PHOSPHORUS EFFICIENCY IN PASTURE SPECIES .4. YIELD, EFFICIENCY PARAMETERS AND PARTITIONING OF P IN 2 WHITE CLOVER ACCESSIONS UNDER VARYING LEVELS OF P SUPPLY IN SOLUTION CULTURE

An experiment was conducted under controlled environment conditions in nutrient solution over four P levels (1,4,8,16-mu-mol P m-3) to evaluate the yield performance and P uptake of two accessions of white clover (Trifolium repens). The two accessions used were Naturalized, which was grown from a seed collection made from a low P soil on the Northern Tablelands of N.S.W., and cv. Ladino. Nutrient solutions were renewed when the P concentrations fell by a maximum of 10% and were pumped so that there was a flow rate of 4.4 L min-1 over the plant roots. The shoot and root fresh weight yield of Ladino was higher than Naturalized at all P levels. At 16-mu-mol P m-3 the shoot yield of Ladino was fourfold that of Naturalized; however, the relative yield response of Ladino between 1 and 16-mu-mol P m-3 was only 20% of the response of that in Naturalized. Phosphorus uptake was highest in Ladino at all P levels. The minimum shoot labile P concentrations recorded in this study were 1.1 and 2.5-mu-mol P g F.wt-1 for Ladino and Naturalized respectively, indicating that Naturalized is capable of maintaining relatively high soluble tissue P levels under low P supply. The generally higher labile, lipid and residue P levels in new leaf, old leaf, stem and root in the Naturalized white clover accession indicate a lower efficiency of nutrient utilization in this collection compared to Ladino. In contrast to Ladino, Naturalized white clover may be able to control its P accumulation at high levels of supply, as toxic levels do not accumulate in old leaf. Ladino was the more efficient accession when P efficiency was defined as shoot yield per unit of P in solution. When P efficiency was defined as shoot or plant yield per unit of P absorbed or as the inverse of P concentration (utilization quotient), Ladino was the more efficient accession at 1 and 4-mu-mol P m-3, with only small differences between the accessions at 8 and 16-mu-mol P m-3. The results suggest that Naturalized may be adapted to low P supply through its low inherent growth rate which lowers its demand for exogenous P.