Transpiration efficiency of a tropical pioneer tree (Ficus insipida) in relation to soil fertility

Abstract

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.

Responses of Legume Versus Nonlegume Tropical Tree Seedlings to Elevated CO2 Concentration

“…This extractable P concentration is in the high range of values typically observed for neotropical rainforest soils (Clinebell et al, 1995;Turner and Engelbrecht, 2011). We previously observed that the addition of rice husks to the soil mixture differentially reduced N availability relative to P availability, presumably due to microbial immobilization of available N (Cernusak et al, 2007b). Based on these considerations, the unfertilized soil treatments in our experiments likely had relatively low N availability compared with P availability.…”

“…Thus, nodule formation can be suppressed by increasing the availability of N in the soil, as demonstrated for fertilized O. macrocalyx in this study, and by growth under low irradiance (McHargue, 1999). The unfertilized treatments in our experiments replicated a low-N gap environment, insofar as there was high irradiance (approximately 80% of full sunlight) and low soil N availability, caused by the addition of rice husks to the soil mixture (Cernusak et al, 2007b). Therefore, these experimental conditions should have led to maximal expression of nodule mass ratio and N 2 fixation, when compared with the range of conditions likely to be encountered by these species in their native environments.…”

Photosynthetic physiology of eucalypts along a sub-continental rainfall gradient in northern Australia

“…. The d was previously estimated to have a mean value of about 3‰ in 15 tropical tree species (Cernusak et al, 2008), with other estimates in the literature ranging from about 0 to 4‰ (Cernusak et al, 2007;Hubick, 1990;Hubick and Farquhar, 1989;Hubick et al, 1986). The c i /c a is controlled by the balance between photosynthesis (A) and stomatal conductance (g s ).…”

Trait-based community assembly of understory palms along a soil nutrient gradient in a lower montane tropical forest

“…The 5 N ratio of plant leaves generally represents the (5 15 N ratio of the soil source the plant takes up, with nitrate having a lower (5 15 N ratio than ammonium in most cases (Hogberg 1997;but see Houlton 2007). Carbon isotope ratios have been used to represent integrated water-use efficiency over the leaf lifespan (Farquhar et al 1989;Baltzer et al 2005;Cernusak et al 2007). Leaf 5 C ratios arise from differences in the diffusivity of C and C in air and through the stomatal pathway, as well as from the enzymatic discrimination of the isotopes.…”

“…Leaf 5 C ratios arise from differences in the diffusivity of C and C in air and through the stomatal pathway, as well as from the enzymatic discrimination of the isotopes. Plants that use water more efficiently exhibit less negative (5 13 C ratios (Farquhar et al 1989;Baltzer et al 2005;Cernusak et al 2007). Instantaneous water use efficiency (WUE) was calculated from A max ( area ) and transpiration according to Lambers et al (1998) where A max (area ) -area-based photosynthetic rate (umol CO2 m~ s~ ) and N area -area-based leaf N content (g N cm -2 ).…”