The growth and phosphorus utilisation of plants in sterile media when supplied with inositol hexaphosphate, glucose 1-phosphate or inorganic phosphate
Seedlings of six temperate pasture species, three grasses and three legumes, were grown for 19–24 days in sterile agar or sand-vermiculite media, in the presence of inorganic phosphate (P i ), glucose 1-phosphate (G1P) or inositol hexaphosphate (IHP). Agar (pH 5.0) had a low IHP-sorbing capacity while IHP was almost completely sorbed in sand-vermiculite. P i and G1P were relatively available in both media. Growth of each species was measured in relation to phosphorus (P) supply and levels of P i supply at which shoot yields reached 90% of maximum yield (P crit ) were determined. P crit values were generally higher for the legume species than for the grasses, and were six-fold higher for Trifolium subterraneum L. seedlings when grown in sand-vermiculite relative to agar. When supplied with G1P, seedlings of the six species grew as well as plants supplied with P i . By contrast, IHP was a poor source of P for plant growth, even when supplied in agar at levels up to 40-fold greater than P crit . Using the growth of T. subterraneum in the presence of IHP, it was calculated that roots released approximately 0.09 nkat phytase g -1 root dry wt per day, over 20 days of growth. By supplementing agar containing IHP with phytase from Aspergillus niger (E.C. 188.8.131.52; 0.012 nkat plant -1 , or ∼ 1.3 nkat g -1 root dry wt), sufficient P became available to enable T. subterraneum seedlings to grow as well as P i -supplied plants. These results indicate that while pasture plants can quite effectively use P from some organic P sources (e.g. G1P), the acquisition of phytate-P is limited both by availability of substrate and the capacity of plant roots to hydrolyse available IHP.