Transpiration efficiency of a tropical pioneer tree (Ficus insipida) in relation to soil fertility.
The response of whole-plant water-use efficiency, termed transpiration efficiency (TE), to variation in soil fertility was assessed in a tropical pioneer tree, Ficus insipida Willd. Measurements of stable isotope ratios (delta(13)C, delta(18)O, delta(15)N), elemental concentrations (C, N, P), plant growth, instantaneous leaf gas exchange, and whole-plant water use were used to analyse the mechanisms controlling TE. Plants were grown individually in 19 l pots with non-limiting soil moisture. Soil fertility was altered by mixing soil with varying proportions of rice husks, and applying a slow release fertilizer. A large variation was observed in leaf photosynthetic rate, mean relative growth rate (RGR), and TE in response to experimental treatments; these traits were well correlated with variation in leaf N concentration. Variation in TE showed a strong dependence on the ratio of intercellular to ambient CO(2) mole fractions (c(i)/c(a)); both for instantaneous measurements of c(i)/c(a) (R(2)=0.69, P <0.0001, n=30), and integrated estimates based on C isotope discrimination (R(2)=0.88, P <0.0001, n=30). On the other hand, variations in the leaf-to-air humidity gradient, unproductive water loss, and respiratory C use probably played only minor roles in modulating TE in the face of variable soil fertility. The pronounced variation in TE resulted from a combination of the strong response of c(i)/c(a) to leaf N, and inherently high values of c(i)/c(a) for this tropical tree species; these two factors conspired to cause a 4-fold variation among treatments in (1-c(i)/c(a)), the term that actually modifies TE. Results suggest that variation in plant N status could have important implications for the coupling between C and water exchange in tropical forest trees.
Smithsonian Tropical Research Institute, PO Box 0843-03092, Balboa, Ancon, Republic of Panama. cernusakl@si.edu
Lucas A
LA
Cernusak
Klaus
K
Winter
Jorge
J
Aranda
Benjamin L
BL
Turner
John D
JD
Marshall
D002244:Carbon / Q000032:analysis; D030681:Ficus / Q000502:physiology*; D009584:Nitrogen / Q000032:analysis; D010100:Oxygen; D010176:Panama; D018515:Plant Leaves / Q000502:physiology; D018526:Plant Transpiration / Q000502:physiology*; D012987:Soil / Q000032:analysis*; D013696:Temperature; D014329:Tropical Climate; D014867:Water
D012987:Soil; D014867:Water; D002244:Carbon; D009584:Nitrogen; D010100:Oxygen