Phosphorus Uptake and Utilization by Tree Seedlings

Abstract

Seedlings of five native forest species and of Pinus radiata D. Don were grown in pots in a forest soil at a range of levels of added phosphate for up to 20 weeks. At intervals of 3 weeks, roots were examined for root hairs and mycorrhizas and the weights of plant parts and their phosphorus contents were measured. Components of growth and response were calculated from three-dimensional surfaces which had level of phosphate and time as the independent variables. Except for Banksia grandis Willd., the species responded to phosphate but the time at which the response appeared was affected by seed reserves of phosphorus. Thus response appeared much earlier for the small-seeded Eucalyptus divevsicolor F. Muell. and Acacia pulchella R. Br. than for the large-seeded Eucalyptus calophylla R. Br. and Eucalyptus marginata Donn. ex Sm. The absence of a response by the banksia seemed to arise because of a large seed reserve of phosphorus, a low relative growth rate and a very low concentration of phosphorus in the leaves for maximum photosynthesis. The rates of uptake of phosphorus per unit weight of roots were generally low but increased at a time coincident with the development of mycorrhizas. Relative growth rates and the net assimilation rates were also low and reached maximum values at low concentrations of phosphorus in the leaf. The outcome was that the level of applied phosphate needed for good early growth did not differ greatly from that of Trifolium subterraneum L.