Estimates of nitrogen fixation by trees on an aridity gradient in Namibia

Summary Nitrogen (N₂) fixation was estimated along an aridity gradient in Namibia from the natural abundance of ¹⁵N (¹⁵N value) in 11 woody species of the Mimosacease which were compared with the ¹⁵N values in 11 woody non-Mimosaceae. Averaging all species and habitats the calculated contribution of N₂ fixation (N _f ) to leaf nitrogen (N) concentration of Mimosaceae averaged about 30%, with large variation between and within species. While in Acacia albida N _f was only 2%, it was 49% in Acacia hereroensis and Dichrostachys cinerea, and reached 71% in Acacia melifera. In the majority of species N _f was 10–30%. There was a marked variation in background ¹⁵N values along the aridity gradient, with the highest ¹⁵N values in the lowland savanna. The difference between ¹⁵N values of Mimosaceae and non-Mimosaceae, which is assumed to result mainly from N₂ fixation, was also largest in the lowland savanna. Variations in ¹⁵N of Mimosaceae did not affect N concentrations, but higher ¹⁵N-values of Mimosaeae are associated with lower carbon isotope ratios (¹³C value). N₂ fixation was associated with reduced intrinsic water use efficiency. The opposite trends were found in non-Mimosaceae, in which N-concentration increased with ¹⁵N, but ¹³C was unaffected. The large variation among species and sites is discussed.This paper is prepared in memory of J. Visser, who took part in the collection of species, but died in 1990     

Crassulacean acid metabolism in Kalanchoë species collected in various climatic zones of Madagascar: a survey by δ13C analysis

Summary The carbon isotope compositions of samples of Kalanchoë species collected at the natural stands in Madagascar were determined. The results suggest that all species of the genus Kalanchoë are capable of crassulacean acid metabolism. The observed ¹³C values cover the whole range from –10 to –30. This high diversity of the ¹³C values was found among the species of the genus as well as, in certain cases, within a single species. This suggest that the CAM patterns in Kalanchoë are generally very flexible. The ¹³C values show a clear correlation with the climate of the habitats from where the samples derived. Values indicative of CO₂ fixation taking place exclusively during the night were found in the dry regions of Madagascar, whereas ¹³C values indicative of mixed CO₂ fixation during night and day or of CO₂ fixation entirely during the day are distributed in the humid zones.     

Variable carbon isotope ratios of Dudleya species growing in natural environments

Summary Measurements are reported of carbon isotope ratios of Dudleya species growing in natural plant communities. Considerable variation in the ¹³C values are interpreted as indicating substantial flexibility of the photosynthetic pathways between C₃ and CAM. The variability in photosynthetic pathway was in response to genetic factors, stage of plant development, life-form, and environmental conditions. Species active during drought periods have less carbon isotope fractionation than species that are summer-dormant. Summer-active species from drier habitats have less negative ¹³C values than those from more mesic sites. On the same plant, leaf tissue had more negative ¹³C values than tissue from the inflorescence. The less negative carbon isotope ratios are indicative of an increased proportion of exogenous CO₂ fixed in dark vs light. The ecological significance of these results is discussed.CIW-DPB Publication No. 522     

Comparisons of δ13C values in leaves of aquatic macrophytes from different habitats in Britain and Finland; some implications for photosynthetic processes in aquatic plants

Summary The ¹³C values of submerged aquatic plants from contrasting but relatively defined habitats, and the ¹³C values of emergent, floating and submerged leaves of dimorphic aquatic plants, were measured. In many instances the ¹³C values of dissolved inorganic carbon in the water were also measured. Plant ¹³C values in the vicinity of-40 to-50 were found in rapidly flowing spring waters with carbonate ¹³C values of-16 to-21, consistent with the notion that species such as Fontinalis antipyretica almost exclusively assimilate free CO₂ via RuP₂ carboxylase. Plant ¹³C values in the vicinity of-10 to-15 in sluggish water with carbonate ¹³C values of about-5 were observed, consistent with the notion that boundary layer diffusion and/or HCO₃ ^- uptake may determine the ¹³C value of submerged aquatic plants in these circumstances. Comparisons of ¹³C values of the same or related species growing in waters of similar carbonate ¹³C value but different flow rates confirmed this view; more negative ¹³C values were frequently associated with plants in fast moving water. In Britain, but not in Finland, the ¹³C values of submerged leaves of dimorphic plants were almost invariably more negative than in aerial leaves. The ¹³C value of carbonate from chalk streams and in acid springs indicate substantial inputs of respiratory CO₂, as opposed to atmospheric carbon. The contributions of these variations in ¹³C of the carbon source, and of isotope fractionation in diffusion, to the ¹³C value of submerged parts of dimorphic plants is discussed.     

Carbon-isotope discrimination by leaves of Flaveria species exhibiting different amounts of C3-and C4-cycle co-function

Carbon-isotope ratios were examined as ¹³C values in several C₃, C₄, and C₃–C₄ Flaveria species, and compared to predicted ¹³C, values generated from theoretical models. The measured ¹³C values were within 4 of those predicted from the models. The models were used to identify factors that contribute to C₃-like ¹³C values in C₃–C₄ species that exhibit considerable C₄-cycle activity. Two of the factors contributing to C₃-like ¹³C values are high CO₂ leakiness from the C₄ pathway and pi/pa values that were higher than C₄ congeners. A marked break occurred in the relationship between the percentage of atmospheric CO₂ assimilated through the C₄ cycle and the ¹³C value. Below 50% C₄-cycle assimialtion there was no significant relationship between the variables, but above 50% the ¹³C values became less negative. These results demonstrate that the level of C₄-cycle expression can increase from, 0 to 50% with little integration of carbon transfer from the C₄ to the C₃ cycle. As expression increaces above 50%, however, increased integration of C₃- and C₄-cycle co-function occurs.Abbreviations and symbols RuBP carboxylase ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) - PEP carboxylase phosphoenolpyruvate carboxylase (EC 4.1.1.31) - pa atmospheric CO₂ partial pressure - pi intercellular CO₂ partial pressure - isotope ratio - quantum yield for CO₂ uptake     

Ecophysiological studies on orchids of Madagascar: incidence and plasticity of crassulacean acid metabolism in species of the genus Angraecum Bory

The present study is an investigation on photosynthetic options in an orchid taxon and deals with the mainly epiphytes comprising genus Angraecum Bory. The incidence of crassulacean acid metabolism (CAM) in Angraecum species collected at various habitats in Madagascar was surveyed by analysis of stable carbon isotope composition (¹³C values). The values showed both inter- and intraspecific variability and suggest that in situ about 50% of the analysed species perform C3 photosynthesis, 20% moderate CAM (fixation of external CO₂ during day-and night-time) and 30% pronounced CAM (CO₂ uptake entirely during the night). The photosynthetic behaviour of the species indicated by the ¹³C values was clearly related to the habitat from where the samples derived. In A. eburneum, A. sororium and particularly in A. sesquipedale the stable carbon isotope analysis was complemented by measurements of CAM performance under controlled conditions. The experiments with A. sesquipedale revealed that drought and temperature are important factors modulating CAM, whereas variation of the leaf-to-air water vapor pressure difference was less effective. Altogether, the results of the study support the view that the high biological adaptability and thus the ecological success of the genus Angraecum is largely based on genotypic diversity and intraspecific plasticity of the photosynthetic behaviour.     

Long-term steady-state labelling of wheat plants by use of natural 13CO2/12CO2 mixtures in an open,rapidly turned-over system

A photosynthate labelling method is presented which takes advantage of the natural difference in carbon-isotope composition () which exists between atmospheric CO₂ (-8) and commercially available compressed CO₂. Carbon dioxide with -4.0 and –27.9%., respectively, has been used for labelling. A plant growth cabinet served as the labelling compartment. CO₂-free air was continuously injected at a rate of up to 54m³·h^–1. Dilution of cabinet CO₂ by CO₂-free air was counterbalanced by addition of CO₂ with known constant . Since the labelling-cabinet atmosphere was continuously exchanged at a high rate, photosynthetic carbon-isotope discrimination was fully expressed. In order to study the distribution of carbon acquired by the plant during a defined growth period, the of CO₂ was modified by replacing, for example, atmospheric CO₂ by CO₂ with –27.9%. and the weight and 5 of plant carbon pools was monitored over time. In such an experiment the change of CO₂ was followed by a rapid change of the of sucrose in mature flag-leaf blades of wheat (Triticum aestivum L.). The 5 of sucrose stabilized near –51%., indicating complete exchange by current photosynthate. In contrast 83% of the total carbon in mature flag-leaf blades was not exchanged after 14 d continuous labelling. Differential labelling of pre- and post-anthesis photosynthate indicated that 13% of grain carbon originated from pre-anthesis photosynthesis. Carbon-isotope discrimination and its consideration in experimentation and labelling data evaluation are discussed in detail. Since the air supplied to the labelling cabinet is dry and free of CO₂, carbon-isotope discrimination and carbon turnover and partitioning can be studied over a wide range of CO₂ concentrations (0–2600 cm³ · m^–3) and vapor-pressure deficits.Abbreviation and Symbol PPFD photosynthetic photon flux density - carbon-isotope composition Dr. G. Schleser (Forschungszentrum Jülich, FRG) and Professor S. Hoernes (Mineralogisch-Petrologisches Institut, Universität Bonn) for valuable help and advice during the initial stages of the project and Professor W. Kühbauch (Institut für Pflanzenbau, Universität Bonn) for continuing support. Technical assistance of Ute Labusch, Petra Biermann, Ludwig Schmitz and Thomas Gebbing is gratefully acknowleged.

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15N natural abundance studies in Australian commercial sugarcane

The measurement of natural ¹⁵N abundance is a well-established technique for the identification and quantification of biological N₂ fixation in plants. Associative N₂ fixing bacteria have been isolated from sugarcane and reported to contribute potentially significant amounts of N to plant growth and development. It has not been established whether Australian commercial sugarcane receives significant input from biological N₂ fixation, even though high populations of N₂ fixing bacteria have been isolated from Australian commercial sugarcane fields and plants. In this study, ¹⁵N measurements were used as a primary measure to identify whether Australian commercial sugarcane was obtaining significant inputs of N via biological N₂ fixation. Quantification of N input, via biological N₂ fixation, was not possible since suitable non-N₂ fixing reference plants were not present in commercial cane fields. The survey of Australian commercially grown sugarcane crops showed the majority had positive leaf ¹⁵N values (73% >3.00, 63% of which were >5.00), which was not indicative of biological N₂ fixation being the major source of N for these crops. However, a small number of sites had low or negative leaf ¹⁵N values. These crops had received high N fertiliser applications in the weeks prior to sampling. Two possible pathways that could result in low ¹⁵N values for sugarcane leaves (other than N₂ fixation) are proposed; high external N concentrations and foliar uptake of volatilised NH₃. The leaf ¹⁵N value of sugarcane grown in aerated solution culture was shown to decrease by approximately 5 with increasing external N concentration (0.5–8.0 mM), with both NO₃ ^– and NH₄ ⁺ nitrogen forms. Foliar uptake of atmospheric NH₃ has been shown to result in depleted leaf ¹⁵N values in many plant species. Acid traps collected atmospheric N with negative ¹⁵N value (–24.45±0.90) from above a field recently surface fertilised with urea. The ¹⁵N of leaves of sugarcane plants either growing directly in the soil or isolated from soil in pots dropped by 3.00 in the same field after the fertiliser application. Both the high concentration of external N in the root zone (following the application of N-fertilisers) and/or subsequent foliar uptake of volatilised NH₃ could have caused the depleted leaf ¹⁵N values measured in the sugarcane crops at these sites.     

Natural abundance of 15N in tropical plants with emphasis on tree legumes

Natural abundance of ¹⁵N ( ¹⁵N) of leaves harvested from tropical plants in Brazil and Thailand was analyzed. The ¹⁵N values of non-N₂-fixing trees in Brazil were +4.5±1.9, which is lower than those of soil nitrogen (+8.0±2.2). In contrast, mimosa and kudzu had very low ¹⁵N values (–1.4+0.5). The ¹⁵N values of Panicum maximum and leguminous trees, except Leucaena leucocephala, were similar to those of non-N₂-fixing trees, suggesting that the contribution of fixed N in these plants is negligible. The ¹⁵N values of non-N₂-fixing trees in Thailand were +4.9±2.0. Leucaena leucocephala, Sesbania grandiflora, Casuarina spp. and Cycas spp. had low ¹⁵N values, close to the value of atmospheric N₂ (0), pointing to a major contribution of N₂ fixation in these plants. Cassia spp. and Tamarindus indica had high ¹⁵N values, which confirms that these species are non-nodulating legumes. The ¹⁵N values of Acacia spp. and Gliricidia sepium and other potentially nodulating tree legumes were, on average, slightly lower than those of non-N₂-fixing trees, indicating a small contribution of N₂ fixation in these legumes.     

Autotrophic carbon sources for heterotrophic bacterioplankton in a floodplain lake of central Amazon

The relative contribution of autotrophic carbon sources (aquatic macrophytes, flooded forest, phytoplankton) for heterotrophic bacterioplankton was evaluated in a floodplain lake of the Central Amazon. Stable carbon isotopes (¹³C) were used as tracers. Values of ¹³C of different autotrophic sources were compared to those of dissolved organic carbon (DOC) and those of bacterially produced CO₂.The percentage of carbon derived from C₄ macrophytes for bacterially produced CO₂ was the highest, on average 89%. The average ¹³C value of CO₂ from bacterial respiration was –18.5 ± 3.3. Considering a fractionation of CO₂ of 3 by bacterial respiration, ¹³C value was –15.5, near C₄ macrophyte ¹³C value (–13.1).The average value of total DOC ¹³C was –26.8 ± 2.4. The percentage of C₄ macrophytes carbon for total DOC was on average 17%. Considering that bacteria consume mainly carbon from macrophytes, the dominance of C₃ plants for total DOC probably reflects a faster consumption of the former source, rather than a major contribution of the latter source.Heterotrophic bacterioplankton in the floodplain may be an important link in the aquatic food web, transferring the carbon from C₄ macrophytes to the consumers.     

Interpretations of gradients in δ13C value in thick photosynthetic tissues of plants with Crassulacean acid metabolism

In Ceropegia dichotoma, Crassula argentea, Esheveria colorata, Kalanchoë beharensis, Opuntia ficus-indica, Sansveria stuckyi and Opuntia inermis the carbon-isotope ratio ( ¹³C) of tissues close to the epidermis is 2–4.3 more negative than those in the centre of the leaf or cladode. The greatest change in ¹³C value occurs between the epidermal layer and the layer of mesophyll tissue immediately underneath. Analysis of major metabolic and structural components in successive layers of Crassula argentea grown under controlled environmental conditions conducive to Crassulacean acid metabolism confirmed that much of the variation in ¹³C values of bulk carbon is caused by differences in chemical composition. Thus the steep gradient in ¹³C value at the epidermis reflects, in part, the contribution of more-negative ¹³C values of lipids in these tissues. Moreover, during nocturnal CO₂ fixation the amount of malic acid synthesised decreases with depth and the ¹³C value of the methanol-soluble fraction is less negative with distance away from the upper epidermis. These results are consistent with diffusion limitation to CO₂ uptake in these thick leaf tissues, which also contributes to the observed gradients in ¹³C value.Abbreviation CAM Crassulacean acid metabolism - FW fresh weight - PEPCase phospoenolpyruvate carboxylase - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - ¹³C carbon-isotope ratio This work was supported by grants from the North Carolina Biotechnology Centre to Duke University, National Science Foundation (NSF) grant DCB90-06830, Department of Energy, Division of Energy Biosciences grant DE-FG05-89 ER 14005, and NSF grant BSR 87-06429 to Duke University Phytotron.     

Environmental control of crassulacean acid metabolism in Welwitschia mirabilis Hook. Fil. in its range of natural distribution in the Namib desert

Summary Within the area of its natural distribution in South West Africa, Welwitschia mirabilis has a less negative ¹³C value than C₃ plants and a more negative ¹³C value than C₄ species. This indicates that Welwitschia m. assimilates CO₂ partially via CAM when growing in its natural habitat. The difference between the ¹³C values of Welwitschia m. and of the C₃ species is significant in the savanna, whereas it is only small and statistically not significant in the grassland zone. The proportion of CO₂ fixed via CAM is largest in the coastal desert zone. There was no correlation between the ¹³C values and the Cl^- or ash content of the tissue. Thus, CAM in Welwitschia m. seems not to be induced by salt stress. There is no change in the ¹³C values along the persistent Welwitschia m. leaf. The present data indicate that on a broad geographical scale in the area of distribution temperature regime, and water stress as a modifying factor, determine CAM in Welwitschia m. The ecological implications are discussed by comparing the behaviour of Welwitschia m. with other CAM, C₃ and C₄ species of the accompanying flora.     

Foliar δ13C within a temperate deciduous forest: spatial,temporal, and species sources of variation

Summary Foliar ¹³C-abundance (¹³C) was analyzed in the dominant trees of a temperate deciduous forest in east Tennessee (Walker Branch Watershed) to investigate the variation in foliar ¹³C as a function of time (within-year and between years), space (canopy height, watershed topography and habitat) and species (deciduous and coniferous taxa). Various hypotheses were tested by analyzing (i) samples collected from the field during the growing season and (ii) foliar tissues maintained in an archived collection. The ¹³C-value for leaves from the tops of trees was 2 to 3%. more positive than for leaves sampled at lower heights in the canopy. Quercus prinus leaves sampled just prior to autumn leaf fall had significantly more negative ¹³C-values than those sampled during midsummer. On the more xeric ridges, needles of Pinus spp. had more positive ¹³C-values than leaves from deciduous species. Foliar ¹³C-values differed significantly as a function of topography. Deciduous leaves from xeric sites (ridges and slopes) had more positive ¹³C-values than those from mesic (riparian and cove) environments. On the more xeric sites, foliar ¹³C was significantly more positive in 1988 (a dry year) relative to that in 1989 (a year with above-normal precipitation). In contrast, leaf ¹³C in trees from mesic valley bottoms did not differ significantly among years with disparate precipitation. Patterns in foliar ¹³C indicated a higher ratio of net CO₂ assimilation to transpiration (A/E) for trees in more xeric versus mesic habitats, and for trees in xeric habitats during years of drought versus years of normal precipitation. However, A/E (units of mmol CO₂ fixed/mol H₂O transpired) calculated on the basis of ¹³C-values for leaves from the more xeric sites was higher in a wet year (6.6±1.2) versus a dry year (3.4±0.4). This difference was attributed to higher transpiration (and therefore lower A/E) in the year with lower relative humidity and higher average daily temperature. The calculated A/E values for the forest in 1988–89, based on ¹³C, were within ±55% of estimates made over a 17 day period at this site in 1984 using micrometeorological methods.     

Inorganic carbon acquisition by red seaweeds grown under dynamic light regimes

Palmaria palmata, which is able to use HCO _inf3 ^sup– as a carbon source for photosynthesis, and Lomentaria articulata, which is dependent on diffusive uptake of dissolved CO₂, were grown under constant light and light with sunflecks designed to model wave-induced fluctuations of near-shore underwater light. Both species exhibited significantly increased stable carbon isotope discrimination (more negative values of ¹³C relative to PDB) when grown with sunflecks. More negative ¹³C values were associated with decreased growth rate of P. palmata but not of L. articulata. The contrasting effects of sunflecks on the carbon-use characteristics of the two species are discussed in terms of the energetic cost of HCO _inf3 ^sup– use and the susceptibility of CO₂ diffusion-dependent species to photoinhibition.     

Feeding level and individual metabolic rate affect δ13C and δ15N values in carp: implications for food web studies

Stable isotope analyses are often used to calculate relative contributions of multiple food sources in an animals diet. One prerequisite for a precise calculation is the determination of the diet-tissue fractionation factor. Isotopic ratios in animals are not only affected by the composition of the diet, but also by the amount of food consumed. Previous findings regarding the latter point are controversial. As stable isotope analyses have often been used to investigate aquatic food webs, an experiment with carp (Cyprinus carpio L.) was carried out to test the influence of the feeding level and individual metabolic rate on ¹³C and ¹⁵N values of the whole body. After an initial phase, 49 carp were assigned randomly to four groups and fed the same diet at different levels for 8 weeks. For 15 fish, the energy budget was determined by indirect calorimetry. Feed and individual fish were analysed for their proximate composition, gross energy content and ¹³C and ¹⁵N values. ¹³C and ¹⁵N values differed significantly at different feeding levels. While ¹³C values of the lipids and ¹⁵N values decreased with increasing feeding rate, ¹³C values of the lipid-free matter showed a non-linear pattern. Data obtained from fish held in the respirometric system revealed a relationship between ¹³C values and the percentage retention of metabolizable energy. Our results show that reconstructing the diets of fish from the isotopic ratios when the feeding level and individual metabolic rates are unknown would introduce an error into the data used for back-calculation of up to 1 for both ¹³C and ¹⁵N values and may have substantial effects on the results of calculated diets. As other workers have pointed out, the development and application of stable isotopes to nutritional ecology studies is a field in its infancy and gives rise to erroneous, misleading results without nutritional, physiological and ecological knowledge.     

Crassulacean acid metabolism in the shade. Studies on an epiphytic fern,Pyrrosia longifolia,and other rainforest species from Australia

Summary Crassulacean acid metabolism (CAM) was studied in a tropical epiphytic fern, Pyrrosia longifolia, from a fully sun-exposed and from a very shaded site in Northern Queensland, Australia. Measurements of instantaneous net CO₂ exchange showed carbon gain via CO₂ dark fixation with some net CO₂ uptake also occuring during late afternoon, in both sun and shade fronds. Maximum rates of net CO₂ uptake and the nocturnal increase in titratable acidity were lower in shade than in sun fronds. ¹³C values of sun and shade fronds were not significantly different, and ranged between-14 and-15 suggesting that, in the long term, carbon gain was mainly via CO₂ dark fixation. Sun fronds had a higher light compensation point of photosynthesis than shade fronds but the same quantum yield. Yet there was no acclimation of photosynthetic O₂ evolution, (measured at 5% CO₂) in sun and shade fronds and photosynthesis saturated at between 200 and 400 mol quanta m^-2 s^-1. Use of higher light intensities for photosynthesis of sun fronds was probably precluded by low nutrient availability. Total nitrogen was less than 1% of dry weight in fully expanded sun and shade fronds. Exposure of shade fronds to full sunlight for 6 h led to a 60% decline in the quantum yield of photosynthesis and to a decline in variable fluorescence measured at room temperature. Photoinhibition by high light was also observed in Hoya nicholsoniae, a rainforest climber growing in deep shade. This species also exhibited CAM as demonstrated by nocturnal net CO₂ uptake, nocturnal acidification and a ¹³C value of-14. Photosynthetic O₂ evolution in this species was saturated at 2.5% of full sunlight. Two species of Dendrobium (Orchidaceae) from sun-exposed sites, one species exhibiting CAM and the other one exhibiting net CO₂ uptake exclusively during daytime via conventional C₃ photosynthesis, showed similar light response curves and the same quantum yield for photosynthetic O₂ evolution.     

Using stable hydrogen and oxygen isotope measurements of feathers to infer geographical origins of migrating European birds

Successful application of stable-hydrogen isotope measurements (D_f) of feathers to track origins of migratory birds and other wildlife requires a fundamental understanding of the correlation between D_f and deuterium patterns in rainfall (D_p) over continental scales. A strong correlation between D_p and D_f has been confirmed for birds and insects in North America, but not yet for other continents. Here, we compare D_f data from resident European birds to new D_p basemaps for Europe. Three maps, representing growing-season and mean annual D_p estimates from an elevation-explicit, detrended interpolation model and growing-season D_p estimates from simple Kriging, all indicate that strong isotope gradients occur across Europe with a general depletion occurring in a northeast direction. The feather data, representing 141 individuals of 25 avian species from 38 sites, ranged from –131 to –38. Regression analysis showed that strong correlations existed between both mean annual and growing-season D_p estimated by detrended interpolation and D_f of non-aquatic and non-corvid birds (r²=0.66 and 0.65, respectively). We also examined mean annual and growing-season ¹⁸O_p vs. ¹⁸O_f for our samples. Both oxygen regressions were similar (r²=0.56 and 0.57, respectively) but poorer than for deuterium. Our study reveals that D measurements of feathers from migratory birds in Europe may be used to track their origin and movements, and so provide a powerful investigative tool for avian migration research in Europe.     

Uptake of organic matter by the roots of sweet potato: Analysis of the δ14C value of plants

Summary To determine whether sweet potato (Ipomoea batatas (L.) Poir.) takes up organic matter through the roots from the medium, the concentrations of natural¹⁴C (¹⁴C) in plant organic matter, atmospheric CO₂ and compost applied to media were examined under soil and sand culture conditions. In these experiments, three kinds of composts of different ¹⁴C were used. CO₂ derived from the mineralization of compost was continuously pumped out from the pots and its direct uptake by the leaves was prevented.¹⁴C of plant parts harvested after the 43 days experimental period were affected by the ¹⁴C of the compost in the treatments where the compost of rice straw was applied, and which suggested that a significant amount of plant carbon was derived from the compost.     

Carbon assimilation patterns and growth of the introduced CAM plant Opuntia inermis in Eastern Australia

Summary The daily course of CO₂ and H₂O exchange in cladodes of Opuntia inermis was studied at four sites in Eastern Australia. On most occasions cladode water contents were high and nocturnal stomatal opening resulted in substantial uptake of CO₂ and synthesis of about 130 equiv cm^-2 of malic acid during the night. Under water stress nocturnal stomatal opening was confined to the latter part of the night and acid synthesis was reduced to about 40 equiv cm^-2. Night temperature had little effect on acid synthesis, which responded primarily to rainfall and changed from the stressed condition within 2–3 days in irrigation experiments. On many occasions following summer rainfall stomata opened for 4 h in the late afternoon permitting net CO₂ fixation which may contribute about 25% of the total carbon assimilated. This CO₂ fixation was insufficient to have a marked impact on the ₁₃C value of the Opuntia cladodes. CO₂ fixation in the light occurred in conjunction with maximum dark CO₂ fixation under mesic conditions. Dark CO₂ fixation rates were 3 to 5 times greater than those recorded in desert cacti under favorable conditions. Relative growth rates calculated on the basic of CO₂ exchange correspond to measured relative growth rates of 0.05 g g^-1 dry wt day^-1 which prevailed for 60–90 days in summer. The capacity for very active CO₂ fixation in the dark and light following summer rainfall and the capacity to persist at low levels of metabolic activity through summer drought are discussed in relation to the success of this introduced species in this habitat.     

Stable carbon and oxygen isotopic compositions of recent charophyte oosporangia and water from Malham Tarn,U.K.: palaeontological implications

Charophyte oosporangia and water samples from a highly calcareous lake were measured for stable carbon and oxygen isotopic composition. The time period over which the oosporangia calcify is short, thus any biochemical relationship between the water and oosporangia"s calcite represents only one time window (late Summer in Malham Tam). This important temporal restraint must also apply to interpretations of all fossil material measured. The ¹⁸O_c of the charophyte oosporangia is deduced to be in equilibrium with the ¹⁸O of the water for a given temperature. The ¹³ C_c of the charophyte oosporangia was approximately 2.5 per mil lower than the ¹³C_DIC in the water we measured. With the release Of CO₂ with phosphoric acid from the charophyte oosporangia, there was no significant difference in the ¹⁸O_c values obtained, regardless of whether or not the carbonate was separated from the organic center, however ¹³C_c values were marginally lower for carbonate plus organic center measurements. Our results indicate that fossil charophyte gyrogonites can be used to elucidate the geochemistry of the ancient water body in which they lived.