Effect of rhizobial inoculation on germination, seedling vigor and growth of four cotton varieties

Seeds of four cotton varieties viz. NIBGE-1, Karishma-99, NIAB-98 and FH- 901 were subjected to inoculation with Rhizobium leguminosarum bv. trifolii E11, Rhizobium leguminosarum bv. Viciae PS1, Bradyrhizobium japonicum MnS and Bradyrhizobium japonicum TAL-102, in addition to uninoculated control. All (Brady) rhizobium strains produce indole acetic acid (IAA). In addition, Azotobacter sp. S8, a plant growth promoting rhizobacterium (PGPR) isolated from root zone of cotton was also included as positive control. The experiment was laid out in completely randomized design with factorial arrangement and each treatment was replicated thrice. For germination studies, ten seeds were sown in each petriplate filled with sterilized sand and watered with an equal volume of distilled water and data were recorded upto a period of 12 days. For studying growth, another experiment was conducted in plastic pots filled with sterilized sand supplemented with an equal volume of 1/2 strength nutrient solution with nitrogen source with an interval of 3 days and data were taken after 45 days of sowing. A day/night temperature of 30 plus minus 2 degree centigrade / 25 plus minus 2 degree centigrade and day length of 16 hours were maintained. Germination percentage did not differ significantly, however, it remained significantly higher in cotton variety FH-901. Cumulative emergence rate (CER) was found to be higher in cotton variety FH-901 in response to inoculation with B. japonicum TAL-102. Nitrogenase activity of cotton roots was found to be negative with all the bacterial strains used. Rhizobium and Azotobacter strains showed a decline in their population size as recorded at 45 DAP (days after planting) while Bradyrhizobium strains showed an increasing trend in their population size. Cotton variety NIBGE-1 showed good performance regarding shoot weight and shoot nitrogen content in response to inoculation with Azotobacter, sp. S8. Cotton variety Karishma-99 gave highest shoot length in control plants. R. leguminosarum bv. trifolii E11 produced maximum biomass, root weight and root width in cotton variety NIBGE-1, as well as, maximum root area and total root length in cotton variety Karishma-99. However minimum biomass was recorded in uninoculated control. Maximum concentration of Ca++ and K+ were recorded in cotton varieties NIAB- 98 and Karishma-99, respectively, in response to inoculation with R. leguminosarum bv, viciae PS1, whereas control x NIAB-98 interaction showed highest Na+ concentration. Stimulation of root growth and thereby enhanced nutrient uptake by inoculation with (Brady) rhizobium strains seems to be due to IAA produced by these strains, rather than biological nitrogen fixation. However, increased nitrogen uptake and shoot growth by Azotobacter sp. S8 leads us to the conclusion that this PGPR strain, besides producing IAA, might also fix nitrogen at some stage during its growth: Keeping in view the overall growth stimulation performance, R. leguminosarum bv. trifolii E11 and Azotobacter sp. S8 were found to be the best and it is advisable that these strains should be used in the production of bio-fertilizer for enhanced growth and yield of cotton. Cotton varieties NIBGE-1 and Karishma-99 showed good response to these strains.