1. Foliar phosphorus (P) concentration is the sum of the concentrations of P fractions in cells, such as inorganic P and various P-containing biochemical compounds (e.g. nucleic acids, lipids and sugar phosphates). Plants generally reduce foliar P concentration and enhance P-use efficiency in response to low soil P availability. However, how plants allocate P among foliar P fractions to reduce foliar P concentration remains unclear.
2. We investigated foliar P fractions and leaf traits of 21 tropical tree species along a soil P availability gradient across three tropical montane rain forests on Mount Kinabalu, Borneo. We chemically and sequentially fractionated foliar P into the following four fractions: structural P (i.e. phospholipids), metabolic P (collectively including Pi, ATP and sugar phosphates), nucleic acid P and residual P (phosphoproteins and unidentified residue).
3. With decreasing soil P availability, foliar P concentration decreased and leaf mass per area (LMA) increased. The reduction in foliar P concentration strongly correlated with the reduction in the concentrations of both metabolic P and nucleic acid P. Although increased LMA implies an increased allocation to structural tissues, there was no trade-off in P allocation between metabolic P and structural P with increasing LMA. This suggests that tropical tree species on P-poor soils increase the toughness of leaves (i.e. prolonged leaf life span) and also maintain high photosynthetic P-use efficiency (PPUE) without increasing the cost of P for structural tissues.
4. Phosphorus resorption efficiency increased with decreasing soil P availability. The amount of P resorbed before leaf abscission on P-poor soils exceeded that of metabolic P. This suggests that tropical tree species achieve the high P resorption efficiency by withdrawing immobile fractions (i.e. nucleic acid P and structural P) in addition to metabolic P.
5. Synthesis. We conclude that tree species on P-poor soils reduce the demand for foliar P by reducing concentrations of both metabolic P and nucleic acid P, which may potentially limit growth and productivity. However, these tree species can maintain high whole-plant P-use efficiency, because such responses in foliar P fractions do not decrease PPUE, leaf life span and P resorption efficiency.