The use of invitro hydroponic culture is one of the useful and practical methods for selection of superior isolates of plant growth promoting rhizobacteria with high colonization potential at root surface. In this regard, the present study was designed to select superior isolates isolated from dry-land farming of Qazvin and Zanjan provinces during the previous stages of this investigation.
Materials and Methods:
This experiment was carried out based on a completely randomized design (CRD), including: inoculation of wheat seeds with 16 bacterial isolates and culture of seedlings in Hoagland nutrient solution (EC 8 dS/m and osmotic potential (OP) -0.54 MPa) containing tricalcium phosphate (source of low-soluble phosphorus), as well as non-inoculated seedlings was cultured in Hoagland nutrient solutions containing monopotassium phosphate (control with source of soluble phosphorus) and tricalcium phosphate (control with source of low-soluble phosphorus) at three replications in growth chamber for 45 days. The length of the light (with 196 μmol photons m−2 s-1 light intensity) and dark period in these times was 16 and 8 h, respectively, The day and night temperature were 25±2 and 20±2 °C, respectively. At end of experiment (45 days after transportation of seedlings into invitro hydroponic culture) shoots and roots of plants were harvested separately; then, Population of phosphate solubilizing bacteria (PSB) on root surface, pH and concentration of available phosphorus in nutrient solution, fresh weight, dry weight and length of both section of plants, P-concentration and P-uptake of plant, and relative water content (RWC) of leaf were measured. Ultimately, correlation (Pearson) among measured traits were calculated.
Results and Discussion:
Results showed that inoculated wheat seedlings with B18 isolate perished. It seems that B18 isolate was either a highly pathogenic plant or its metabolites were highly inhibitory to the seedling which in short time killed the wheat seedling. Except B (14, 17, and 32) isolates; the rest of isolates had over 6×106 CFU/g root fresh weight. All bacterial isolates compared to the control with source of insoluble phosphorus, increased available phosphorus and decreased pH of nutrient solution. Dissolution of inorganic phosphates by PSB with chelating processes, secretion of inorganic and organic acids has been proven topic. Thus, reducing pH and increasing soluble phosphorus in nutrient solution at PSB treatments are reasonable. B (1, 2, 3, 4, 5, 6, 14, 17, 32) isolates treatments compared to control with tricalcium phosphate, increased RWC of leaf, and also increased length, fresh weight and dry weight of root and shoot of wheat plants. Previous research has shown that inoculation of plants with PGPR under drought and salinity conditions improved RWC of plant leaves and plant growth indices; Of course, result of some experiment show that some of PGPR do not always improve plant growth under all conditions. There was a significant positive correlation among P-uptake, RWC, root length, shoot height, fresh and dry weight of root and shoot of wheat plant. Also, the pH of nutrient solution had a positive correlation with root length, but it had a negative and significant correlation with P-concentration of nutrient solution and P-concentration of plant. PSB population at root surface had a negative correlation with pH of nutrient solution and root length of plant, but with P-concentration of nutrient solution had positive and significant correlation.
All of screened and selected PSB isolates in lab (in respect of PGP traits) could not promote plant growth; even some of these isolates (such as B15 and B18) had negative effects on wheat growth indices. Inoculation of wheat with PSB isolates significantly increased the use of insoluble phosphorus. Solubility of tricalcium phosphate by isolates and increased phosphorus availability cannot lonely improve wheat growth indices; it seems that the outcome of the set of PGP effects of these bacteria and the secretion of their metabolites in the presence of plant root are determinative agents for improvement of wheat growth indices. Generally, B (1, 2, 3, 4, 5, 6, and 32) isolates were superior isolates in respect of colonization at root surface, relative water content (RWC) and growth indices of wheat plant; and these isolates are recommended for further experiments in greenhouse and field (in order to production of suitable biofertilizer for dry-land farming of wheat).
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