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1.
Carbon isotope discrimination in C 3–C 4 intermediates is determined by fractionations during diffusion and the biochemical fractionations occurring during CO 2 fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO 2 fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO 2 fixation occurring in the bundle sheath, the extent of bundle-sheath leakiness and the contribution which C 4-cycle activity makes to the CO 2 pool there. In most instances, carbon isotope discrimination in C 3–C 4 intermediates is C 3-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 CO 2 into C 4-acids. In C 3–C 4 intermediates that refix photorespiratory CO 2 alone, it is possible for carbon isotope discrimination to be greater than in C 3-species, particularly at low CO 2 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 C 3-C 4 intermediates and their hybrids as has recently been done for C 3 and C 4 species. 相似文献
2.
Abstract. The photosynthetic responses to temperature in C 3, C 3-C 4 intermediate, and C 4 species in the genus Flaveria were examined in an effort to identify whether the reduced photorespiration rates characteristic of C 3-C 4 intermediate photosynthesis result in adaptive advantages at warm leaf temperatures. Reduced photorespiration rates were reflected in lower CO 2 compensation points at all temperatures examined in the C 3-C 4 intermediate, Flaveria floridana, compared to the C 3 species, F. cronquistii. The C 3-C 4 intermediate, F. floridana, exhibited a C 3-like photosynthetic temperature dependence, except for relatively higher photosynthesis rates at warm leaf temperatures compared to the C 3 species, F. cronquistii. Using models of C 3 and C 3-C 4 intermediate photosynthesis, it was predicted that by recycling photorespired CO 2 in bundle-sheath cells, as occurs in many C 3-C 4 intermediates, photosynthesis rates at 35°C could be increased by 28%, compared to a C 3 plant. Without recycling photorespired CO 2, it was calculated that in order to improve photosynthesis rates at 35°C by this amount in C 3 plants, (1) intercellular CO 2 partial pressures would have to be increased from 25 to 31 Pa, resulting in a 57% decrease in water-use efficiency, or (2) the activity of RuBP carboxylase would have to be increased by 32%, resulting in a 22% decrease in nitrogen-use efficiency. In addition to the recycling of photorespired CO 2, leaves of F. floridana appear to effectively concentrate CO 2 at the active site of RuBP carboxylase, increasing the apparent carboxylation efficiency per unit of in vitro RuBP carboxylase activity. The CO 2-concentrating activity also appears to reduce the temperature sensitivity of the carboxylation efficiency in F. floridana compared to F. cronquistii. The carboxylation efficiency per unit of RuBP carboxylase activity decreased by only 38% in F. floridana, compared to 50% in F. cronquistii, as leaf temperature was raised from 25 to 35°C. The C 3-C 4 intermediate, F. ramosissima, exhibited a photosynthetic temperature temperature response curve that was more similar to the C 4 species, F. trinervia, than the C 3 species, F. cronquistii. The C 4-like pattern is probably related to the advanced nature of C 4-like biochemical traits in F. ramosissima The results demonstrate that reductions in photorespiration rates in C 3-C 4 intermediate plants create photosynthetic advantages at warm leaf temperatures that in C 3 plants could only be achieved through substantial costs to water-use efficiency and/or nitrogen-use efficiency. 相似文献
3.
Abstract Evidence is drawn from previous studies to argue that C 3—C 4 intermediate plants are evolutionary intermediates, evolving from fully-expressed C 3 plants towards fully-expressed C 4 plants. On the basis of this conclusion, C 3—C 4 intermediates are examined to elucidate possible patterns that have been followed during the evolution of C 4 photosynthesis. An hypothesis is proposed that the initial step in C 4-evolution was the development of bundle-sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO 2 using RuBP carboxylase. The CO 2-recycling mechanism appears to involve the differential compartmentation of glycine decarboxylase between mesophyll and bundle-sheath cells, such that most of the activity is in the bundlesheath cells. Subsequently, elevated phosphoenolpyruvate (PEP) carboxylase activities are proposed to have evolved as a means of enhancing the recycling of photorespired CO 2. As the activity of PEP carboxylase increased to higher values, other enzymes in the C 4-pathway are proposed to have increased in activity to facilitate the processing of the products of C 4-assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C 4-cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully-expressed C 4 plants. Such metabolism would have limited benefit in terms of concentrating CO 2 at RuBP carboxylase and, therefore, also be of little benefit for improving water- and nitrogen-use efficiencies. However, the development of such a limited C 4-cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully-expressed C 4 plants. Thus, during the initial stages of C 4-evolution it is proposed that improvements in photorespiratory CO 2-loss and their influence on increasing the rate of net CO 2 assimilation per unit leaf area represented the evolutionary ‘driving-force’. Improved resourceuse efficiency resulting from an efficient CO 2-concentrating mechanism is proposed as the driving force during the later stages. 相似文献
4.
Attempts are being made to introduce C 4 photosynthetic characteristics into C 3 crop plants by genetic manipulation. This research has focused on engineering single‐celled C 4‐type CO 2 concentrating mechanisms into C 3 plants such as rice. Herein the pros and cons of such approaches are discussed with a focus on CO 2 diffusion, utilizing a mathematical model of single‐cell C 4 photosynthesis. It is shown that a high bundle sheath resistance to CO 2 diffusion is an essential feature of energy‐efficient C 4 photosynthesis. The large chloroplast surface area appressed to the intercellular airspace in C 3 leaves generates low internal resistance to CO 2 diffusion, thereby limiting the energy efficiency of a single‐cell C 4 concentrating mechanism, which relies on concentrating CO 2 within chloroplasts of C 3 leaves. Nevertheless the model demonstrates that the drop in CO 2 partial pressure, pCO 2, that exists between intercellular airspace and chloroplasts in C 3 leaves at high photosynthetic rates, can be reversed under high irradiance when energy is not limiting. The model shows that this is particularly effective at lower intercellular pCO 2. Such a system may therefore be of benefit in water‐limited conditions when stomata are closed and low intercellular pCO 2 increases photorespiration. 相似文献
5.
The carbon isotope composition of terrestrial C 4 plants depends on the primary carboxylation of phosphoenolpyruvate (PEP) and on the diffusion of CO 2 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 C 4 photosynthesis. As a new feature, the model considers four types of limitation of the overall CO 2 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 CO 2, irradiance and water stress on short-term discrimination by leaves of several C 4 species measured simultaneously with CO 2 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 C 4 plants seasonal and developmental changes in carbon isotope composition, studies on the systematic and geographic distribution of C 4 plants, evolutionary and genetical aspects, and some ecological implications are reviewed. 相似文献
7.
Immediate export in leaves of C 3‐C 4 intermediates were compared with their C 3 and C 4 relatives within the Panicum and Flaveria genera. At 35 Pa CO 2, photosynthesis and export were highest in C 4 species in each genera. Within the Panicum, photosynthesis and export in ‘type I’ C 3‐C 4 intermediates were greater than those in C 3 species. However, ‘type I’ C 3‐C 4 intermediates exported a similar proportion of newly fixed 14C as did C 4 species. Within the Flaveria, ‘type II’ C 3‐C 4 intermediate species had the lowest export rather than the C 3 species. At ambient CO 2, immediate export was strongly correlated with photosynthesis. However, at 90 Pa CO 2, when photosynthesis and immediate export increased in all C 3 and C 3‐C 4 intermediate species, proportionally less C was exported in all photosynthetic types than that at ambient CO 2. All species accumulated starch and sugars at both CO 2 levels. There was no correlation between immediate export and the pattern of 14C‐labelling into sugars and starch among the photosynthetic types within each genus. However, during CO 2 enrichment, C 4Panicum species accumulated sugars above the level of sugars and starch normally made at ambient CO 2, whereas the C 4Flaveria species accumulated only additional starch. 相似文献
8.
The 18O content of CO 2 is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO 2 and 18O-enriched leaf water and retrodiffusion of most of this CO 2 back to the atmosphere, leaves effectively discriminate against 18O during photosynthesis. Discrimination against 18O ( Δ 18O) is expected to be lower in C 4 plants because of low ci and hence low retrodiffusing CO 2 flux. C 4 plants also generally show lower levels of carbonic anhydrase (CA) activities than C 3 plants. Low CA may limit the extent of 18O exchange and further reduce Δ 18O. We investigated CO 2–H 2O isotopic equilibrium in plants with naturally low CA activity, including two C 4 ( Zea mays, Sorghum bicolor) and one C 3 ( Phragmites australis) species. The results confirmed experimentally the occurrence of low Δ 18O in C 4, as well as in some C 3, plants. Variations in CA activity and in the extent of CO 2–H 2O isotopic equilibrium ( θ eq) estimated from on-line measurements of Δ 18O showed large range of 0–100% isotopic equilibrium ( θ eq = 0–1). This was consistent with direct estimates based on assays of CA activity and measurements of CO 2 concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Δ 18O as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of θ eq < 1 (incomplete isotopic equilibrium) on 18O of atmospheric CO 2 can be similar for C 3 and C 4 plants and in both cases it increases with natural enrichment of 18O in leaf water. 相似文献
10.
Abstract. The similarities between the component reactions of the presently known variants of photosynthetic carbon metabolism (crassulacean acid metabolism, the acid metabolism of Tillandsia usneoides , aquatic acid metabolism, and C 4 photosynthesis) when considered along with their widely scattered taxonomic distribution strongly suggest polyphyletic origins resulting from evolutionary modification of a common, universally distributed metabolic sequence. The synthesis and consumption of four-carbon acids in the cation-balancing reactions involved in the regulation of stomatal aperture appear to exhibit all of the characteristics likely to be displayed by such a metabolic progenitor. The present status of the proposal that the expression of aspects of stomatal metabolism in photosynthetic mesophyll cells represents the basis for the evolution of the variant of photosynthetic carbon metabolism is discussed. The prospects of experimental approaches which may yield information relevant to the proposal are also explored. 相似文献
11.
There is continuing controversy over whether a degree of C 4 photosynthetic metabolism exists in ears of C 3 cereals. In this context, CO 2 exchange and the initial products of photosynthesis were examined in flag leaf blades and various ear parts of two durum wheat ( Triticum durum Desf.) and two six-rowed barley ( Hordeum vulgare L.) cultivars. Three weeks after anthesis, the CO 2 compensation concentration at 210 mmol mol ?1 O 2 in durum wheat and barley ear parts was similar to or greater than that in flag leaves. The O 2 dependence of the CO 2 compensation concentration in durum wheat ear parts, as well as in the flag leaf blade, was linear, as expected for C 3 photosynthesis. In a complementary experiment, intact and attached ears and flag leaf blades of barley and durum wheat were radio-labelled with 14CO 2 during a 10s pulse, and the initial products of fixation were studied in various parts of the ears (awns, glumes, inner bracts and grains) and in the flag leaf blade. All tissues assimilated CO 2 mainly by the Calvin (C 3) cycle, with little fixation of 14CO 2 into the C 4 acids malate and aspartate (about 10% or less). These collective data support the conclusion that in the ear parts of these C 3 cereals C 4 photosynthetic metabolism is nil. 相似文献
13.
Abstract. In this review we relate the physiological significance of C 4 photosynthesis to plant performance in nature. We begin with an examination of the physiological consequences of the C 4 pathway on photosynthesis, then discuss the ecophysiological performance of C 4 plants in contrasting environments. We then compare the performance of C 3 and C 4 plants when they occur together in similar habitats, and finally discuss the distribution of C 4 photosynthesis with respect to the physical environment, phylogeny, and life form. 相似文献
15.
Aim Numerous studies have examined the climatic factors that influence the abundance of C 4 species within the grass flora (C 4 relative species richness) in various regions throughout the world, but very few have examined the relative abundance of C 4 vs. C 3 grasses (C 4 relative abundance). We sought to determine the climatic factors that influence C 4 relative abundance throughout Australia. Location Australia (including Tasmania). Methods We measured C 4 relative abundance at 168 locations and measured δ 13C (the abundance of 13C relative to 12C) of the bone collagen of 779 kangaroos collected throughout Australia, as bone collagen δ 13C was assumed to be proportional to the relative abundance of C 4 grasses in the diet. Results Both C 4 relative abundance and kangaroo bone collagen δ 13C were found to have a strong positive relationship with seasonal water availability, i.e. the distribution of rainfall in the C 4 vs. C 3 growing seasons (76% and 69% of deviance explained, respectively). There was clear evidence that seasonal water availability was a better predictor of both C 4 relative abundance and bone collagen δ 13C than other climate variables such as mean annual temperature and January daily minimum temperature. However, seasonal water availability appeared to be a relatively poor predictor of C 4 relative species richness, which was most closely related to January daily minimum temperature (90% of deviance explained). Main conclusions Our results highlight the relatively poor relationship between C 4 relative abundance and C 4 relative species richness, and suggest that these two variables may be related to different climatic factors. They also suggest that caution is required when using C 4 relative species richness to infer the relative biomass and productivity of C 4 grasses on a global scale. 相似文献
17.
The atmospheric CO 2 concentration has increased from the pre-industrial concentration of about 280 μmol mol −1 to its present concentration of over 350 μmol mol −1, and continues to increase. As the rate of photosynthesis in C 3 plants is strongly dependent on CO 2 concentration, this should have a marked effect on photosynthesis, and hence on plant growth and productivity. The magnitude of photo-synthetic responses can be calculated based on the well-developed theory of photosynthetic response to intercellular CO 2 concentration. A simple biochemically based model of photosynthesis was coupled to a model of stomatal conductance to calculate photosynthetic responses to ambient CO 2 concentration. In the combined model, photosynthesis was much more responsive to CO 2 at high than at low temperatures. At 350 μmol mol −1, photosynthesis at 35°C reached 51% of the rate that would have been possible with non-limiting CO 2, whereas at 5°C, 77% of the CO 2 non-limited rate was attained. Relative CO 2 sensitivity also became smaller at elevated CO 2, as CO 2 concentration increased towards saturation. As photosynthesis was far from being saturated at the current ambient CO 2 concentration, considerable further gains in photosynthesis were predicted through continuing increases in CO 2 concentration. The strong interaction with temperature also leads to photosynthesis in different global regions experiencing very different sensitivities to increasing CO 2 concentrations. 相似文献
18.
Panicum milioides, a naturally occurring species with C 4-like Kranz leaf anatomy, is intermediate between C 3 and C 4 plants with respect to photorespiration and the associated oxygen inhibition of photosynthesis. This paper presents direct evidence for a limited degree of C 4 photosynthesis in this C 3-C 4 intermediate species based on: 1. (a) the appearance of 24% of the total 14C fixed following 4 s photosynthesis in 14CO2-air by excised leaves in malate and aspartate and the complete transfer of label from the C4 acids to Calvin cycle intermediates within a 15 s chase in 12CO2-air; 2. (b) pyruvate- or alanine-enhanced light-dependent CO2 fixation and pyruvate stimulation of oxaloacetate- or 3-phosphoglycerate-dependent O2 evolution by illuminated mesophyll protoplasts, but not bundle sheath strands; and 3. (c) NAD-malic enzyme-dependent decarboxylation of C4 acids at the C-4 carboxyl position, C4 acid-dependent O2 evolution, and 14CO2 donation from [4-14C]C4 acids to Calvin cycle intermediates during photosynthesis by bundle sheath strands, but not mesophyll protoplasts.
However, P. milioides differs from C4 plants in that the activity of the C4 cycle enzymes is only 15 to 30% of a C4 Panicum species and the Calvin cycle and phosphoenolpyruvate carboxylase are present in both cell types. From these and related studies (Rathnam, C.K.M. and Chollet, R. (1979) Arch. Biochem. Biophys. 193, 346–354; (1978) Biochem. Biophys. Res. Commun. 85, 801–808) we conclude that reduced photorespiration in P. milioides is due to a limited degree of NAD-malic enzyme-type C4 photosynthesis permitting an increase in pCO2 at the site of bundle sheath, but not mesophyll, ribulosebisphosphate carboxylase-oxygenase. 相似文献
19.
We analyzed the δ13C of soil organic matter (SOM) and fine roots from 55 native grassland sites widely distributed across the US and Canadian Great Plains to examine the relative production of C 3 vs. C 4 plants (hereafter %C 4) at the continental scale. Our climate vs. %C 4 results agreed well with North American field studies on %C 4, but showed bias with respect to %C 4 from a US vegetation database (statsgo ) and weak agreement with a physiologically based prediction that depends on crossover temperature. Although monthly average temperatures have been used in many studies to predict %C 4, our analysis shows that high temperatures are better predictors of %C 4. In particular, we found that July climate (average of daily high temperature and month's total rainfall) predicted %C 4 better than other months, seasons or annual averages, suggesting that the outcome of competition between C 3 and C 4 plants in North American grasslands was particularly sensitive to climate during this narrow window of time. Root δ13C increased about 1‰ between the A and B horizon, suggesting that C 4 roots become relatively more common than C 3 roots with depth. These differences in depth distribution likely contribute to the isotopic enrichment with depth in SOM where both C 3 and C 4 grasses are present. 相似文献
20.
Grasses with the C 3 photosynthetic pathway are commonly considered to be more nutritious host plants than C 4 grasses, but the nutritional quality of C 3 grasses is also more greatly impacted by elevated atmospheric CO 2 than is that of C 4 grasses; C 3 grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C 4 grasses under elevated CO 2. Will C 3 grasses remain nutritionally superior to C 4 grasses under elevated CO 2 levels? We addressed this question by determining whether levels of protein in C 3 grasses decline to similar levels as in C 4 grasses, and whether total carbohydrate : protein ratios become similar in C 3 and C 4 grasses under elevated CO 2. In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C 3 grasses, levels of fructan respond most strongly to elevated CO 2. Five C 3 and five C 4 grass species were grown from seed in outdoor open‐top chambers at ambient (370 ppm) or elevated (740 ppm) CO 2 for 2 months. As expected, a significant increase in sugars, starch and fructan in the C 3 grasses under elevated CO 2 was associated with a significant reduction in their protein levels, while protein levels in most C 4 grasses were little affected by elevated CO 2. However, this differential response of the two types of grasses was insufficient to reduce protein in C 3 grasses to the levels in C 4 grasses. Although levels of fructan in the C 3 grasses tripled under elevated CO 2, the amounts produced remained relatively low, both in absolute terms and as a fraction of the total nonstructural carbohydrates in the C 3 grasses. We conclude that C 3 grasses will generally remain more nutritious than C 4 grasses at elevated CO 2 concentrations, having higher levels of protein, nonstructural carbohydrates, and water, but lower levels of fiber and toughness, and lower total carbohydrate : protein ratios than C 4 grasses. 相似文献
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