Leaf phosphorus and nitrogen concentrations and net photosynthesis in Eucalyptus seedlings

Seedlings of Eucalyptus grandis Hill ex Maiden, E. pilularis Smith and E. gummifera (Sol. ex Gaertner) Hochr. were grown in solution culture with 100 micromol phosphorus (P). After eight weeks, half of the seedlings were transferred to solution cultures containing 1 micromol P. After a further four weeks, growth, net photosynthesis and foliar P and nitrogen (N) concentrations were measured. The seeds of E. grandis came from a relatively fertile site and those of the other two species from phosphorus-deficient sites. Growth and net photosynthesis did not change in E. pilularis subjected to the low-P treatment, whereas in E. grandis, and to a limited extent in E. gummifera, the low-P treatment resulted in an increase in net photosynthesis that was associated with higher foliar N concentrations (especially protein-N), possibly as a result of nitrogen being retranslocated from the roots. In response to the low-P treatment, leaf phosphorus concentration was reduced by 50-60% in E. grandis and E. pilularis and by 20-30% in E. gummifera. Of the chemical fractions examined, the greatest decrease occurred in the inorganic-P pool. The data suggest that photosynthesis is not limited by leaf phosphorus concentrations typical of those found in Eucalyptus seedlings growing on phosphorus-deficient sites.