In view of anthropogenic global warming, heat tolerance of a neotropical pioneer tree, Ficus insipida Willd., was determined. Sections of sun leaves from a mature tree and from seedlings cultivated at ambient and elevated temperatures were heated to 42–53°C. Leaves from a late-successional tree species, Virola sebifera Aubl., were also studied. Widely used chlorophyll a fluorescence methods based on heat-induced rise of initial fluorescence emission, Fo, and decrease in the ratio of variable to maximum fluorescence, Fv/Fm, were reassessed. Fv/Fm determined 24 h after heat treatment was the fluorescence parameter most suitable to assess the lethal temperature causing permanent tissue damage. Thermo-tolerance was underestimated when Fo and Fv/Fm were recorded immediately after the heat treatment. The limit of thermo-tolerance was between 50 and 53°C, only a few °C above peak leaf temperatures measured in situ. The absence of seasonal changes in thermo-tolerance and only marginal increases in thermo-tolerance of plants grown under elevated temperatures suggest little capacity for further heat acclimation. Heat-stress experiments with intact potted seedlings also revealed irreversible leaf damage at 51–53°C, but plants survived and developed new leaves during post-culture.

Summary• It is well known that whole-plant water-use efficiency (transpiration efficiency of carbon gain, TE C ) varies among plant species with different photosynthetic pathways. However, less is known of such variation among tree species within the C 3 group. Here we measured the TE C of seven C 3 tropical tree species. Isotopic analyses ( δ 13 C, δ 18O, and δ 15 N) and elemental analyses (carbon and nitrogen) were undertaken to provide insight into sources of variation in TE C .• Plants were grown over several months in approx. 80% full sunlight in individual 38-l containers in the Republic of Panama. Soil moisture content was nonlimiting.• Significant variation was observed in TE C among the C 3 tree species. Values ranged from 1.6 mmol C mol − 1 H 2 O for teak ( Tectona grandis ) to 4.0 mmol C mol − 1 H 2 O for a legume, Platymiscium pinnatum .• Variation in TE C was correlated with both leaf N concentration, a proxy for photosynthetic capacity, and oxygen-isotope enrichment, a proxy for stomatal conductance. The TE C varied with C-isotope discrimination within species, but the relationship broke down among species, reflecting the existence of species-specific offsets.

Seedlings of several species of gymnosperm trees, angiosperm trees, and angiosperm lianas were grown under tropical field conditions in the Republic of Panama; physiological processes controlling plant C and water fluxes were assessed across this functionally diverse range of species. Relative growth rate, r, was primarily controlled by the ratio of leaf area to plant mass, of which specific leaf area was a key component. Instantaneous photosynthesis, when expressed on a leaf-mass basis, explained 69% of variation in r (P , 0.0001, n 5 94). Mean r of angiosperms was significantly higher than that of the gymnosperms; within angiosperms, mean r of lianas was higher than that of trees. Whole-plant nitrogen use efficiency was also significantly higher in angiosperm than in gymnosperm species, and was primarily controlled by the rate of photosynthesis for a given amount of leaf nitrogen. Whole-plant water use efficiency, TE c , varied significantly among species, and was primarily controlled by c i /c a , the ratio of intercellular to ambient CO 2 partial pressures during photosynthesis. Instantaneous measurements of c i /c a explained 51% of variation in TE c (P , 0.0001, n 5 94). Whole-plant 13 C discrimination also varied significantly as a function of c i /c a (R 2 5 0.57, P , 0.0001, n 5 94), and was, accordingly, a good predictor of TE c . The 18O enrichment of stem dry matter was primarily controlled by the predicted 18 O enrichment of evaporative sites within leaves (R 2 5 0.61, P , 0.0001, n 5 94), with some residual variation explained by mean transpiration rate. Measurements of carbon and oxygen stable isotope ratios could provide a useful means of parameterizing physiological models of tropical forest trees.Tropical forest ecosystems have been subject to extensive perturbations associated with anthropogenic activity in recent decades, and such perturbations will likely continue into the foreseeable future (Laurance et al., 2004;Wright, 2005). Effective environmental management requires knowledge of how such perturbations impact upon cycling of carbon (C) and water between forest trees and the atmosphere, and how these C and water fluxes relate to plant nutrient status. A sound, mechanistic understanding of the physiological processes that control photosynthesis and transpiration in tropical trees is therefore essential for understanding and managing the human impact upon tropical forests. In this study, we analyzed the physiological controls over growth (the relative rate of C accumulation), nitrogen (N) use efficiency (NUE; the rate of C accumulation for a given N content), water use efficiency (the ratio of whole-plant C gain to water loss), and stable isotope composition (d 13

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.

The genus Clusia is notable in that it contains arborescent crassulacean acid metabolism (CAM) plants. As part of a study of CAM in Clusia, titratable acidities were measured in 25 species and í i^c values were measured for 38 species from Panamá, including seven undescribed species, and 11 species from Colombia, Costa Rica and Honduras. CAM was detected in 12 species. Clusia flava, C rosea and C. uvitana exhibited Í l^C values or diurnal fluctuations in acidity indicative of strong CAM. In C. croatii, C cylindrica, C. fructiangusta, C lineata, C odorata, C pratensis, C quadrangula, C. valerioi and C. sp. D diurnal fluctuations in acidity were consistent with weak CAM but the Í i^C values were Cß-like. AU of the species that exhibited strong or weak CAM were in the C. flava or C. minor species groups. CAM was not detected in any member of the C. multiflora species group. Strong CAM species were not collected at altitudes above 680 m a. s.l. On the basis of ^^^C values, the expression of CAM was similar in terrestrial, hemiepiphytic and epiphytic species and did not differ between individuals of the same species http://www.springerlink.com/media/ECXX5U63F83JUG8CA...butions/LA'/N/J/LVNJBJQ2KGNTAVP7_html/fulltext.html (1 of 39)6/5/2004 1:09:18 PM 10.1007/s00468-004-0342-y that exhibited different Hfe-forms. This study indicates that phylogenetic affihation may be a predictor of an ability to exhibit CAM in Clusia species from the Panamanian region, and that weak CAM is probably a common photosynthetic option in many Clusia species. í l^C value is not a particularly good indicator of a potential of Clusia species growing in the field to exhibit CAM because it appears that the contribution in most species of CAM to carbon gain is generally rather small when integrated over the life-time of leaves.