, A. lipoferum AC45-II, A. brasilense AC46-I, A. irakense AC49-VII and A. irakense AC51-VI were tested for nitrate reductase activity (NRA). In a pot culture experiment using a sandy loam soil, wheat plants (Triticum aestivum L. cv. Sardari) were inoculated with these bacterial strains and three ranges of soil water potential (W1: -10 to -20, W2: -40 to -50 and W3: -65 to -75 kPa) were applied to the pots. All strains were positive in NRA test and the highest (7.63mg NO2-N.L-1.48h-1) was recorded for AC49-VII and the least (0.23mg NO2-N.L-1.48h-1) was belong to AC51-VI. Leaf and root NRA, root and shoot nitrogen concentrations, and dry weights of root and shoot decreased by increasing water deficit stress. All four bacterial strains caused a significant enhancement in root NRA and in each water deficit level, the higher root NRA was recorded in AC46-I and AC49-VII inoculated plants. The highest leaf NRA was achieved by AC49-VII. The mean increment of root NRA by bacterial strains was 171% compared to the non-bacterial plants. Moreover, at the highest level of water deficit stress, the highest dry weight and nitrogen concentration in root and shoot were obtained by AC46-I and AC49-VII treatments.