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. subterrane um) 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 s and-vermiculite media (pH 5.0). The total P content of shoots from IHP-supp lied plants grown in agar was between 20% and 34% of that for seedlings sup plied 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 an d 10% of control plants. The poor ability of plants to utilize P from IHP r esulted in significantly lower tissue P concentrations and, in general, red uced plant dry weight accumulation. In contrast, the P nutrition of plants supplied with IHP was significantly improved by inoculating media with eith er a cultured population of total soil micro-organisms or with a specific i solate of Pseudomonas sp., selected for its ability to release phosphate fr om IHP (strain CCAR59; Richardson and Hadobas, 1997 Can. J. Micro. 43, 509- 516). In agar and sand-vermiculite media, respectively, the P content of IH P-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-o rganisms used. In sand-vermiculite, the cultured population of soil micro-o rganisms was effective when IHP was supplied at an equivalent level of P-i required for maximum plant growth. By comparison, inoculation of plants wit h the Pseudomonas strain was only effective at very high levels of IHP supp ly (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, theref ore, influenced by the availability of IHP substrate, which was further aff ected by the presence of soil micro-organisms. Our results show that in add ition to having an effect on the sorption characteristics of the growth med ia, soil micro-organisms also provided a source of phytase for the dephosph orylation of phytate for subsequent utilization of P-i by plants.