Plant growth promoting rhizobacteria (PGPR) can enhance plant growth by alleviating soil stresses. Although previously investigated, some new interesting details are presented regarding the alleviating affects of Azospirillum sp. on wheat growth under drought stress in this research work. We hypothesized that the isolated strains of Azospirillum sp. may alleviate the adverse effects of drought stress on wheat (Triticum aestivum L.) growth. Three different strains of Azospirillum lipoferum (B1, B2 and B3) were used to inoculate wheat seedlings under drought. During the flowering stage the seedlings were subjected to three drought levels with five different time longevity, including control. Pots were water stressed at 80% (S0), 50% (S1) and 25% (S2) of field capacity moisture in a 25 day-period. Soil and plant water properties including water potential and water content, along with their effects on bacterial inoculum and wheat growth, were completely monitored during the experiment. While stress intensity significantly affected bacterial population and wheat growth, stress longevity only affected wheat water potential and water content. Compared to uninoculated treatments strain B3 (fixing and producing the highest amounts of N and auxin, respectively, with P solubilizing and ACC-deaminase activities) increased wheat yield at S1 and S2 by 43 and 109%, respectively. However, strain B2 (producing siderophore) was the most resistant strain under drought stress. The results of this experiment may elucidate the more efficient strains of Azospirillum sp. for wheat inoculation under drought stress and the mechanisms by which they alleviate the stress.
Plant growth promoting rhizobacteria (PGPR) is referred to a heterogeneous group of beneficial rhizosphere bacteria that could enhance plant yield through one or more mechanisms. Flavobacterium has been noted as PGPR in almost all review articles. However, there are a few studies regarding plant growth promotion imposed by them. Some of Plant growth promoting characteristics such as Phosphate solubilizing capacity, ability to use of 1-Amino Cyclopropan-1-Carboxylate (ACC) as sole nitrogen source and production of auxin, siderophore, salicylic acid, chitinase and hydrogen cyanide were evaluated in forty-four flavobacteria isolated from rhizosphere of wheat. Results showed that none of the isolates were able to produce siderophore, salicylic acid and chitinase and they were not able to use ACC as well. Determining the siderophore showed that none isolates did not grow on Chrome Azurol S (CAS) Agar medium. The results of this part were further analyzed using CAS Agar Diffusion (CASAD) method, but the results were also negative. HCN production was observed in all isolates, but in lowest limit. Thirty-four isolates were capable to solubilize insoluble inorganic Phosphate (P) sources. The average rate of P-solubilization was 3.54 µg Pml -1 , ranging from zero to 37.48 µgPml -1 . There was a significant negative correlation (r = -0.81**) between solubilized P and the final pH of the growth medium. In this study, all the isolates were able to produce auxin, ranging from 0.27 to 12.03 µgml -1 averaged by 2.03. Considering the ability of the isolates to produce auxin and for P-solubilization, it is necessary to evaluate their effect on growth and yield of different crop plants.
We have recently indicated the plant growth promoting activities of Pseudomonas sp. as well as their alleviating effects on some soil stressors such as salinity. This is because in recent years, biological fertilizers have received special attention by scientists in sustainable agriculture. Accordingly, it is pertinent to specify the beneficiary level of such soil bacteria on plant growth including phosphorous (P) uptake. Hence, the objectives were to determine: (1) the plant growth promoting effects of the tested Pseudomonas sp., and (2) its combined effects with different P fertilization rates on the nutrient uptake (N, P, and K) and yield of wheat (Triticum aestivum L.) under greenhouse and field conditions. The experiments were factorially arranged on the basis of a completely randomized block design with three replicates and were conducted at the Research Farm of Agriculture and Natural Resources Research Center of Khorasan, Mashhad, Iran. P was fertilized at three levels including 0, 25 and 50 kg/ha P 2 O 5 . Pseudomonas sp. including Pseudomonas fluorescens 153, P. fluorescens 169, P. putida 4, and P. putida 108 were tested. Activities such as production of ACC deaminase and IAA-like products, as well as P solubilization were among the most important activities of the tested Pseudomonas sp. Such bacterial effects greatly enhanced wheat growth and yield under greenhouse and field conditions. The results also showed that the effects of Pseudomonas sp. on wheat nutrient uptake and the effects of bacteria as well as P fertilization on wheat yield were significant. P. putida 108 was the most effective strain enhancing wheat P uptake and grain yield under greenhouse (96 and 58%) and field (80 and 37%) conditions, respectively. Hence, although Pseudomonas sp. could be a suitable replacement for high P fertilization, however, the optimum wheat yield resulted when the bioinoculants are combined with 50% (25 kg/ha P 2 O 5 ) P fertilization. This finding has great agricultural and environmental implications.
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