Carbon-Isotope Ratios and Photosynthetic Pathways in the Neotropical Family Rapateaceae

Abstract: The Rapateaceae is a small, mainly Neotropical family of terrestrial or occasionally epiphytic herbs that grow on mesic, nutrient-poor sites. Some recent studies suggest that the Rapateaceae may be closely related to the Bromeliaceae, one of the major families containing CAM plants. To investigate the photosynthetic pathway in Rapateaceae, the plant carbon-isotope ratio (δ¹³C) was determined for samples from dried herbarium specimens for 85 of the approximately 100 species in the family. The δ¹³C values ranged from - 37.7 to - 19.8 ‰. Most Rapateaceae showed δ¹³C values typical of C₃ plants. However, six species ( Kunhardtia rhodantha Maguire, Marahuacaea schomburgkii (Maguire) Maguire, Saxofridericia compressa Maguire, Stegolepis grandis Maguire, St. guianensis Klotzsch ex Körn. and St. squarrosa Maguire) showed δ¹³C values less negative than - 23 ‰, i.e., at the higher end of the range for C₃ plants and at the lower end of the distribution for plants exhibiting CAM. The δ¹³C values became significantly less negative with increasing altitude (regression analysis indicating a change from about - 30.7 ‰ at sea level to - 22.5 ‰ at 2500 m). Although other environmental factors and the type of tissue analysed may also influence δ¹³C values, these results suggest that some Rapateaceae may be capable of performing CAM. Further studies, including measurements of diel gas exchange patterns and leaf organic-acid fluctuations, would be needed to demonstrate CAM in Rapateaceae unequivocally, but living material of many of these enigmatic plants is difficult to obtain.     

Diurnal and seasonal variation in the carbon isotope composition of leaf dark-respired CO2 in velvet mesquite (Prosopis velutina)

We evaluated diurnal and seasonal patterns of carbon isotope composition of leaf dark-respired CO₂ ( δ ¹³C_l) in the C₃ perennial shrub velvet mesquite ( Prosopis velutina ) across flood plain and upland savanna ecosystems in the south-western USA. δ ¹³C_l of darkened leaves increased to maximum values late during daytime periods and declined gradually over night-time periods to minimum values at pre-dawn. The magnitude of the diurnal shift in δ ¹³C_l was strongly influenced by seasonal and habitat-related differences in soil water availability and leaf surface vapour pressure deficit. δ ¹³C_l and the cumulative flux-weighted δ ¹³C value of photosynthates were positively correlated, suggesting that progressive ¹³C enrichment of the CO₂ evolved by darkened leaves during the daytime mainly resulted from short-term changes in photosynthetic ¹³C discrimination and associated shifts in the δ ¹³C signature of primary respiratory substrates. The ¹³C enrichment of dark-respired CO₂ relative to photosynthates across habitats and seasons was 4 to 6‰ at the end of the daytime period (1800 h), but progressively declined to 0‰ by pre-dawn (0300 h). The origin of night-time and daytime variations in δ ¹³C_l is discussed in terms of the carbon source(s) feeding respiration and the drought-induced changes in carbon metabolism.     

Elevated pyruvate,orthophosphate dikinase (PPDK) activity alters carbon metabolism in C3 transgenic potatoes with a C4 maize PPDK gene

Changes in carbon metabolism and δ¹³C value of transgenic potato plants with a maize pyruvate,orthophosphate dikinase (PPDK; EC 2.7.9.1) gene are reported. PPDK catalyzes the formation of phospho enol pyruvate (PEP), the initial acceptor of CO₂ in the C₄ photosynthetic pathway. PPDK activities in the leases of transgenic potatoes were up to 5.4‐fold higher than those of control potato plants (wild‐type and treated control plants). In the transgenic potato plants, PPDK activity in leaves was negatively correlated with pyruvate content (r²= 0.81), and was positively correlated with malate content (r²= 0.88). A significant increase in the δ¹³C value was observed in the transgenic potato plants, suggesting a certain contribution of PEP carboxylase as the initial acceptor of atmospheric CO₂. These data suggest that elevated PPDK activity may alter carbon metabolism and lead to a partial operation of C₄‐type carbon metabolism. However, since parameters associated with CO₂ gas exchange were not affected, the altered carbon metabolism had only a small effect on the total photosynthetic characteristics of the transgenic plants.     

Carbon isotope ratios of plants and occurrences of C4 species under different soil moisture regimes in arid region of Northwest China

Carbon isotope ratio of leaf dry matter, δ ¹³C, was measured on species occurring within Baiyin desert community, consisting of valley, slope and ridge microhabitats, and within Shandan desert community, consisting of Gobi desert and seasonal flooded creek microhabitats, in Northwest China. δ ¹³C of C₃ species increased with a decrease in soil water availability, suggesting that water-use efficiency (WUE) increased with decreasing soil moisture, whereas for all C₄ species, δ ¹³C tended to decrease with decreasing soil water availability, suggesting that WUE also increased with decreasing soil moisture. Above results indicated that water-use pattern was conservative under drought for C₄ and C₃ plants. In this present study, C₄ species' occurrences within different microhabitats were investigated and C₄ plants were observed to be absent and/or scarce within relatively lower soil moisture microhabitats, whereas they occurred and/or even had a high abundance within relatively higher soil moisture microhabitats, suggesting limited moisture available was a key factor of limiting C₄ distribution in arid region of Northwest China.     

Leaf anatomy and photosynthetic characteristics of succulent alpine plants growing at high elevations

Sempervivum monlanum and Saxifraga particulate are succulent alpine plants, with flat leaves, growing at up to 3300 m; Sedum atraium , with cylindrical leaves, is only present at the highest elevations. We have made a preliminary study of the photosynthetic mechanism (C, or CAM) of these species in relation to morphological and ultrastructural differences. The following physiological parameters were determined in plants collected at 3000 m: δ¹³C, night/day variations of titratable acidity and kinetic properties (Vmax, Km, S_0.5, n) of phosphoenolpyruvate carboxylase (EC 4.1.1.31). The results suggest that, even at the highest elevations, the photosynthetic system can be obligate CAM ( Sempervivum monlanum ), facultative C₃-CAM ( Sedum atratum ) or obligate C, ( Saxifraga paniculata ). Other parameters such as chlorophyll a/b ratio, succulence and succulence of mesophyll were measured and the results were consistent with the above mechanisms. Morphological analyses were performed by using LM, SEM and TEM. Sempervivum monlanum (CAM) leaves were found to differ from those of Saxifraga paniculata (C,) in having fewer thyreoids per granum, more plastoglobules per chloroplast, a higher plastoglobule relative density and a lower stomatal density. In Sedum atratum (facultative C₃-CAM) corresponding values were intermediate.     

Leaf δ13C reflects ecosystem patterns and responses of alpine plants to the environments on the Tibetan Plateau

Leaf δ¹³C is an indicator of water-use efficiency and provides useful information on the carbon and water balance of plants over longer periods. Variation in leaf δ¹³C between or within species is determined by plant physiological characteristics and environmental factors. We hypothesized that variation in leaf δ¹³C values among dominant species reflected ecosystem patterns controlled by large-scale environmental gradients, and that within-species variation indicates plant adaptability to environmental conditions. To test these hypotheses, we collected leaves of dominant species from six ecosystems across a horizontal vegetation transect on the Tibetan Plateau, as well as leaves of Kobresia pygmaea (herbaceous) throughout its distribution and leaves of two coniferous tree species ( Picea crassifolia, Abies fabri ) along an elevation gradient throughout their distribution in the Qilian Mountains and Gongga Mountains, respectively. Leaf δ¹³C of dominant species in the six ecosystems differed significantly, with values for evergreen coniferous13C values of the dominant species and of K. pygmaea were negatively correlated with annual precipitation along a water gradient, but leaf δ¹³C of A. fabri was not significantly correlated with precipitation in habitats without water-stress. This confirms that variation of δ¹³C between or within species reflects plant responses to environmental conditions. Leaf δ¹³C of the dominant species also reflected water patterns on the Tibetan Plateau, providing evidence that precipitation plays a primary role in controlling ecosystem changes from southeast to northwest on the Tibetan Plateau.     

CAM-cycling in the cycad Dioon edule Lindl. in its natural tropical deciduous forest habitat in central Veracruz, Mexico

The cycad Dioon edule Lindl. inhabits a seasonally-dry tropical forest along with associated CAM plants such as bromeliads and cacti. To test the hypothesis that D . edule might also be a CAM plant, diel total-acid fluctuation was measured through the dry to wet seasons of 4 consecutive years on adult D . edule plants in their natural forest habitat in Veracruz, Mexico. Correlations between acid fluctuation index and climatic data, and also soil water potential were determined over this period. Laboratory trials were followed up to estimate diel patterns of CO₂ exchange and estimation of δ¹³C value. A comparison of stomatal density cm⁻² with other C₃, CAM and CAM-facultative plants was made. The diel total titratable-acid fluctuation values, although variable, were found to be consistent and significant for the dry season. Carbon dioxide exchange was found to be typical of C₃ plants when hydrated but when the plant was stressed by withholding water, although the leaf remained healthy, there was no significant dark-period CO₂ output. Stomatal density was comparable to other CAM and CAM-facultative plants. It was concluded that D. edule is a C₃ plant that shows CAM-cycling metabolism when water stressed. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 138 , 155–161.     

Stable isotope ecology of fossil hippopotamids from the Lake Turkana Basin of East Africa

The diet of African hippopotamids can be documented through δ ¹³C analyses of enamel and other tissues. Analysis of a 10-million-year sequence of hippopotamids in and near the Lake Turkana Basin of northern Kenya shows that hippos have included a substantial fraction of C₃ vegetation in their diets since the late Miocene when C₄ vegetation first appears in hippo diet as a measurable fraction. The C₄ component of vegetation becomes dominant (>50%) by Upper Burgi time ( c . 2 million years ago) but does not reach 100% for all individuals. It is therefore not unexpected that the δ ¹³C values of modern hippopotamids show a higher fraction of dietary C₃ biomass than has been estimated from traditional observations. Analysis of δ ¹⁸O of hippos from different stratigraphic levels shows no systematic trend over time; the average value for fossil hippos over the last 10 million years is similar to that of modern hippos from the Omo River system.     

δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

Correlations Between Foliar Stable Carbon Isotope Composition and Environmental Factors in Desert Plant Reaumuria soongorica （Pall.） Maxim.

Leaves of 407 individuals of Reaumuria soongorica （Pall.） Maxim. collected from the major distribution areas were measured to investigate the distribution characteristics of the stable carbon isotope in this desert plant, as well as correlations between δ^13C values and environmental factors. Results showed that δ^13C values in R. soongorica ranged from -22.77‰ to -29.85‰ and that the mean δ^13C value （-26.52‰） was higher than a previously reported δ^13C value for a different desert ecosystem. This indicates that R. soongorica belongs to the C3 photosynthetic pathway and has higher water use efficiency than other species. The correlations between δ^13C values and environmental factors demonstrated that the foliar δ^13C values in R. soongorica increased significantly with decreasing mean annual precipitation and mean relative humidity, and decreased with decreasing duration of sunshine and evaporation. The spatial distribution trend of δ^13C values in R. soongorica was not obvious and there was no significant correlation between the δ^13C values and mean annual temperature. We conclude that different distribution trends in δ^13C values for R. soongorica were likely caused by stomatal limitation rather than by nutrient-related changes in photosynthetic efficiency and that precipitation played an important role in the wide distribution range of R. soongorica. This pattern of δ^13C values for R. soongorica reinforced that it is a super-xerophil in terms of its adaptive strategies to a desert environment.     

Carbon dioxide fixation in orchid aerial roots

Acidity fluctuation, CO₂ gas exchange, δ¹³C value, PEP carboxylase and RuBP carboxylase activities in aerial roots of selected thick-leaved orchid hybrids ( Arachnis and Aranthera ) were studied. Both aerial roots and leaves showed acidity fluctuation over a 24 h period. Dark acidification in aerial roots was enhanced at low temperature (15°C). Aerial roots had δ¹³C values close to those of leaves which have been previously demonstrated to possess crassulacean acid metabolism. Variation in δ¹³C values along the length of the roots was observed; the root tip having a less negative δ¹³C value (—13.34%‰) than the older portions of the roots (—14.55%‰). There was no net CO₂ fixation by aerial root, although ¹⁴³²CO₂ fixation was observed in light and in darkness. The pattern of fluctuation in activities of PEP carboxylase and RuBP carboxylase in aerial roots was similar to that obtained for the leaves. In both aerial roots and leaves, PEP carboxylase activity was several times higher than that of RuBP carboxylase.     

Effects of water stress and nocturnal temperature on carbon allocation in the perennial grass, Leymus chinensis

Water deficit and high temperature often occur simultaneously, but their effects on plants are usually investigated separately. The aim of this study was to test how interactions between water stress and nocturnal warming affect carbon allocation in the perennial grass, Leymus chinensis . Plant biomass, dry mass allocation, ¹⁴C partitioning and carbon isotope composition (δ¹³C) were measured. Severe and extreme water stress during nocturnal warming decreased the allocation of dry mass and ¹⁴C partitioning below ground to the roots, but moderate water stress significantly increased the below-ground allocation of dry mass and ¹⁴C, especially at the lower night temperature. The δ¹³C values were more positive at day/night temperatures of 30/20°C than at 30/25°C, and greater in the roots than in the leaves. By plotting the δ¹³C values of the leaves against the δ¹³C values of the roots, the slopes of regressions were steeper at low than at high night temperature, also indicating that nocturnal warming reduces carbon allocation below ground to the roots. The results suggest that nocturnal warming may weaken acclimation during water stress in this species by regulating carbon allocation between source and sink organs.     

Photosynthesis in Salsola Species (Chenopodiaceae) from Southern Africa Relative to their C4 Syndrome Origin and their African-Asian Arid Zone Migration Pathways

Abstract: Over 60 Salsola species of Chenopodiaceae from South Africa were studied for their photosynthesis type, using δ¹³C analysis and light microscopy of leaf anatomy. These species cover about 70 % of the total list of Southern African Salsola species and grow naturally in South and Southwest African desert regions. All species are shrubby forms and belong to the single subsection Caroxylon. Only C₄ photosynthesis was found in the Salsola species determined with ¹³C/¹²C carbon isotope discrimination values that ranged from - 11.04 to - 14.03 % (PDB), plus the presence of a Kranz type assimilation tissue anatomy. The apparent absence of C₃ in Salsola in South and Southwest Africa and the known presence of C₃ and C₃ - C₄ intermediate photosynthesis in Caroxylon, Salsola species in Asia strongly indicate that the genus Salsola originated in Asia and later migrated to South Africa.     

Seasonal and annual changes in leaf δ13C in two co-occurring Mediterranean oaks: relations to leaf growth and drought progression

1. Changes of δ¹³C and its relation to leaf development, biochemical content and water stress were monitored over a 2 year period in two co-occurring Mediterranean oak species: the deciduous Quercus pubescens and the evergreen Quercus ilex .
2. The time course of leaf δ¹³C showed different patterns in the two species. Young Q. pubescens leaves had a high δ¹³C and a marked decrease occurred during leaf growth. In contrast, leaves at budburst and maturity did not differ significantly in the case of Q. ilex . We suggest that the difference between δ¹³C of young leaves was linked to differential use of reserves of carbon compounds in the two species.
3. δ¹³C values of mature leaves were negatively correlated with minimum seasonal values of predawn water potential, suggesting that a functional adjustment to water resources occurred.
4. There was a significant correlation between individual δ¹³C values for two successive years. This interannual dependence showed that δ¹³C rankings between trees were constant through time.     

A survey of photosynthetic carbon metabolism in 4 ecotypes of Phragmites australis in northwest China: Leaf anatomy, ultrastructure, and activities of ribulose 1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase and glycollate oxidase

Four ecotypes of Phragmites australis from different habitats in northwest China were examined to compare their photosynthetic characteristics. In a swamp ecotype, the Δ ¹³C value of leaf materials was −34.0‰, and bundle sheath cells contained a small amount of organelles and round-shaped chloroplasts, as being similar to typical C₃ plants. In a dune ecotype, the Δ ¹³C value was −20.9‰ and bundle sheath cells contained oval-shaped chloroplasts with poorly-developed grana. In light and heavy salt meadow ecotypes, Δ ¹³C values were −30.6‰ and −35.6‰, respectively. The shape of bundle sheath chloroplasts in the light salt meadow ecotype was intermediate between those of the swamp and dune ecotypes. Abundance of bundle sheath organelles in the heavy salt meadow ecotype was intermediate. The swamp ecotype had photosynthetic enzyme activities typical of C₃ type plants, whereas the dune ecotype had an increased activity of phosphoenolpyruvate carboxylase (PEPC), a key C₄ enzyme, and a decreased ribulose 1,5-bisphosphate carboxylase (Rubisco) activity. The light salt meadow and heavy salt meadow ecotypes had substantial activities of PEPC, which indicates potential for C₄ photosynthesis. These data suggest that this species evolved the C₃-like ecotype in swamp environments and the C₄-like C₃-C₄ intermediate in dune desert environments, and C₃-like C₃-C₄ intermediates in salt environments.     

A Remarkable New Leaf Type With Unusual Photosynthetic Tissue in a Central Asiatic Genus of Chenopodiaceae

Abstract: From the hygrohalophyte Borszczowia aralocaspica Bunge (Chenopodiaceae), a new leaf type with 1-layered chlorenchyma is described as "borszczovoid" and compared with other leaf types in subfamily Salsoloideae. The chlorenchyma is suspected to represent a unique C₄ type. Evidence is cited from anatomical studies and documented by micrographs and Carbon isotope determinations (ä¹³C values). The 1-layered photosynthetic tissue combines all essential anatomical characters of a 2-layered chlorenchyma of regular C₄ plants and is in intimate contact with concentrically arranged peripheral bundles. The ä¹³C values are − 13.03 ‰ from young stems and − 13.78 ‰ from leaves. The results are discussed in the anatomical, physiological and taxonomic framework. In addition, from distantly-related Suaeda species of section Conosperma the conospermoid leaf type is re-described. It is characterized by typical palisade and Kranz layers and differs from the C₄ suaedoid type by an external water-storaging hypodermis and an arrangement of Kranz cells reminiscent of the atriplicoid type from subfamily Chenopodioideae. From eight other species of Chenopodiaceae ä¹³C values are given for the first time.     

Relationships of grain δ13C and δ18O with wheat phenology and yield under water-limited conditions

Stable carbon isotope composition (δ¹³C) of dry matter has been widely investigated as a selection tool in cereal breeding programmes. However, reports on the possibilities of using stable oxygen isotope composition (δ¹⁸O) as a yield predictor are very scarce and only in the absence of water stress. Indeed, it remains to be tested whether changes in phenology and stomatal conductance in response to water stress overrule the use of either δ¹³C or δ¹⁸O when water is limited. To answer this question, a set of 24 genotypes of bread wheat ( Triticum aestivum ) were assayed in two trials with different levels of deficit irrigation and a third trial under rainfed conditions in a Mediterranean climate (northwest Syria). Grain yield (GY) and phenology (duration from planting to anthesis and from anthesis to maturity) were recorded, and the δ¹³C and δ¹⁸O of grains were analysed to assess their suitability as GY predictors. Both δ¹³C and δ¹⁸O showed higher broad-sense heritabilities ( H ²) than GY. Genotype means of GY across trials were negatively correlated with δ¹³C, as previously reported, but not with δ¹⁸O. Both isotopes were correlated with grain filling duration, whereas δ¹⁸O was also strongly affected by crop duration from planting to anthesis. We concluded that δ¹⁸O of grains is not a proper physiological trait to breed for suboptimal water conditions, as its variability is almost entirely determined by crop phenology. In contrast, δ¹³C of grains, despite being also affected by phenology, still provides complementary information associated with GY.     

Seasonal Variation of δ^13C of Four Tree Species： A Biological Integrator of Environmental Variables

Foliar δ^13C values, an indicator of long-term intercellular carbon dioxide concentration and, thus, of long-term water use efficiency （WUE） in plants, were measured for Pinus massoniana Lamb., P. elliottii Engelm., Cunninghamia laceolata （Lamb.） Hook., and Schima superba Gardn. et Champ. in a restored forest ecosystem in the Jiazhu River Basin. Seasonal variation and the relationship between the foliar δ^13C values of the four species and environmental factors （monthly total precipitation, monthly average air temperature, relative humidity, atmospheric pressure, and monthly total solar radiation and evaporation） were investigated. The monthly δ^13C values and WUE of the four species increased with increasing precipitation, air temperature, solar radiation, and evaporation, whereas δ^13C values of the four species decreased with increasing relative humidity and atmospheric pressure. Despite significant differences in δ^13C seasonal means for the four species, our results demonstrate a significant convergence in the responses of δ^13C values and WUE to seasonal variations in environmental factors among the species investigated and that the δ^13C signature for each species gives a strong indication of environmental variables.     

Stable isotope evidence (δ13C, δ18O) for winter feeding on seaweed by Neolithic sheep of Scotland

The antiquity of the use of seaweed to feed domestic animals was investigated through carbon ( δ ¹³C) and oxygen ( δ ¹⁸O) isotope analysis of tooth enamel bioapatite. The analysis was performed on sheep and cattle teeth from two Neolithic sites in Orkney (Scotland). At the Knap of Howar, c . 3600 bc , carbon isotopes reflect grazing on terrestrial plants throughout the year for both sheep and cattle, with no contribution of seaweed to their diet. At the Holm of Papa Westray North (HPWN), c . 3000 bc , significant contribution of seaweed to the sheep diet during winter is indicated by bioapatite δ ¹³C values as high as −5.7‰, far outside of the range of values expected for the feeding on terrestrial C₃ plants, and δ ¹⁸O values higher than expected during winter, possibly caused by ingestion of oceanic water with seaweed. Ingestion of seaweed by sheep at HPWN might have been necessitated by severe reduction of pastures during winter. Results suggest that sheep ingested fresh seaweed rather than dry fodder, perhaps directly on the shore as sheep do nowadays on North Ronaldsay. A significant difference between the two populations is the exclusive reliance on seaweed by the North Ronaldsay sheep, which have developed physiological adaptations to this diet. Contribution of seaweed to the sheep winter diet at HPWN might have been a first step towards this adaptation.     

Natural 13C and 15N abundance of field-collected fungi and their ecological implications

The natural abundance of ¹³C and ¹⁵N was measured in basidiocarps of at least 115 species in 88 genera of ectomycorrhizal, wood-decomposing and litter-decomposing fungi from Japan and Malaysia. The natural abundance of ¹³C and ¹⁵N was also measured in leaves, litter, soil and wood from three different sites. ¹⁵N and ¹³C were enriched in ectomycorrhizal and wood-decomposing fungi, respectively, relative to their substrates. Ectomycorrhizal and wood-decomposing fungi could be distinguished on the basis of their δ¹³C and δ¹⁵N signatures. Although there was high variability in the isotopic composition of fungi, the following isotope- enrichment factors (ε, mean±SD) of the fungi relative to substrates were observed:
ε_{ectomycorrhizal fungi/litter} = 6.1±0.4‰¹⁵N
ε_{ectomycorrhizal fungi/wood} = 1.4±0.8‰¹³C
ε_{wood-decomposing fungi/wood} = −0.6±0.7‰¹⁵N
ε_{wood-decomposing fungi/wood} = 3.5±0.9‰¹³C
The basis of isotope fractionation in C metabolism from wood to wood-decomposing fungus is discussed.