Utilization of phosphorus by pasture plants supplied with myo-inositol hexaphosphate is enhanced by the presence of soil micro-organisms

A range of pasture grass (Danthonia richardsonii and Phalaris aquatica) and legume (Medicago polymorpha, M. sativa, Trifolium repens and T. subterraneum) species showed limited capacity to obtain phosphorus (P) from inositol hexaphosphate (IHP), when grown in either sterile agar (pH 5.0 or 5.5) or sand-vermiculite media (pH 5.0). The total P content of shoots from IHP-supplied plants grown in agar was between 20% and 34% of that for seedlings supplied with an equivalent amount of P as inorganic phosphate (P-i), while in sand-vermiculite, the total P content of IHP-grown plants was between 5 and 10% of control plants. The poor ability of plants to utilize P from IHP resulted in significantly lower tissue P concentrations and, in general, reduced plant dry weight accumulation. In contrast, the P nutrition of plants supplied with IHP was significantly improved by inoculating media with either a cultured population of total soil micro-organisms or with a specific isolate of Pseudomonas sp., selected for its ability to release phosphate from IHP (strain CCAR59; Richardson and Hadobas, 1997 Can. J. Micro. 43, 509-516). In agar and sand-vermiculite media, respectively, the P content of IHP-grown plants increased with inoculation by up to 3.9- and 6.8-fold, such that the dry weight and P content of the plant material were equivalent to those observed for control plants supplied with P-i. However, the response to inoculation was dependent on the growth medium and the source of micro-organisms used. In sand-vermiculite, the cultured population of soil micro-organisms was effective when IHP was supplied at an equivalent level of P-i required for maximum plant growth. By comparison, inoculation of plants with the Pseudomonas strain was only effective at very high levels of IHP supply (x36), whereas in agar a response to inoculation occurred at all levels of IHP. The ability of pasture plants to acquire P from phytate was, therefore, influenced by the availability of IHP substrate, which was further affected by the presence of soil micro-organisms. Our results show that in addition to having an effect on the sorption characteristics of the growth media, soil micro-organisms also provided a source of phytase for the dephosphorylation of phytate for subsequent utilization of P-i by plants.