Carbon isotope fractionation in plants

Plants with the C₃, C₄, and crassulacean acid metabolism (CAM) photosynthetic pathways show characteristically different discriminations against ¹³C during photosynthesis. For each photosynthetic type, no more than slight variations are observed within or among species. CAM plants show large variations in isotope fractionation with temperature, but other plants do not. Different plant organs, subcellular fractions and metabolises can show widely varying isotopic compositions. The isotopic composition of respired carbon is often different from that of plant carbon, but it is not currently possible to describe this effect in detail. The principal components which will affect the overall isotope discrimination during photosynthesis are diffusion of CO₂, interconversion of CO₂ and HCO^?₃, incorporation of CO₂ by phosphoenolpyruvate carboxylase or ribulose bisphosphate carboxylase, and respiration. Theisotope fractionations associated with these processes are summarized. Mathematical models are presented which permit prediction of the overall isotope discrimination in terms of these components. These models also permit a correlation of isotope fractionations with internal CO₂ concentrations. Analysis of existing data in terms of these models reveals that CO₂ incorporation in C₃ plants is limited principally by ribulose bisphosphate carboxylase, but CO₂ diffusion also contributes. In C₄ plants, carbon fixation is principally limited by the rate of CO₂ diffusion into the leaf. There is probably a small fractionation in C₄ plants due to ribulose bisphosphate carboxylase.     

Photorespiration Contributes to Stomatal Regulation and Carbon Isotope Fractionation: A Study with Barley,Potato and Arabidopsis Plants Deficient in Glycine Decarboxylase

The rates of respiration in light and darkness, C _i/C _a and carbon isotope fractionation were investigated in glycine decarboxylase-deficient plants of barley, potato and Arabidopsis thaliana grown in climate chambers with controlled light intensity, temperature, humidity, irradiation and different CO₂ concentrations (360, 700 and 1400 µl l^–1) and compared to the wild-type plants. All photorespiration-impaired plants exhibited higher C _i/C _a and corresponding lower apparent water-use efficiencies, which were more expressed under high irradiance and elevated temperature. The mutants were depleted in ¹³C as compared to the wild-type plants, with a difference of up to 6 following growth in 360 µl l^–1 CO₂. We determined the carbon isotope content at different CO₂ concentrations to calculate the contribution of both C _i/C _a and photorespiration for ¹³C/¹²C fractionation. The direct effect of photorespiration was in the range of 0.7–1.0, from which we calculated the value of fractionation at the site of glycine decarboxylation as being 10–13, which is in agreement with the previously reported carbon isotope discrimination exerted by the glycine decarboxylase. Respiratory rates, particularly in the light, were increased in the glycine decarboxylase mutants. The necessity of the maintenance of a high CO₂ concentration near the site of carboxylation in chloroplasts in plants deficient in photorespiratory enzymes, requires an increased opening of the stomata with a corresponding decrease in water-use efficiency. It is concluded that photorespiration participates in the regulation of C _i/C _a and contributes to carbon isotope fractionation, both via effects on stomata and via discrimination of ¹³C in the glycine decarboxylase reaction.This revised version was published online in October 2005 with corrections to the Cover Date.     

Photosynthesis in intact leaves of C3 plants: Physics,physiology and rate limitations

The instantaneous rate of photosynthetic CO₂ assimilation in C₃ plants has generally been studied in model systems such as isolated chloroplasts and algae. From these studies and from theoretical analyses of gas exchange behavior it is now possible to study the biochemistry of photosynthesis in intact leaves using a combination of methods, most of which are nondestructive. The limitations to the rate of photosynthesis can be divided among three general classes: (1) the supply or utilization of CO₂, (2) the supply or utilization of light, and (3) the supply or utilization of phosphate. The first limitation is most readily studied by determining how the CO₂ assimilation rate varies with the partial pressure of CO₂ inside the leaf. The second limitation can be studied by determining the quantum requirement of photosynthesis. The third limitation is most easily detected as a loss of O₂ sensitivity of photosynthesis. Measurement of fluorescence from intact leaves can give additional information about the various limitations. These methods are all non-destructive and so can be observed repeatedly as the environment of a leaf is changed. In addition, leaves can be quick-frozen and metabolite concentrations then measured to give more information about the limitations to intact leaf photosynthesis rates. In this review the physics and biochemistry of photosynthesis in intact C₃ leaves, and the interface between physiology and photosynthesis—triose phosphate utilization—are discussed.     

Carbon: terrestrial C4 plants

The carbon isotope composition of terrestrial C₄ plants depends on the primary carboxylation of phosphoenolpyruvate (PEP) and on the diffusion of CO₂ to the carboxylation sites, but is also influenced by the final carboxylation of ribulose-1,5-bisphosphate (RuBP). Several models have been used for reproducing this complex situation. In the present review, a particular model is applied as a means to interpret the effects of environmental and genetically determined factors on carbon isotope discrimination during C₄ photosynthesis. As a new feature, the model considers four types of limitation of the overall CO₂ assimilation rate. Both carboxylation reactions are assumed to be limited by either maximum enzyme activity or maximum substrate regeneration rate. The model is applied to experimental data on the effects of CO₂, irradiance and water stress on short-term discrimination by leaves of several C₄ species measured simultaneously with CO₂ gas exchange characteristics. In particular, different patterns of the influence of low irradiances on carbon isotope discrimination are interpreted as due to variations in that irradiance at which a transition from limitation by PEP regeneration rate and RuBP carboxylase activity to limitation by the regeneration rates of both substrates occurs. After discussing literature data on the effects of environmental conditions on carbon isotope discrimination by C₄ plants seasonal and developmental changes in carbon isotope composition, studies on the systematic and geographic distribution of C₄ plants, evolutionary and genetical aspects, and some ecological implications are reviewed.     

Isotopic Fractionation of Hydrogen in Plants

The naturally-occurring stable isotopes deuterium and hydrogen are fractionated by a number of physical and biological processes. Deuterium has a tendency to precipitate out first from a moist air mass. Thus ground water will become isotopically lighter with an increase in latitude, altitude, or distance inland. Water taken up by the plant from the soil undergoes little change until evapotranspiration results in leaf water becoming isotopically heavier. Thus hydrogen isotopes in plants can reveal something of geography (groundwater) and climate. Hydrogen isotopes undergo little fractionation by passage through the food chain, although plant parasites tend to be enriched in D as compared to their hosts, possibly due to higher rates of transpiration in the parasitic plants. The splitting of water in photosynthesis results in the lighter isotope being incorporated into organic matter. An even larger isotopic fractionation results during lipid synthesis and other processes involving the pyruvate dehydrogenase complex. Differences in metabolic pathway between species can be detected by D/H ratios. Hydrogen isotopic differences can be detected between CAM, C₄, and C₃ species. Within C₄ plants, the NADP-ME plants are isotopically distinguishable from NAD-ME and PEP-CK plants.     

δ<Superscript>13</Superscript>C values,photosynthetic pathways,and plant functional types for some plants from saline meadows,Northeastern China

Based on stable carbon isotope ratio (δ¹³C) measurements, photosynthetic pathway types were determined for 61 species in 54 genera and 24 families of flowering plants from the saline meadows of Northeastern China. Of these total vascular plants, 18 species in 17 genera from 6 families were found to have C₄ photosynthesis; 43 species in 38 genera from 20 families had C₃ photosynthesis. Six dicotyledonous species exhibited C₄ pathway, 12 monocotyledonous species were found with C₄ photosynthesis. The dicotyledonous C₄ species had relative greater mean δ¹³C value and less total carbon content than both monocotyledonous C₄ and C₃ species. Most dicotyledonous C₄ species were annual forbs and halophytes. Some C₄ species had been previously documented, but their δ¹³C values varied remarkably from those of the present study. Even though there are some fluctuations for the δ¹³C values of some C₄ species, δ¹³C value was still more reliable for C₃ and C₄ identification than the use of the enzyme ratio method and of low CO₂ compensation concentration.     

Role of Photorespiration and Cyclic Electron Transport in C<Subscript>4</Subscript> Photosynthesis Evolution in the C<Subscript>3</Subscript>–C<Subscript>4</Subscript> Intermediate Species <Emphasis Type="Italic">Sedobassia sedoides</Emphasis>

Plants from two Sedobassia sedoides (Pall.) Aschers populations (Makan and Valitovo) (Chenopodiaceae) with C₂ photosynthesis (precursor of C₄ photosynthesis in phylogenesis) and photorespiratory CO₂-concentrating mechanism were studied. Genetic polymorphism and isotope discrimination (δ¹³С) levels of the plants were determined under natural conditions, and their morpho-physiological parameters such as fresh and dry biomass of the above ground parts of plants, functioning of photosystem I (PSI) and photosystem II (PSII), intensity of net photosynthesis (A), transpiration (E), photorespiration and water use efficiency (WUE) of plants were calculated under control and salinine conditions (0 and 200 mM NaCl). Results of the population-genetic analysis showed that the Makan population is polymorphic (plastic) and the Valitovo population is monomorphic (narrowly specialized). There were no significant differences between the populations based on δ¹³С values or growth parameters, PSII, A, E and WUE under control conditions. Under saline conditions, dry biomass accumulation decreased in the Makan population by 15% and by more than 2- fold in the Valitovo population. Population differences were revealed in terms of photorespiration intensity and P700 oxidation kinetics under control and saline conditions. Under control conditions, Makan plants were characterized by a higher photorespiration intensity, which decreased by 2-fold under saline conditions to the photorespiration level of Valitovo plants. Cyclic electron transport activity was minimal in the control Makan plants, and it increased by almost 2-fold under saline conditions to the level of that in Valitovo plants under control and saline conditions. Under control conditions, photosynthesis in Makan plants can be specified as the proto-Kranz type (transitional type from C₃ to C₂) and that in Valitovo plants can be specified as the C₂ type (C₄ photosynthesis with photorespiratory CO₂-concentrating mechanism), based on their photorespiration level and cyclic electron transport activity. Under saline conditions, Makan plants exhibited features of C₂ photosynthesis. Intraspecific functional differences of photosynthesis were revealed in different populations of intermediate C₃–C₄ plant species S. sedoides which reflect the initial stages of formation of a photorespiratory CO₂-concentrating mechanism during C₄ photosynthesis evolution, accompanied by decrease in salt tolerance.     

Carbon isotope ratios and crop analyses of Arphia (Orthoptera: Acrididae) species in southeastern Wyoming Grassland

Summary Food habits of Arphia conspersa Scudder and Arphia pseudonietana (Thomas) were studied along an altitudinal transect in southeastern Wyoming shortgrass mixed prairie. Stable carbon isotope ratios indicated that diets were significantly different between study sites, between species, and between sexes. These differences were found to be primarily related to the availability of different food plants along the transect, although species with the C₃ pathway of photosynthesis were consumed in greater proportion than their availability in the habitat. The preference for C₃ species is presumably related to their higher nutritional value and digestibility, in spite of the fact that more time and energy must be spent to locate these food plants in some of the habitats studied. This study demonstrates the utility of the carbon isotope method in studying plant-animal interactions in habitats containing both C₃ and C₄ plants.     

The mechanism of carbon isotope fractionation in photosynthesis and carbon dioxide component of the greenhouse effect

The relationship between the global climate warming, which is largely induced by increased CO₂ atmospheric concentration, and the changes in carbon isotope fractionation in plants was explained in terms of the previously proposed oscillatory mechanism of photosynthesis, according to which CO₂ assimilation and photorespiration are two reciprocally coupled oscillating mechanisms controlled by ribulose bisphosphate carboxylase/oxygenase switching. This explanation is confirmed by the changes in carbon isotope fractionation in the annual rings of trees and demonstrates that the light carbon isotope ¹²C enrichment before 1990s resulted from increased photosynthetic assimilation of CO₂. The subsequent sharp ¹³C enrichment of the tree ring carbon until the present time suggests that the compensatory role of photosynthesis in boreal forests has been lost for the global climate.     

Carbon isotope composition as a tool to control the quality of herbs and medicinal plants

Isotope screening is a simple test for determining the photosynthetic pathway used by plants. The scope of this work was to classify the photosynthetic type of some herbs and medicinal plants through studies of the carbon isotope composition (δ¹³C). Also, we propose the use of carbon isotope composition as a tool to control the quality of herbs and medicinal plants. For studies of δ¹³C, δ¹³C‰ = [R (sample)/R (standard) − 1] × 10⁻³, dry leaves powdered in cryogenic mill were analyzed in a mass spectrometer coupled with an elemental analyzer for determining the ratio R = ¹³CO₂/¹²CO₂. In investigation of δ¹³C of 55 species, 23 botanical families, and 44 species possessed a C₃ photosynthetic type. Six species found among the botanical families Euphorbiaceae and Poaceae were C₄ plants, and 5 species found among the botanical families Agavaceae, Euphorbiaceae, and Liliaceae possessed CAM-type photosynthesis. Carbon isotope composition of plants can be used as quality control of herbs and medicinal plants, allowing the identification of frauds or contaminations. Also, the information about the photosynthetic type found for these plants can help in introducing and cultivating exotic and wild herbs and medicinal plants.     

Oxygen effects on photosynthesis and C metabolism in desert plants

The effect of 1% and 21% O₂ upon ¹⁴CO₂ assimilation by desert plants exposed for 10 to 90 seconds has been studied. The plants studied can be divided into three groups with respect to O₂. The C₃ plants display the usual Warburg effect. No changes could be observed in the intensity of photosynthesis as a function of O₂ content in another group of plants (showing signs of Crassulacean acid metabolism). In still another group of plants (C₄ plants) the stimulating effect of O₂ on photosynthesis could be detected. In C₃ plants, O₂ inhibits the processing of carbon through the Calvin cycle intermediates. The involvement of carbon in the glycolate pathway fails to explain completely the inhibiting effect of O₂ on photosynthesis. It is assumed that O₂ inhibits the enzymes of the Calvin cycle. In C₄ plants O₂ stimulates the incorporation of ¹⁴C into malate and aspartate. The incorporation of ¹⁴C into the intermediates of the Calvin cycle in C₄ plants is inhibited much like that in typical C₃ plants.     

Occurrence of C3 and C4 photosynthesis in cotyledons and leaves of Salsola species (Chenopodiaceae)

Most species of the genus Salsola (Chenopodiaceae) that have been examined exhibit C₄ photosynthesis in leaves. Four Salsola species from Central Asia were investigated in this study to determine the structural and functional relationships in photosynthesis of cotyledons compared to leaves, using anatomical (Kranz versus non-Kranz anatomy, chloroplast ultrastructure) and biochemical (activities of photosynthetic enzymes of the C₃ and C₄ pathways, ¹⁴C labeling of primary photosynthesis products and ¹³C/¹²C carbon isotope fractionation) criteria. The species included S. paulsenii from section Salsola, S. richteri from section Coccosalsola, S. laricina from section Caroxylon, and S. gemmascens from section Malpigipila. The results show that all four species have a C₄ type of photosynthesis in leaves with a Salsoloid type Kranz anatomy, whereas both C₃ and C₄ types of photosynthesis were found in cotyledons. S. paulsenii and S. richteri have NADP- (NADP-ME) C₄ type biochemistry with Salsoloid Kranz anatomy in both leaves and cotyledons. In S. laricina, both cotyledons and leaves have NAD-malic enzyme (NAD-ME) C₄ type photosynthesis; however, while the leaves have Salsoloid type Kranz anatomy, cotyledons have Atriplicoid type Kranz anatomy. In S. gemmascens, cotyledons exhibit C₃ type photosynthesis, while leaves perform NAD-ME type photosynthesis. Since the four species studied belong to different Salsola sections, this suggests that differences in photosynthetic types of leaves and cotyledons may be used as a basis or studies of the origin and evolution of C₄ photosynthesis in the family Chenopodiaceae.This revised version was published online in October 2005 with corrections to the Cover Date.     

C4 plants use fluctuating light less efficiently than do C3 plants: a study of growth,photosynthesis and carbon isotope discrimination

Plants in the field are commonly exposed to fluctuating light intensity, caused by variable cloud cover, self‐shading of leaves in the canopy and/or leaf movement due to turbulence. In contrast to C₃ plant species, only little is known about the effects of dynamic light (DL) on photosynthesis and growth in C₄ plants. Two C₄ and two C₃ monocot and eudicot species were grown under steady light or DL conditions with equal sum of daily incident photon flux. We measured leaf gas exchange, plant growth and dry matter carbon isotope discrimination to infer CO₂ bundle sheath leakiness in C₄ plants. The growth of all species was reduced by DL, despite only small changes in steady‐state gas exchange characteristics, and this effect was more pronounced in C₄ than C₃ species due to lower assimilation at light transitions. This was partially attributed to increased bundle sheath leakiness in C₄ plants under the simulated lightfleck conditions. We hypothesize that DL leads to imbalances in the coordination of C₄ and C₃ cycles and increasing leakiness, thereby decreasing the quantum efficiency of photosynthesis. In addition to their other constraints, the inability of C₄ plants to efficiently utilize fluctuating light likely contributes to their absence in such environments as forest understoreys.     

Enzymatic fractionation of carbon isotopes by phosphoenolpyruvate carboxylase from c(4) plants

The carbon atoms of glucose and malate in C₄ plants are 2 to 3‰ enriched in ¹²C with respect to atmospheric CO₂; whereas these intermediates in C₃ plants are 15 to 18‰ enriched with ¹²C with respect to atmospheric CO₂. The enzymatic synthesis of malate from phosphoenolpyruvate and bicarbonate in preparations of leaves of Sorghum bicolor, Haygrazer result in a carbon isotope fractionation of about 3‰. The enzymatic synthesis of phosphoglyceric acid from ribulose 1,5-diP and CO₂ in these preparations (contaminated with carbonic anhydrase) at 24 C and 37 C result in a carbon isotope fractionation of 33.7‰ and 18.3‰, respectively. These data are consistent with the conclusion that the small enrichment of ¹²C in the carbon atoms of malate and glucose (with respect to atmospheric CO₂) in leaves of Sorghum bicolor, Haygrazer occurs at the phosphoenolpyruvate carboxylase step.     

The intramolecular ¹³C-distribution in ethanol reveals the influence of the CO₂ -fixation pathway and environmental conditions on the site-specific ¹³C variation in glucose

Efforts to understand the cause of ¹²C versus ¹³C isotope fractionation in plants during photosynthesis and post‐photosynthetic metabolism are frustrated by the lack of data on the intramolecular ¹³C‐distribution in metabolites and its variation with environmental conditions. We have exploited isotopic carbon‐13 nuclear magnetic resonance (¹³C NMR) spectrometry to measure the positional isotope composition (δ¹³C_i, ‰) in ethanol samples from different origins: European wines, liquors and sugars from C₃, C₄ and crassulacean acid metabolism (CAM) plants. In C₃‐ethanol samples, the methylene group was always ¹³C‐enriched (～2‰) relative to the methyl group. In wines, this pattern was correlated with both air temperature and δ¹⁸O of wine water, indicating that water vapour deficit may be a critical defining factor. Furthermore, in C₄‐ethanol, the reverse relationship was observed (methylene‐C relatively ¹³C‐depleted), supporting the concept that photorespiration is the key metabolic process leading to the ¹³C distribution in C₃‐ethanol. By contrast, in CAM‐ethanol, the isotopic pattern was similar to but stronger than C₃‐ethanol, with a relative ¹³C‐enrichment in the methylene‐C of up to 13‰. Plausible causes of this ¹³C‐pattern are briefly discussed. As the intramolecular δ¹³C_i‐values in ethanol reflect that in source glucose, our data point out the crucial impact on the ratio of metabolic pathways sustaining glucose synthesis.     

Isotope ratios of cellulose from plants having different photosynthetic pathways

Hydrogen and carbon isotope ratios of cellulose nitrate and oxygen isotope ratios of cellulose from C₃, C₄, and Crassulacean acid metabolism (CAM) plants were determined for plants growing within a small area in Val Verde County, Texas. Plants having CAM had distinctly higher deuterium/hydrogen (D/H) ratios than plants having C₃ and C₄ metabolism. When hydrogen isotope ratios are plotted against carbon isotope ratios, each photosynthetic mode separates into a distinct cluster of points. C₄ plants had many D/H ratios similar to those of C₃ plants, so that hydrogen isotope ratios cannot be used to distinguish between these two photosynthetic modes. Portulaca mundula, which may have a modified photosynthetic mode between C₄ and CAM, had a hydrogen isotope ratio between those of the C₄ and CAM plants. When oxygen isotope ratios are plotted against carbon isotope ratios, no distinct clustering of the C₄ and CAM plants occurs. Thus, oxygen isotope ratios are not useful in distinguishing between these metabolic modes. A plot of hydrogen isotope ratios versus oxygen isotope ratios for this sample set shows considerable overlap between oxygen isotope ratios of the different photosynthetic modes without a concomitant overlap in the hydrogen isotope ratios of CAM and the other two photosynthetic modes. This observation is consistent with the hypothesis that higher D/H ratios in CAM plants relative to C₃ and C₄ plants are due to isotopic fractionations occurring during biochemical reactions.     

The efficiency of C4 photosynthesis under low light conditions in Zea mays,Miscanthus x giganteus and Flaveria bidentis

The efficiency of C₄ photosynthesis in Zea mays, Miscanthus x giganteus and Flaveria bidentis in response to light was determined using measurements of gas exchange, ¹³CO₂ photosynthetic discrimination, metabolite pools and spectroscopic assays, with models of C₄ photosynthesis and leaf ¹³CO₂ discrimination. Spectroscopic and metabolite assays suggested constant energy partitioning between the C₄ and C₃ cycles across photosynthetically active radiation (PAR). Leakiness (φ), modelled using C₄ light‐limited photosynthesis equations (φ_mod), matched values from the isotope method without simplifications (φ_is) and increased slightly from high to low PAR in all species. However, simplifications of bundle‐sheath [CO₂] and respiratory fractionation lead to large overestimations of φ at low PAR with the isotope method. These species used different strategies to maintain similar φ. For example, Z. mays had large rates of the C₄ cycle and low bundle‐sheath cells CO₂ conductance (g_bs). While F. bidentis had larger g_bs but lower respiration rates and M. giganteus had less C₄ cycle capacity but low g_bs, which resulted in similar φ. This demonstrates that low g_bs is important for efficient C₄ photosynthesis but it is not the only factor determining φ. Additionally, these C₄ species are able to optimize photosynthesis and minimize φ over a range of PARs, including low light.     

Carbon isotope discrimination in C3-C4 intermediates

Carbon isotope discrimination in C₃–C₄ intermediates is determined by fractionations during diffusion and the biochemical fractionations occurring during CO₂ fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO₂ fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO₂ fixation occurring in the bundle sheath, the extent of bundle-sheath leakiness and the contribution which C₄-cycle activity makes to the CO₂ pool there. In most instances, carbon isotope discrimination in C₃–C₄ intermediates is C₃-like because only a small fraction of the total carbon fixed is fixed in the bundle sheath. In particular, this must be the case for Flaveria intermediates which initially fix substantial amounts of CO₂ into C₄-acids. In C₃–C₄ intermediates that refix photorespiratory CO₂ alone, it is possible for carbon isotope discrimination to be greater than in C₃-species, particularly at low CO₂ pressures or at high leaf temperatures. Short-term measurements of carbon isotope discrimination and gas exchange of leaves can be used to study the photosynthetic pathways of C₃-C₄ intermediates and their hybrids as has recently been done for C₃ and C₄ species.     

Temperature response of photosynthesis in C3, C4, and CAM plants: temperature acclimation and temperature adaptation

Most plants show considerable capacity to adjust their photosynthetic characteristics to their growth temperatures (temperature acclimation). The most typical case is a shift in the optimum temperature for photosynthesis, which can maximize the photosynthetic rate at the growth temperature. These plastic adjustments can allow plants to photosynthesize more efficiently at their new growth temperatures. In this review article, we summarize the basic differences in photosynthetic reactions in C₃, C₄, and CAM plants. We review the current understanding of the temperature responses of C₃, C₄, and CAM photosynthesis, and then discuss the underlying physiological and biochemical mechanisms for temperature acclimation of photosynthesis in each photosynthetic type. Finally, we use the published data to evaluate the extent of photosynthetic temperature acclimation in higher plants, and analyze which plant groups (i.e., photosynthetic types and functional types) have a greater inherent ability for photosynthetic acclimation to temperature than others, since there have been reported interspecific variations in this ability. We found that the inherent ability for temperature acclimation of photosynthesis was different: (1) among C₃, C₄, and CAM species; and (2) among functional types within C₃ plants. C₃ plants generally had a greater ability for temperature acclimation of photosynthesis across a broad temperature range, CAM plants acclimated day and night photosynthetic process differentially to temperature, and C₄ plants was adapted to warm environments. Moreover, within C₃ species, evergreen woody plants and perennial herbaceous plants showed greater temperature homeostasis of photosynthesis (i.e., the photosynthetic rate at high-growth temperature divided by that at low-growth temperature was close to 1.0) than deciduous woody plants and annual herbaceous plants, indicating that photosynthetic acclimation would be particularly important in perennial, long-lived species that would experience a rise in growing season temperatures over their lifespan. Interestingly, across growth temperatures, the extent of temperature homeostasis of photosynthesis was maintained irrespective of the extent of the change in the optimum temperature for photosynthesis (T _opt), indicating that some plants achieve greater photosynthesis at the growth temperature by shifting T _opt, whereas others can also achieve greater photosynthesis at the growth temperature by changing the shape of the photosynthesis–temperature curve without shifting T _opt. It is considered that these differences in the inherent stability of temperature acclimation of photosynthesis would be reflected by differences in the limiting steps of photosynthetic rate.     

Compensation point and isotopic characteristics of c(3)/c(4) intermediates and hybrids in panicum

Leaf CO₂ compensation points and stable hydrogen, oxygen and carbon isotope ratios were determined for Panicum species including C₃/C₄ intermediate photosynthesis plants, hybrids between C₃/C₄ intermediates and C₃ plants, C₃ and C₄ plants in the Panicum genus as well as several other C₃ and C₄ plants. C₃ plants had the highest compensation points, followed by hybrids, C₃/C₄ intermediates, and C₄ plants. δ¹³C values of cellulose nitrate and saponifiable lipids from C₄ plants were about 10‰ higher than those observed for cellulose nitrate and saponifiable lipids of C₃/C₄ intermediates, hybrids, and C₃ plants. Oxygen isotope ratios of cellulose as well as those of leaf water were similar for all plants. There was substantial variability in the δD values of cellulose nitrate among the plants studied. In contrast, such variability was not observed in δD values of water distilled from the leaves, nor in the δD values of the saponifiable lipids. Variability in δD values of cellulose nitrate from C₃/C₄ intermediates, hybrids, C₃, and C₄ plants is due to fractionations occurring during biochemical reactions specific to leaf carbohydrate metabolism.