Screening soil samples collected from a diverse range of slightly alkaline soil types, we have isolated 22 competent phosphate solubilizing bacteria (PSB). Three isolates identified as Pantoea agglomerans strain P5, Microbacterium laevaniformans strain P7 and Pseudomonas putida strain P13 hydrolyzed inorganic and organic phosphate compounds effectively. Bacterial growth rates and phosphate solubilization activities were measured quantitatively under various environmental conditions. In general, a close association was evident between phosphate solubilizing ability and growth rate which is an indicator of active metabolism. All three PSB were able to withstand temperature as high as 42°C, high concentration of NaCl upto 5% and a wide range of initial pH from 5 to 11 while hydrolyzing phosphate compounds actively. Such criteria make these isolates superior candidates for biofertilizers that are capable of utilizing both organic and mineral phosphate substrates to release absorbable phosphate ion for plants.

Three phosphate solubilizing bacterial isolates identified as Pantoea agglomerans strain P5, Microbacterium laevaniformans strain P7 and Pseudomonas putida strain P13 were assessed for mutual relationships among them, competitiveness with soil microorganisms and associations with plant root using luxAB reporter genes for follow-up studies. Synergism between either P. agglomerans or M. laevaniformans, as acid-producing bacteria, and P. putida, as a strong phosphatase producer, was consistently observed both in liquid culture medium and in root rhizosphere. All laboratory, greenhouse and field experiments proved that these three isolates compete well with naturally occurring soil microorganisms. Consistently, the combinations of either P. agglomerans or M. laevaniformans strains with Pseudomonas putida led to higher biomass and potato tuber in greenhouse and in field trials. It is conceivable that combinations of an acid-and a phosphatase-producing bacterium would allow simultaneous utilization of both inorganic and organic phosphorus compounds preserving the soil structure.

This is the first report describing culture conditions necessary to induce secondary embryogenesis in two carnation cultivars, Nelson and Spirit. In the first step, embryogenic calli were induced on petal explants followed by development of primary somatic embryos from the calli. In the second stage, secondary somatic embryos were obtained when precotyledonary and cotyledonary primary embryos were isolated and transferred onto a series of culture media all containing MS basal salt mixture, and supplemented with different concentrations of 2,4-D, BA, sucrose and mannitol. The highest rate of secondary embryogenesis occurred on mannitol containing media. Secondary somatic embryos were converted into plantlets when they were transferred onto growth regulator-free half-strength MS medium and successfully acclimated in the greenhouse.

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