Allocation of foliar phosphorus fractions and leaf traits of tropical tree species in response to decreased soil phosphorus availability on Mount Kinabalu, Borneo

Summary 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.

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