Phosphorus concentrations and chemical fractions in Eucalyptus seedlings grown for a prolonged period under nutrient-deficient conditions

Seedlings of Eucalyptus grandis Hill ex Maiden, two provenances of E. pilularis Smith (Coffs Harbour (CH) and Myall Lakes (ML)), and E. gummifera (Sol. ex Gaertner) Hochr. were grown for 60 weeks in sand culture under two nutrient-limiting regimes. Under both levels of nutrient deficiency, E. pilularis (ML) and E. gummifera, the two species from phosphorus-deficient sites, produced more dry matter than the other two species which originated in more fertile soils. Phosphorus partitioning among different organs, concentrations of phosphorus (and hence efficiencies of phosphorus use) and pool sizes of lipid-P, Pi, ester-P, nucleic acid-P and residue-P (bound P) were assessed in each organ at harvest. Concentrations were expressed on a per unit protein-N basis (reflecting the amount of cytoplasm) as well as on a dry weight basis. At the more extreme conditions of nutrient deficiency (Treatment A), there was little difference between the leaves of the species in terms of efficiency of phosphorus use, but under the less severe conditions (Treatment B) efficiency was in the order E. gummifera > E. pilularis > E. grandis. Most of the foliar phosphorus (up to 70%) was present in a nucleic acid and ester form, although E. gummifera (and E. grandis for Treatment B) had a substantial phospholipid pool (approx. 25% of total leaf phosphorus). In the former case at least, this difference in phosphorus distribution among the chemical fractions may be of ecological significance. The stems and roots showed no species differences but noteworthy findings included the high residue-P pool in the roots of Treatment A seedlings (up to 41% of root phosphorus) and the high concentrations of Pi in the stems of Treatment B plants (67-87% of stem phosphorus). The ecologically distinct provenances of E. pilularis differed neither in phosphorus utilization efficiency nor in distribution among the molecular fractions examined. Their differing productivities were thus related to differences in phosphorus acquisition. The comparatively lower productivity of E. grandis was due to a combination of less efficient acquisition and less efficient internal utilization. The success of E. gummifera could be attributed to increased uptake and possibly increased utilization efficiency that in turn may have been related to its different phosphorus allocation pattern among the major chemical fractions.