Effects of gamma-irradiated Acinetobacter calcoaceticus on nitrogen and phosphorus uptake of green mustard (Brassica chinensis)

Several biofertiliser microorganisms are subjected to gamma-irradiation for mutagenesis to improve capabilities of N₂ fixation and P solubilisation, and also to satisfy biofertiliser market demands. The effects of gamma-irradiated Acinetobacter calcoaceticus on N and P uptake of choy sum or green mustard (Brassica chinensis) were investigated in a greenhouse experiment. Eight bacteria isolated from compost, soil and plants which were to be used as biofertilisers were gamma irradiated at 50–400 Gy and screened for the best N-fixing and P-solubilising mutants. The gamma-irradiated A. calcoaceticus M100/200 mutant showed higher N₂ fixation and phosphate solubilisation than those of the wild-type in vitro. The selected mutant (M100/200) and wild-type (M100) A. calcoaceticus were then tested on green mustard in a greenhouse experiment. N, P and K contents in soil, as well as pH, were determined using a soil nutrient analyser. Urea and rock phosphate were used as nitrogen and phosphate sources, respectively. Two-weekold seedlings were treated with biofertiliser (mutant or wild type) with either N or P source alone or in combination. The control treatments comprised of biofertiliser or N or P source alone (positive control) and without treatment (negative control). Crops were harvested after 2 months. Fresh and dry weight, height, chlorophyll content, leaf area and total N and P in the tissue of the crops were measured. Mutant M100/200 with N and P source treatment or with P source only showed superior growth and nutrient uptake in comparison to those of the samples given other treatments in the greenhouse experiment. Nitrogen input did not enhance growth, whereas phosphorus input and administration of phosphate solubiliser increased plant growth indirectly through a better rooting system. Green mustard inoculated with mutant strain demonstrated better nutrient uptake in a greenhouse experiment than those inoculated with the wild type.