Improvement of wheat (Triticum aestivum. L) nutrients by zinc mobilizing plant growth promoting rhizobacteria (PGPR)

The main objective of the proposed PhD project was to get insight and depth of knowledge of deploying Zinc (Zn) solubilizer rhizobacteria to overcome Zn deficiency in wheat crop and ultimately to improve yield and quality of wheat. For this purpose, about 198 bacterial isolates were obtained from rhizosphere of wheat field. Out of 198, five strains FA-2, FA-3, FA-4, FA-9 and FA-11 were found effective for Zn solubilizing activity that was quantified through "Atomic Absorption Spectrophotometer" (AAS). The above mentioned five strains produced a clear zone with diameter of 27.5, 63, 59.5, 51.2, 63 and 17.2 mm, respectively on agar plate having ZnCO3. Similar trend was also observed in ZnO modified agar plates. However, all strains did not show visible activity on ZnS. The aforementioned five strains solubilized Zn concentration by 135, 152, 164, 102 and 45 µg mL-1 respectively, but all the strains showed little solubility in ZnS liquid broth. Based on variation in correlation between qualitative and quantitative screening, it can be stated that agar plate assay might not be so precise to detect the low level of solubilization with various Zn ores by Zinc solubilizing bacteria (ZSB). The ZSB showed positive response for indol acetic acid (IAA), exo-polysaccharides (EPS), siderophores activity, nitrogen (N2) fixation, phosphate (P) solubilization, ACC deaminase, hemolytic and antifungal activities. The ZSB also produced gluconic acid in the range of 330-2500 ppm. The diversity among similar genera of ZSB was observed by amplifying intergenic regions (IGS) through PCR, and restriction length polymorphism (RFLP) analysis. Similarly as far as identification of the ZSB strains is concerned, FA-2, FA-3, FA-4, FA\ud \ud 9 and FA-11 were characterized as Serratia liquefaciens, Bacillus thuringiensis, S. marcescens, Pseudomonas aeruginosa and Enterobacter sp. The PqqC (Co-factor for PQQ), acdS and NifH genes from ZSB were amplified, cloned and sequenced which also confirmed their identity as revealed by 16S rRNA and gyrB gene analysis. The response of three promising ZSB, namely FA-2, FA-3, FA-4 and their consortium was further tested under diverse climatic field condition with four commercial wheat (Triticum aestivum L.) cultivars viz. Inqlab 91, Chakwal-50, Lasani-08 and SH-2002. A significant increase of 54, 68, 57 and 46%, respectively in wheat Zn contents was recorded over chemical Zn fertilizer. Similarly the above-mentioned three ZSB strains and their consortium increased grain yield by 2.4, 0.7, 2.2 and 8.6% over chemical Zn fertilizer. The bacterial strains showed significant interaction with wheat crop cultivars and locations of the field. Moreover, the three strains showed less survival in rhizosphere (45, 38 and 47%) as compared in endophytic environment (91, 90 and 83%). The results of present study are very encouraging and prove that the effect of plant growth promoting rhizobacteria (PGPR) in wheat crop is at least partly. Maintaining suitable density of Zn solubilizers in the soil through field inoculation might be a promising strategy to enhance grain yield and Zn content of wheat. It would also appear as a natural, inexpensive and eco-friendly approach which might reduce the use of synthetic fertilizer to minimize environmental hazards. Commercial field application of this approach among farmers is recommended