The growth and phosphorus utilisation of plants in sterile media when supplied with inositol hexaphosphate, glucose 1-phosphate or inorganic phosphate.

Abstract

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 (Pi), 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. Pi and G1P were relatively available in both media. Growth of each species was measured in relation to phosphorus (P) supply and levels of Pi supply at which shoot yields reached 90% of maximum yield (Pcrit) were determined. Pcrit 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 Pi. 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 Pcrit. 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. 3.1.3.8; 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 Pi-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.