In situ studies on crassulacean acid metabolism in Sedum acre L. and Sedum mite Gil

Summary CO₂ exchange, the diurnal variations in the levels of malic, citric and isocitric acid, and the labelling pattern after ¹⁴CO₂ fixation were measured in Sedum acre and Sedum mite growing in situ. As predicted from laboratory experiments, drought changed the gas exchange pattern from a C₃ type to a crassulacean acid metabolism (CAM) type. This shift correlated with the development of a diurnal rhythm in the malic acid content. The results of ¹⁴CO₂ pulse-chase experiments suggest that in well-watered plants a CAM pattern of carbon flow already exists; hence water stress might enhance latent CAM rather than induce it. The in situ CAM performance by the Sedum species appeared to be highly susceptible to modulation by season and external factors, particularly light and temperature.CAM did not substantially contribute to total carbon gain in S. acre and S. mite. During most of their lifecycles the plants grow under conditions that favour CO₂ uptake by the C₃ pathway rather than by CAM. Hence, despite a capability to feature CAM, the ¹³C values found in S. acre and S. mite are those of C₃ plants.Abbreviations CAM Crassulacean Acid Metabolism - PEP-C Phosphoenolpyruvate-Carboxylase - DW Dry weight Dedicated to Prof. Dr. Dr. h.c. M. Evenari on the occasion of his 75th birthday and to Dr. K.F. Springer     

A Comparative Study by δ13C-Analysis of Crassulacean Acid Metabolism (CAM) in Kalanchoë (Crassulaceae) Species of Africa and Madagascar

The carbon isotope ratios (δ¹³C values) of samples of Kalanchoë species collected in Africa were compared with previous data obtained with species from Madagascar. In contrast to the Malagasy species which cover the whole range of δ¹³C values from ?10 to ? 30%_o, indicating high inter- and intraspecific diversity of CAM performance, in the African species nearly all δ¹³C values were less negative than ?18%₀. Thus, in the African species the CAM behaviour is characterized by CO₂ uptake proceeding mainly during the night. The distribution of δ¹³C values among the species clearly mirrors the taxonomic groups and the three sections of the genus Kalanchoë sensu lato. The Kitchingia section comprises only groups having CAM with a high proportion of carbon acquisition by the C₃-pathway of photosynthesis. The same holds true for the first three groups of the Bryophyllum section, whereas in the following groups of the section CAM with CO₂ proceeding mainly during the night is common. The latter CAM mode is typical also for the majority of groups and species in the section Eukalanchoë. The African Kalanchoë species belong to the Eukalanchoë section, whereas in Madagascar all three sections are abundant. The data support the view that the centre of adaptive radiation of the genus is located in Madagascar. They also suggest that high CAM variability is abundant in the more primitive taxa of the genus, whereas the phylogenetically more derived taxa show a stereotype CAM with CO₂ uptake taking place only during the night.     

Comparative ecophysiology of five species of Sedum (Crassulaceae) under well-watered and drought-stressed conditions

Summary Gas exchange patterns, diurnal malic acid fluctuations, and stable carbon isotope ratios of five species of Sedum were investigated to assess the ecophysiological characteristics of three different photosynthetic pathways under well-watered and drought-stressed conditions. All five species have succulent leaves and stems and were examined under identical environmental conditions. When well-watered, Sedum integrifolium (Raf.) Nels. and S. ternatum Michx. displayed C₃ photosynthesis, S. telephioides Michx. and S. nuttallianum Raf. exhibited CAM-cycling, and S. wrightii A. Gray showed CAM. When grown under a less frequent watering regime, S. integrifolium and S. ternatum exhibited CAM-cycling, whereas S. telephioides and S. nuttallianum displayed CAM-cycling simultaneously with low-level CAM. Sedum wrightii retained its CAM mode of photosynthesis. In general, leaf ¹³C values reflected these variations in photosynthetic pathways. While all values of water-use efficiency (WUE) were greater than those reported for most C₃ and C₄ species, no correlation of malic acid accumulation in the CAM and CAM-cycling (including low-level CAM) species with increased WUE was found. Sedum wrightii (CAM) had the highest WUE value at night, yet its 24-h WUE was not different from S. ternatum when the latter was in the C₃ mode. Thus, relative water-use efficiencies of these species of Sedum were not predictable based on photosynthetic pathways alone.     

Carbon and Hydrogen Isotope Ratios in Bromeliads Growing under Different Light Environments in Natural Conditions

Wild Ananas species in northern South America occur in shady environments and appear to be relatively intolerant to droughts associated with growth under full sun exposure. This behaviour contrasts with the higher productivity of commercial varieties of Ananas comosus when grown under full sun exposure. Such differentiation within the genus offers an opportunity to study the process of adaptation of apparently high light avoiding species into true sun plants. As a first approximation, the analysis of nitrogen content and carbon and hydrogen isotope ratios of bromeliads growing under natural conditions was undertaken to test the following hypotheses: 1. Leaf nitrogen content of plants grown under partial shade is higher than that of the same species in the same habitat growing under full sun exposure, due to the higher availability to nitrogen in the under-canopy, but also to the lower proportion of structural carbohydrates in shade leaves; 2. δ¹³C values are usually more negative in CAM bromeliads growing under partial shade because of the lower contribution of CAM to total carbon gain, and the probable fixation of CO₂ originating from soil respiration; 3. δD values of CAM bromeliads are less negative than those of C3 bromeliads, but CAM bromeliads grown in shady habitats tend to have more negative δD values because of the lower relative accumulation of deuterium in leaf tissue water, and also because of their relatively lower CAM activity. The results show a clear differentiation between CAM and C3 bromeliads based on δ¹³C values, and in general δD values are less negative in CAM bromeliads. However, in several species overlapping δD values between C3 and CAM bromeliads were observed. The analysis of paired samples of the same species grown under contrasting light intensity usually conformed with the expectations. A number of deviations from the hypotheses were observed which appeared to be related to particular environmental conditions. The interpretation of δD values obtained from total organic matter is made difficult by the local variation of hydrogen/deuterium ratios in water available to the plant.     

Implications of Genotypic Diversity and Phenotypic Plasticity in the Ecophysiological Success of CAM Plants,Examined by Studies on the Vegetation of Madagascar1

Abstract: On the basis of δ¹³C‐values, genotypic diversity and phenotypic plasticity of CAM behaviour in plants of the Malagasy vegetation is surveyed. The study compares CAM patterns performed in the wild on the levels of genera (Kalanchoë [Crassulaceae], Angraecum [Orchidaceae], Lissochilus [Orchidaceae] and Rhipsalis [Cactaceae]), on the level of a family (Didiereaceae) and finally on the level of a common growth form, namely in leafless orchids. For Rhipsalis, also non‐Malagasy species were included in the comparison. The genus Kalanchoë was found to be dominated by species representing the CAM‐physiotype with CO₂ fixation taking place only during the night, whereas the CAM/C3‐ and the C3‐physiotypes (with limited intrinsic CAM potential) were less frequent. The opposite holds true for Angraecum. In the genus Rhipsalis, in the Didiereacean family and in the leafless orchids only the CAM‐physiotype is represented. The photosynthetic physiotypes of CAM plants were found to be related to the environmental conditions of the habitat. That is, strong CAM performers are typically abundant in the dry climatic zones or at otherwise dry niches, species of the C3‐physiotype (possibly with weak intrinsic capability of CAM performance) are distributed at humid sites and those of the CAM/C3‐physiotype occupy sites with medium and changing exposure to stress. Phenotypic plasticity of CAM, as indicated by the intraspecific variability of the δ¹³C‐values, was lower in the CAM‐physiotype compared with the CAM/C3‐physiotype. Our data support the view that strong stress leads to the dominance of highly adapted specialists among the CAM plants, with low phenotypic plasticity of the photosynthetic behaviour, whereas medium stress advances the unfolding of plastic CAM behaviour. Moreover, the data suggest that genera comprising all three physiotypes (Kalanchoë, Angraecum) are dispersed all over Madagascar, whilst groups comprising only strong CAM performers are restricted to limited areas or special types of habitats. This suggests that both genotypic diversity and phenotypic plasticity are important factors for the ecophysiological success of CAM.     

Stable isotope analysis indicates trophic differences among forest floor carabids in Japan

Differences in trophic niches among carabid beetles (Coleoptera: Carabidae) co‐occurring on the forest floors of warm temperate forests in central Japan were studied using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analyses. Different carabid species showed similar δ¹⁵N values, which were higher than those of their possible invertebrate prey (herbivores and detritivores) collected from the litter layer, indicating that these species were consumers in the same trophic level. In contrast, δ¹³C values differed among carabid species, indicating interspecific differences in prey animals. The variation in the δ¹³C value was larger in summer than in autumn. In summer, δ¹³C values indicated that some carabids depended highly on either grazing (low δ¹³C values) or detrital sources (high δ¹³C values) within the food chain [Chlaenius posticalis Motschulsky and Haplochlaenius costiger (Chaudoir), respectively], although other species with intermediate δ¹³C values likely depended on both. The latter group of species comprised mostly two dominant genera (Carabus and Synuchus). Although congeners might have similar feeding habits, the stable isotope ratios indicated trophic niche differences between adults of different species and between adults and larvae of the same genus.     

Carbon isotopic abundances in Mesozoic and Cenozoic fossil plants: Palaeoecological implications

Carbon isotopic abundances have been measured for more than one hundred samples of fossil plants ranging in age from middle Triassic to late Tertiary. Most of the plant fossils were identified at the specific or generic level and were selected as representing a variety of continental environments, including xeric and humid habitats. Material analysed included numerous fragments of flowers, seeds, fruits, leaves and wood, as well as a single amorphous lignite sample. The analyses performed for the plant fragments indicate relatively constant isotopic compositions during this time interval, with plant δ¹³C values ranging between -28 and -20%. These values are within the range for living terrestrial plants with C₃, photosynthesis, although values more positive than -23% are rare in C₃ plants and typically found in plants growing under environmental stress. Lower δ¹³C values might have been expected owing to the much higher CO₂, levels of the Cretaceous atmosphere that have been inferred from marine carbonates. No fossils with values indicating C₄, photosynthesis were discovered. Fossil plants from inferred mesic environments showed δ¹³C values ranging between -26.7 and -24.1%. Highest δ¹³C values in angiosperms (up to -20.1%) were measured for Late Cretaceous combretaceous flowers from Portugal. Some cheirolepidiaceous conifers from the Early Cretaceous also showed high δ¹³C values. Values measured for Pseudofrenelopsis varians and Glenrosa taxensis were -21.9%, and values of gymnosperm wood, probably of cheirolepidiaceous affinity, were -19.0%. These high values are in accordance with inferred ecological conditions for the fossil plants. They may suggest a tendency for C₄,-like photosynthesis, although the data are equivocal. Higher values (-17.3%) clearly falling outside the C₃, range were, however, obtained from a single lignite fragment of Late Cretaceous (Maastrichtian) age. The nature of this plant fragment is unknown, but the result suggests that C₄-like photosynthesis was present at least in some latest Cretaceous vegetation. A hadrosaurian dinosaur with well-preserved collagen-like organic matter from the same deposit showed δ¹³C values around-16%, which also suggests the presence of CAM or even C₄ plants in the latest Cretaceous. □Carbon isotopic abundances, δ¹³C values, dinosaurs, plants, photosynthetic pathways, Mesozoic.     

Juvenile tank-bromeliads lacking tanks: do they engage in CAM photosynthesis?

In the epiphytic tillandsioids, Guzmania monostachia, Werauhia sanguinolenta, and Guzmania lingulata (Bromeliaceae), juvenile plants exhibit an atmospheric habit, whereas in adult plants the leaf bases overlap and form water-holding tanks. CO₂ gas-exchange measurements of the whole, intact plants and δ¹³C values of mature leaves demonstrated that C₃ photosynthesis was the principal pathway of CO₂ assimilation in juveniles and adults of all three species. Nonetheless, irrespective of plant size, all three species were able to display features of facultative CAM when exposed to drought stress. The capacity for CAM was the greatest in G. monostachia, allowing drought-stressed juvenile and adult plants to exhibit net CO₂ uptake at night. CAM expression was markedly lower in W. sanguinolenta, and minimal in G. lingulata. In both species, low-level CAM merely sufficed to reduce nocturnal respiratory net loss of CO₂. δ¹³C values were generally less negative in juveniles than in adult plants, probably indicating increased diffusional limitation of CO₂ uptake in juveniles.     

Latitudinal variation in the degree of crassulacean acid metabolism in Puya chilensis

Crassulacean acid metabolism (CAM) is a photosynthetic pathway found in many plant species from arid and semiarid environments. Few studies aiming to characterise plant species as CAM or C₃ account for inter‐population differences in photosynthetic pathway, often relying on samples taken from herbarium material and/or a single plant or population. This may be especially problematic for species growing under contrasting climate conditions, as is the case for species with a wide geographic range. We used Puya chilensis, a species previously reported as CAM and C₃, to study among‐population variation in expression of the CAM pathway within its distribution range, which spans a significant climate gradient. We carried out a wide sampling scheme, including five populations and a combination of analytical methods (quantification of nocturnal acidification and stable isotope measurements). The study populations of P. chilensis encompass the entire latitudinal distribution range, from semi‐arid to temperate oceanic climates. Our results indicate that CAM decreased with latitude. However, even in the southern (wetter) populations, where δ¹³C values were indicative of C₃ metabolism, we found some nocturnal acidification. We stress the value of using two methods along with the use of samples from different populations, as this allows more reliable conclusions on the photosynthetic pathway for ‘probable’ CAM species that face varying climate conditions within their distribution ranges.     

Photosynthetic pathways in Bromeliaceae: phylogenetic and ecological significance of CAM and C3 based on carbon isotope ratios for 1893 species

A comprehensive analysis of photosynthetic pathways in relation to phylogeny and elevational distribution was conducted in Bromeliaceae, an ecologically diverse Neotropical family containing large numbers of both terrestrial and epiphytic species. Tissue carbon isotope ratio (δ¹³C) was used to determine the occurrence of crassulacean acid metabolism (CAM) and C₃ photosynthesis in 1893 species, representing 57% of species and all 56 genera in the family. The frequency of δ¹³C values showed a strongly bimodal distribution: 1074 species (57%) had values more negative than −20‰ (mode = −26.7‰), typical of predominantly daytime carbon fixation via the C₃ pathway, whereas 819 species (43%) possessed values less negative than −20‰ (mode = −13.3‰), indicative of predominantly nocturnal fixation of carbon via the CAM pathway. Amongst the six almost exclusively terrestrial subfamilies in Bromeliaceae, Brocchinioideae, Lindmanioideae and Navioideae consisted entirely of C₃ species, with CAM species being restricted to Hechtioideae (all species of Hechtia tested), Pitcairnioideae (all species belonging to a xeric clade comprising Deuterocohnia, Dyckia and Encholirium) and Puyoideae (21% of Puya spp.). Of the other two subfamilies, in the overwhelmingly epiphytic (plus lithophytic) Tillandsioideae, 28% of species possessed CAM photosynthesis, all restricted to the derived genus Tillandsia and tending towards the more extreme epiphytic ‘atmospheric’ life‐form. In Bromelioideae, with comparable numbers of terrestrial and epiphytic species, 90% of taxa showed CAM; included in these are the first records of CAM photosynthesis in Androlepis, Canistropsis, Deinacanthon, Disteganthus, Edmundoa, Eduandrea, Hohenbergiopsis, Lymania, Pseudananas, Ronnbergia and Ursulaea. With respect to elevational gradients, the greatest number of C₃ bromeliad species were found at mid‐elevations between 500 and 1500 m, whereas the frequency of CAM species declined monotonically with increasing elevation. However, in Puya, at least ten CAM species have been recorded at elevations > 3000 m, showing that CAM photosynthesis is not necessarily incompatible with low temperatures. This survey identifies five major origins of CAM photosynthesis at a higher taxonomic level in Bromeliaceae, but future phylogenetic work is likely to reveal a more fine‐scale pattern of gains and losses of this trait, especially in ecologically diverse and widely distributed genera such as Tillandsia and Puya. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 178, 169–221.     

The nutritional significance of a winter‐flowering succulent for opportunistic avian nectarivores

The winter‐flowering succulent Aloe marlothii provides nectar for many opportunistic avian nectarivores in southern African savannas. We assessed the importance of A. marlothii nectar sugar for opportunistic nectarivores by analysing temporal changes in stable carbon isotope ratios (δ¹³C) in the tissues of birds in Suikerbosrand Nature Reserve, South Africa. The blood of the 11 most common non‐granivorous opportunistic nectarivores at our site was enriched in ¹³C by 3.4 ± 1.5‰ during the flowering period of A. marlothii, reflecting the enriched crassulacean acid metabolism (CAM) isotopic signature of nectar (?12.6 ± 0.5‰). This relatively small contribution of A. marlothii nectar to assimilated carbon in whole blood contrasted with that of exhaled CO₂ in African Red‐eyed Bulbuls Pycnonotus nigricans and Cape White‐eyes Zosterops capensis. In both these species, the δ¹³C of breath samples was significantly enriched compared with blood and feathers, and closely resembled that of the nectar, revealing combustion of ingested nectar rather than assimilation. Although our analysis was complicated by the presence of C₄ grasses, whose δ¹³C values are similar to those of CAM photosynthesizers, when considered with previously published feeding observations our data reveal that opportunistic nectarivores feeding on A. marlothii nectar obtain a relatively small fraction of their assimilated carbon, but most of their metabolized carbon, from this seasonally available carbohydrate food resource. Because the δ¹³C values of insects associated with C₃ plants also became enriched during the flowering season, some insect‐eating opportunistic nectarivores may have assimilated A. marlothii carbon indirectly from insects. This study highlights the importance of understanding isotopic routing when assessing the nutritional significance of specific dietary items to consumer communities.     

Induction by drought of crassulacean acid metabolism in the terrestrial bromeliad, <Emphasis Type="Italic">Puya floccosa</Emphasis>

In the terrestrial bromeliad, Puya floccosa, a value of carbon isotopic composition (δ¹³C) of −22‰ has been previously reported, suggesting the operation of weak and/or intermediate (C₃-CAM) crassulacean acid metabolism (CAM). In order to characterize the operation of CAM in P. floccosa and its possible induction by drought, plants were grown in Caracas and subjected to four independent drought cycles. Additionally, since plants of this species grow in Venezuela in a large range of elevations, leaf samples were collected at elevations ranging from 725 to 2,100 m a.s.l. in the Venezuelan Andes and the Coastal Range, in order to evaluate the effect of elevation on CAM performance. Even though nocturnal acid accumulation occurred in both watered and droughted plants, mean ΔH⁺ was higher in droughted than watered plants [ΔH⁺ = 60.17.5 and 22.9 ± 5.2 μmol g⁻¹(FM), respectively]. The majority of plants from all the natural populations sampled had low values of δ¹³C not differing significantly from those of C₃ plants collected as standards and δ¹³C did not change with elevation. We conclude that P. floccosa is capable of a weak CAM activity, with a large variability among populations and drought experiments probably due to local and temporal differences in microclimatic variables and drought stress; elevation bears no influence on values of δ¹³C in this species.     

Environmental regulation of carbon isotope composition and crassulacean acid metabolism in three plant communities along a water availability gradient

Expression of crassulacean acid metabolism (CAM) is characterized by extreme variability within and between taxa and its sensitivity to environmental variation. In this study, we determined seasonal fluctuations in CAM photosynthesis with measurements of nocturnal tissue acidification and carbon isotopic composition (δ¹³C) of bulk tissue and extracted sugars in three plant communities along a precipitation gradient (500, 700, and 1,000 mm year⁻¹) on the Yucatan Peninsula. We also related the degree of CAM to light habitat and relative abundance of species in the three sites. For all species, the greatest tissue acid accumulation occurred during the rainy season. In the 500 mm site, tissue acidification was greater for the species growing at 30% of daily total photon flux density (PFD) than species growing at 80% PFD. Whereas in the two wetter sites, the species growing at 80% total PFD had greater tissue acidification. All species had values of bulk tissue δ¹³C less negative than −20‰, indicating strong CAM activity. The bulk tissue δ¹³C values in plants from the 500 mm site were 2‰ less negative than in plants from the wetter sites, and the only species growing in the three communities, Acanthocereus tetragonus (Cactaceae), showed a significant negative relationship between both bulk tissue and sugar δ¹³C values and annual rainfall, consistent with greater CO₂ assimilation through the CAM pathway with decreasing water availability. Overall, variation in the use of CAM photosynthesis was related to water and light availability and CAM appeared to be more ecologically important in the tropical dry forests than in the coastal dune.     

<Emphasis Type="Italic">δ</Emphasis><Superscript>13</Superscript>C values and crassulacean acid metabolism in <Emphasis Type="Italic">Clusia</Emphasis> species from Panama

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 ¹³C 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 ¹³C 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 ¹³C values were C₃-like. All 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 ¹³C values, the expression of CAM was similar in terrestrial, hemi-epiphytic and epiphytic species and did not differ between individuals of the same species that exhibited different life-forms. This study indicates that phylogenetic affiliation 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. ¹³C 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.     

Size-Dependent Variation of Carbon and Nitrogen Isotope Abundances in Epiphytic Bromeliads

Abstract: While atmospheric species of bromeliads have narrow leaves, densely covered with water‐absorbing trichomes throughout their life cycles, many tank bromeliads with broad leaves, forming phytotelmata, go through an atmospheric juvenile phase. The effect of the different habits and the phase change in tank‐forming bromeliads on water and nutrient relations was investigated by analysing the relationship between plant size, C/N ratios and the natural abundance of ¹³C and ¹⁵N in five epiphytic bromeliad species or morphospecies of a humid montane forest in Xalapa, Mexico. The atmospheric species Tillandsia juncea and T. butzii exhibited full crassulacean acid metabolism, with δ¹³C values (mean ‐ 15.3 ‰ and ‐ 14.7 ‰, respectively) independent of size. In Tillandsia species with C₃ photosynthesis, δ¹³C decreased with increasing plant size, indicating stronger drought stress in juveniles. The increase of the C/N ratio with size suggests that, at least in heteroblastic bromeliads, the availability of water is more limiting during early growth, and that limitations of nitrogen supply become more important later on, when water stored in the tank helps to bridge dry periods, reducing water shortage. δ¹⁵N values of the two atmospheric species were very negative (‐ 12.6 ‰ and ‐ 12.2 ‰, respectively) and did not change with plant size. Tank‐forming bromeliads had less negative δ¹⁵N values (c ‐ 6 ‰), and, in species with atmospheric juveniles and tank‐forming adults, δ¹⁵N values increased significantly with plant size. These differences do not appear to be an effect of the isotopic composition of N sources, but rather reflect N availability and limitation and stress‐induced changes in ¹⁵N discrimination.     

Diel variations in carbon isotopic composition and concentration of organic acids and their impact on plant dark respiration in different species

Leaf respiration in the dark and its C isotopic composition (δ¹³C_R) contain information about internal metabolic processes and respiratory substrates. δ¹³C_R is known to be less negative compared to potential respiratory substrates, in particular shortly after darkening during light enhanced dark respiration (LEDR). This phenomenon might be driven by respiration of accumulated ¹³C‐enriched organic acids, however, studies simultaneously measuring δ¹³C_R during LEDR and potential respiratory substrates are rare. We determined δ¹³C_R and respiration rates (R) during LEDR, as well as δ¹³C and concentrations of potential respiratory substrates using compound‐specific isotope analyses. The measurements were conducted throughout the diel cycle in several plant species under different environmental conditions. δ¹³C_R and R patterns during LEDR were strongly species‐specific and showed an initial peak, which was followed by a progressive decrease in both values. The species‐specific differences in δ¹³C_R and R during LEDR may be partially explained by the isotopic composition of organic acids (e.g., oxalate, isocitrate, quinate, shikimate, malate), which were ¹³C‐enriched compared to other respiratory substrates (e.g., sugars and amino acids). However, the diel variations in both δ¹³C and concentrations of the organic acids were generally low. Thus, additional factors such as the heterogeneous isotope distribution in organic acids and the relative contribution of the organic acids to respiration are required to explain the strong ¹³C enrichment in leaf dark‐respired CO₂.     

Gradation in Nutrient Composition and Photosynthetic Pathways Across the Restinga Vegetation of Brazil

The restinga comprises coastal vegetation formations which dominate the Atlantic seaboard of Brazil. Exposed sand ridges and associated lagoon systems have poorly developed soils subject to pronounced water deficits. Distinct vegetation zones support a high diversity of life forms, and a comparative study has been undertaken to investigate interactions between degree of exposure, nutrient supply and photosynthetic pathway (C₃, or CAM) in selected species across the restinga. A number of species occurring throughout the restinga were chosen as representative species of different life forms, comprising C₃ pioneer shrubs (Eugenia rotundifolia and Erythroxylum ovalifolium), impounding (tank) terrestrial bromeliad (Neoregelia cruenta: CAM) and the atmospheric epiphyte (Tillandsia stricta: CAM). Comparisons of plant and soil nutrient composition, and airborne deposition were conducted for each zone. Soil nutrient content and organic matter were closely related, reaching a maximum in zone 4, the seaward face of the inner dune. Salt concentration in leaves was independent of atmospheric deposition for the terrestrial species, in contrast to the atmospheric epiphyte T. stricta. In the slack area, vegetation formed characteristic “islands” with the soil beneath enriched in nutrients, suggesting a complex interplay between plants and soil during the development of vegetation succession. Here, two additional trees were investigated, C₃ and CAM members of the Clusiaceae, respectively Clusia lanceolata and C. fluminensis. Stable isotope composition of nitrogen (δ¹⁵N) was generally more negative (depleted in ¹⁵N) in plants with low total nitrogen content. This was exemplified by the atmospheric bromeliad, T. stricta, with an N content of 2.91 g/kg and δ¹⁵N of ?12.3 per mil. Stable isotopes of carbon (δ¹³C) were used to identify the distribution of photosynthetic pathways, and while the majority of bromeliads and orchids were CAM, analysis of the soil organic matter suggested that C₃ plants made the major contribution in each zone of the restinga. Since δ¹³C of plant material also suggested that water supply was optimal in zone 4, we conclude that succession and high diversity in the restinga is dependent on exposure, edaphic factors, and perhaps a critical mass of vegetation required to stabilize nutrient relations of the system.     

Using stable isotopes and C:N ratios to examine the life‐history strategies and nutritional sources of larval lampreys

Natural abundance stable‐isotope analysis (δ¹³C and δ¹⁵N) and C:N ratios were used to study the ammocoete phase of two common non‐parasitic lamprey species (least brook lamprey Lampetra aepyptera and American brook lamprey Lethenteron appendix) in two tributaries of the Ohio River (U.S.A.). The C:N ratios suggest that each species employs different lipid accumulation strategies to support its metamorphosis and recruitment into an adult animal. Ammocoete δ¹³C values generally increased with increasing C:N values. In contrast to δ¹³C, ammocoete δ¹⁵N values were weakly related to the total length (L_T) in L. aepyptera, but positively correlated to both L_T and C:N ratios in L. appendix. In L. appendix, C:N also correlated positively with L_T, and presumably age. A Bayesian mixing model using δ¹³C and δ¹⁵N was used to estimate nutritional subsidies of different potential food resources to ammocoetes at each site. The models suggested that although nutritional subsidies to ammocoetes varied as a function of site, ammocoetes were generally reliant on large contributions (42–62% at three sites) from aquatic plants. Contributions from aquatic sediment organic matter were also important at all sites (32–63%) for ammocoetes, with terrestrially derived plant materials contributing smaller amounts (4‐33%). These findings provide important insights into the feeding ecology and nutrition of two species of lampreys. They also suggest that similar and other quantitative approaches are required to (1) fully understand how the observed stable‐isotopes ratios are established in ammocoetes and (2) better assess ammocoete nutritional subsidies in different natal streams.     

Crassulacean acid metabolism contributes significantly to the in situ carbon budget in a population of the invasive aquatic macrophyte Crassula helmsii

1. The ecophysiological significance of Crassulacean acid metabolism (CAM) in the invasive aquatic macrophyte Crassula helmsii was studied in an English soft‐water lake. The extent and the contribution of CAM to the carbon budget was examined in spring (April) and summer (July) along a depth gradient (0.5–2.2 m), covering the growth range of C. helmsii in the lake. 2. Significant in situ CAM activity (30–80 meq kg⁻¹ FW) was present in all specimens, although it decreased with depth and hence correlated with the decline in photon irradiance. Potential CAM activity (60–161 meq kg⁻¹ FW), measured after exposure to low concentrations of CO₂ in the day and high concentrations at night, were on average 2.7‐times greater than in situ CAM activity. Overall CAM activity increased from April to July, which is consistent with higher potential carbon limitation caused by increased temperature and light availability. 3. CAM activity in C. helmsii appeared to be carbon‐limited at night because night‐time carbon‐fixation increased at raised, compared to ambient, concentrations of CO₂. 4. The high in situ CAM activity in C. helmsii was reflected in the contribution of CAM to the total carbon budget which, independent of depth and season, ranged from 18% to 42%. The amount of CO₂ taken up in the night via CAM was 0.74 to 2.94 times the amount of CO₂ lost in respiration, thus emphasizing the importance of CAM in refixation of potentially lost respiratory CO₂. 5. The onset of decarboxylation in the morning appeared to be under circadian control as there was a delay of up to 5.5 h between the start of the light period and a decline in cell acidity level. 6. There was little variation in δ¹³C content (−21.69 to 23.49‰) with season or depth suggesting, along with the estimated contribution to the carbon‐budget, that CAM is important for the whole population of C. helmsii. CAM may confer a competitive advantage in relation to growth, which may be one of the reasons for the invasiveness of this species.     

Trophic level delineation and resource partitioning in a South African afromontane forest bird community using carbon and nitrogen stable isotopes

Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ¹³C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ¹³C signatures typical of a C₃ photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C₄ signature (?12.0 ± 0.6‰). Most bird species had δ¹³C values representing a predominantly C₃‐based diet (?24.8‰ to ?20.7‰). δ¹⁵N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ¹³C and δ¹⁵N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C₃‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.