CAM photosynthesis in submerged aquatic plants

Crassulacean acid metabolism (CAM) is a CO₂-concentrating mechanism selected in response to aridity in terrestrial habitats, and, in aquatic environments, to ambient limitations of carbon. Evidence is reviewed for its presence in five genera of aquatic vascular plants, includingIsoëtes, Sagittaria, Vallisneria, Crassula, andLittorella. Initially, aquatic CAM was considered by some to be an oxymoron, but some aquatic species have been studied in sufficient detail to say definitively that they possess CAM photosynthesis. CO₂-concentrating mechanisms in photosynthetic organs require a barrier to leakage; e.g., terrestrial C₄ plants have suberized bundle sheath cells and terrestrial CAM plants high stomatal resistance. In aquatic CAM plants the primary barrier to CO₂ leakage is the extremely high difrusional resistance of water. This, coupled with the sink provided by extensive intercellular gas space, generates daytime CO₂(pi) comparable to terrestrial CAM plants. CAM contributes to the carbon budget by both net carbon gain and carbon recycling, and the magnitude of each is environmentally influenced. Aquatic CAM plants inhabit sites where photosynthesis is potentially limited by carbon. Many occupy moderately fertile shallow temporary pools that experience extreme diel fluctuations in carbon availability. CAM plants are able to take advantage of elevated nighttime CO2 levels in these habitats. This gives them a competitive advantage over non-CAM species that are carbon starved during the day and an advantage over species that expend energy in membrane transport of bicarbonate. Some aquatic CAM plants are distributed in highly infertile lakes, where extreme carbon limitation and light are important selective factors. Compilation of reports on diel changes in titratable acidity and malate show 69 out of 180 species have significant overnight accumulation, although evidence is presented discounting CAM in some. It is concluded that similar proportions of the aquatic and terrestrial floras have evolved CAM photosynthesis. AquaticIsoëtes (Lycophyta) represent the oldest lineage of CAM plants and cladistic analysis supports an origin for CAM in seasonal wetlands, from which it has radiated into oligotrophic lakes and into terrestrial habitats. Temperate Zone terrestrial species share many characteristics with amphibious ancestors, which in their temporary terrestrial stage, produce functional stomata and switch from CAM to C₃. Many lacustrineIsoëtes have retained the phenotypic plasticity of amphibious species and can adapt to an aerial environment by development of stomata and switching to C₃. However, in some neotropical alpine species, adaptations to the lacustrine environment are genetically fixed and these constitutive species fail to produce stomata or loose CAM when artificially maintained in an aerial environment. It is hypothesized that neotropical lacustrine species may be more ancient in origin and have given rise to terrestrial species, which have retained most of the characteristics of their aquatic ancestry, including astomatous leaves, CAM and sediment-based carbon nutrition.     

Evolutionary physiology: the extent of C4 and CAM photosynthesis in the genera Anacampseros and Grahamia of the Portulacaceae

The Portulacaceae is one of the few terrestrial plant families known to have both C(4) and Crassulacean acid metabolism (CAM) species. There may be multiple origins of the evolution of CAM within the Portulacaceae but the only clear evidence of C(4) photosynthesis is found in members of the genus Portulaca. In the Portulaca, CAM succulent tissue is overlaid with the C(4) tissue in a unique fashion where both pathways are operating simultaneously. Earlier reports have shown that the clade containing the genera Anacampseros and Grahamia may also contain C(4) photosynthetic species similar to the Portulaca, which would indicate multiple origins of C(4) photosynthesis within the family. The aim of the present study was to ascertain the true photosynthetic nature of these genera. An initial survey of the carbon isotope composition of the Anacampseros ranged from -12.6 per thousand to -24.0 per thousand, indicating very little CAM activity in some species, with other values close to the C(4) range. Anacampseros (=Grahamia) australiana which had been previously identified as a C(4) species had a carbon isotope composition value of -24.0 per thousand, which is more indicative of a C(3) species with a slight contribution of CAM activity. Other Anacampseros species with C(4)-like values have been shown to be CAM plants. The initial isotope analysis of the Grahamia species gave values in the range of -27.1 per thousand to -23.6 per thousand, placing the Grahamia species well towards the C(3) photosynthetic range. Further physiological studies indicated increased night-time CO(2) uptake with imposition of water stress, associated with a large diurnal acid fluctuation and a marked increased phosphoenolpyruvate carboxylase activity. This showed that the Grahamia species are actually facultative CAM plants despite their C(3)-like carbon isotope values. The results indicate that the Grahamia and Anacampseros species do not utilize the C(4) photosynthetic pathway. This is the first to identify that the Grahamia species are facultative CAM plants where CAM can be induced by water stress. This work supports earlier physiological work that indicates that this clade containing Anacampseros and Grahamia species comprises predominantly facultative CAM plants. This report suggests there may be only one clade which contains C(4) photosynthetic members with CAM-like characteristics.     

Carbon: freshwater plants

δ¹³C values for freshwater aquatic plant matter varies from ?11 to ?50‰ and is not a clear indicator of photosynthetic pathway as in terrestrial plants. Several factors affect δ¹³C of aquatic plant matter. These include: (1) The δ¹³C signature of the source carbon has been observed to range from +1‰ for HCO₃^? derived from limestone to ?30‰ for CO₂ derived from respiration. (2) Some plants assimilate HCO₃^?, which is –7 to –11‰ less negative than CO₂. (3) C₃, C₄, and CAM photosynthetic pathways are present in aquatic plants. (4) Diffusional resistances are orders of magnitude greater in the aquatic environment than in the aerial environment. The greater viscosity of water acts to reduce mixing of the carbon pool in the boundary layer with that of the bulk solution. In effect, many aquatic plants draw from a finite carbon pool, and as in terrestrial plants growing in a closed system, biochemical discrimination is reduced. In standing water, this factor results in most aquatic plants having a δ¹³C value similar to the source carbon. Using Farquhar's equation and other physiological data, it is possible to use δ¹³C values to evaluate various parameters affecting photosynthesis, such as limitations imposed by CO₂ diffusion and carbon source.     

Is C4 photosynthesis less phenotypically plastic than C3 photosynthesis?

C4 photosynthesis is a complex specialization that enhances carbon gain in hot, often arid habitats where photorespiration rates can be high. Certain features unique to C4 photosynthesis may reduce the potential for phenotypic plasticity and photosynthetic acclimation to environmental change relative to what is possible with C3 photosynthesis. During acclimation, the structural and physiological integrity of the mesophyll-bundle sheath (M-BS) complex has to be maintained if C4 photosynthesis is to function efficiently in the new environment. Disruption of the M-BS structure could interfere with metabolic co-ordination between the C3 and C4 cycles, decrease metabolite flow rate between the tissues, increase CO2 leakage from the bundle sheath, and slow enzyme activity. C4 plants have substantial acclimation potential, but in most cases lag behind the acclimation responses in C3 plants. For example, some C4 species are unable to maintain high quantum yields when grown in low-light conditions. Others fail to reduce carboxylase content in shade, leaving substantial over-capacity of Rubisco and PEP carboxylase in place. Shade-tolerant C4 grasses lack the capacity for maintaining a high state of photosynthetic induction following sunflecks, and thus may be poorly suited to exploit subsequent sunflecks compared with C3 species. In total, the evidence indicates that C4 photosynthesis is less phenotypically plastic than C3 photosynthesis, and this may contribute to the more restricted ecological and geographical distribution of C4 plants across the Earth.     

CO<Subscript>2</Subscript> sequestration in plants: lesson from divergent strategies

Most organisms inhabiting earth feed directly or indirectly on the products synthesized by the reaction of photosynthesis, which at the current atmospheric CO₂ levels operates only at two thirds of its peak efficiency. Restricting the photorespiratory loss of carbon and thereby improving the efficiency of photosynthesis is seen by many as a good option to enhance productivity of food crops. Research during last half a century has shown that several plant species developed CO₂-concentrating mechanism (CCM) to restrict photorespiration under lower concentration of available CO₂. CCMs are now known to be operative in several terrestrial and aquatic plants, ranging from most advanced higher plants to algae, cyanobacteria and diatoms. Plants with C₄ pathway of photosynthesis (where four-carbon compound is the first product of photosynthesis) or crassulacean acid metabolism (CAM) may consistently operate CCM. Some plants however can undergo a shift in photosynthetic metabolism only with change in environmental variables. More recently, a shift in plant photosynthetic metabolism is reported at high altitude where improved efficiency of CO₂ uptake is related to the recapture of photorespiratory loss of carbon. Of the divergent CO₂ assimilation strategies operative in different oraganisms, the capacity to recapture photorespiratory CO₂ could be an important approach to develop plants with efficient photosynthetic capacity.     

Ectopic expression of C 4 photosynthetic pathway genes improves carbon assimilation and alleviate stress tolerance for future climate change

Alteration in atmospheric carbon dioxide concentration and other environmental factors are the significant cues of global climate change. Environmental factors affect the most fundamental biological process including photosynthesis and different metabolic pathways. The feeding of the rapidly growing world population is another challenge which imposes pressure to improve productivity and quality of the existing crops. C4 plants are considered the most productive, containing lower photorespiration, and higher water-use & N-assimilation efficiencies, compared to C3 plants. Besides, the C4-photosynthetic genes not only play an important role in carbon assimilation but also modulate abiotic stresses. In this review, fundamental three metabolic processes (C4, C3, and CAM) of carbon dioxide assimilation, the evolution of C4-photosynthetic genes, effect of elevated CO2 on photosynthesis, and overexpression of C4-photosynthetic genes for higher photosynthesis were discussed. Kranz-anatomy is considered an essential prerequisite for the terrestrial C4 carbon assimilation, but single-celled C4 plant species changed this well-established paradigm. C4 plants are insensitive to an elevated CO2 stress condition but performed better under stress conditions. Overexpression of essential C4-photosynthetic genes such as PEPC, PPDK, and NADP-ME in C3 plants like Arabidopsis, tobacco, rice, wheat, and potato not only improved photosynthesis but also provided tolerance to various environmental stresses, especially drought. The review provides useful information for sustainable productivity and yield under elevated CO2 environment, which to be explored further for CO2 assimilation and also abiotic stress tolerance. Additionally, it provides a better understanding to explore C4-photosynthetic gene(s) to cope with global warming and prospective adverse climatic changes.     

Quantum yield variation across the three pathways of photosynthesis: not yet out of the dark

The convergent quantum yield hypothesis (CQY) assumes that thermodynamics and natural selection jointly limit variation in the maximum energetic efficiency of photosynthesis in low light under otherwise specified conditions (e.g. temperature and CO(2) concentration). A literature survey of photosynthetic quantum yield (phi) studies in terrestrial plants from C(3), C(4), and CAM photosynthetic types was conducted to test the CQY hypothesis. Broad variation in phi values from C(3) plants could partially be explained by accounting for whether the measuring conditions were permissive or restrictive for photorespiration. Assimilatory quotients (AQ), calculated from the CO(2) phi:O(2) phi ratios, indicated that 49% and 29% of absorbed light energy was allocated to carbon fixation and photorespiration in C(3) plants, respectively. The unexplained remainder (22%) may represent diversion to various other energy-demanding processes (e.g. starch synthesis, nitrogen assimilation). Individual and cumulative effects of these other processes on photosynthetic efficiency are poorly quantified. In C(4) plants, little variation in phi values was observed, consistent with the fact that C(4) plants exhibit little photorespiration. As before, AQ values indicate that 22% of absorbed light energy cannot be accounted for by carbon fixation in C(4) plants. Among all three photosynthetic types, the phi of photosynthesis in CAM plants is the least studied, appears to be highly variable, and may present the greatest challenge to the CQY hypothesis. The high amount of energy diverted to processes other than carbon fixation in C(3) and C(4) plants and the poor characterization of photosynthetic efficiency in CAM plants are significant deficiencies in our otherwise robust understanding of the energetics of terrestrial photoautotrophy.     

Variations in Kinetic Properties of Ribulose-1,5-bisphosphate Carboxylases among Plants

Studies of ribulose-1,5-bisphosphate (RuBP) carboxylase from taxonomically diverse plants show that the enzyme from C(3) and crassulacean acid metabolism pathway species exhibits lower K(m)(CO(2)) values (12-25 micromolar) than does that from C(4) species (28-34 micromolar). RuBP carboxylase from aquatic angiosperms, an aquatic bryophyte, fresh water and marine algae has yielded consistently high K(m)(CO(2)) values (30-70 micromolar), similar in range to that of the enzyme from C(4) terrestrial plants. This variation in K(m)(CO(2)) is discussed in relation to the correlation between the existence of CO(2)-concentrating mechanisms for photosynthesis and the affinity of the enzyme for CO(2). The K(m)(RuBP) of the enzyme from various sources ranges from 10 to 136 micromolar; mean +/- sd = 36 +/- 20 micromolar. This variation in K(m)(RuBP) does not correlate with different photosynthetic pathways, but shows taxonomic patterns. Among the dicotyledons, the enzyme from crassinucellate species exhibits lower K(m)(RuBP) (18 +/- 4 micromolar) than does that from tenuinucellate species (25 +/- 7 micromolar). Among the Poaceae, RuBP carboxylase from Triticeae, chloridoids, andropogonoids, Microlaena, and Tetrarrhena has yielded lower K(m)(RuBP) values (29 +/- 11 micromolar) than has that from other members of the grass family (46 +/- 10 micromolar).     

Climate change and the evolution of C(4) photosynthesis

Plants assimilate carbon by one of three photosynthetic pathways, commonly called the C(3), C(4), and CAM pathways. The C(4) photosynthetic pathway, found only among the angiosperms, represents a modification of C(3) metabolism that is most effective at low concentrations of CO(2). Today, C(4) plants are most common in hot, open ecosystems, and it is commonly felt that they evolved under these conditions. However, high light and high temperature, by themselves, are not sufficient to favor the evolution of C(4) photosynthesis at atmospheric CO(2) levels significantly above the current ambient values. A review of evidence suggests that C(4) plants evolved in response to a reduction in atmospheric CO(2) levels that began during the Cretaceous and continued until the Miocene.     

Proof of C4 photosynthesis without Kranz anatomy in Bienertia cycloptera (Chenopodiaceae)

Kranz anatomy, with its separation of elements of the C4 pathway between two cells, has been an accepted criterion for function of C4 photosynthesis in terrestrial plants. However, Bienertia cycloptera (Chenopodiaceae), which grows in salty depressions of Central Asian semi-deserts, has unusual chlorenchyma, lacks Kranz anatomy, but has photosynthetic features of C4 plants. Its photosynthetic response to varying CO2 and O2 is typical of C4 plants having Kranz anatomy. Lack of night-time CO2 fixation indicates it is not acquiring carbon by Crassulacean acid metabolism. This species exhibits an independent, novel solution to function of the C4 mechanism through spatial compartmentation of dimorphic chloroplasts, other organelles and photosynthetic enzymes in distinct positions within a single chlorenchyma cell. The chlorenchyma cells have a large, spherical central cytoplasmic compartment interconnected by cytoplasmic channels through the vacuole to the peripheral cytoplasm. This compartment is filled with mitochondria and granal chloroplasts, while the peripheral cytoplasm apparently lacks mitochondria and has grana-deficient chloroplasts. Immunolocalization studies show enzymes compartmentalized selectively in the CC compartment, including Rubisco in chloroplasts, and NAD-malic enzyme and glycine decarboxylase in mitochondria, whereas pyruvate, Pi dikinase of the C4 cycle is localized selectively in peripheral chloroplasts. Phosphoenolpyruvate carboxylase, a cytosolic C4 cycle enzyme, is enriched in the peripheral cytoplasm. Our results show Bienertia utilizes strict compartmentation of organelles and enzymes within a single cell to effectively mimic the spatial separation of Kranz anatomy, allowing it to function as a C4 plant having suppressed photorespiration; this raises interesting questions about evolution of C4 mechanisms.     

Molecular evolution and genetic engineering of C4 photosynthetic enzymes

The majority of terrestrial plants, including many important crops such as rice, wheat, soybean, and potato, are classified as C(3) plants that assimilate atmospheric CO(2) directly through the C(3) photosynthetic pathway. C(4) plants, such as maize and sugarcane, evolved from C(3) plants, acquiring the C(4) photosynthetic pathway in addition to the C(3) pathway to achieve high photosynthetic performance and high water- and nitrogen-use efficiencies. Consequently, the transfer of C(4) traits to C(3) plants is one strategy being adopted for improving the photosynthetic performance of C(3) plants. The recent application of recombinant DNA technology has made considerable progress in the molecular engineering of photosynthetic genes in the past ten years. It has deepened understanding of the evolutionary scenario of the C(4) photosynthetic genes. The strategy, based on the evolutionary scenario, has enabled enzymes involved in the C(4) pathway to be expressed at high levels and in desired locations in the leaves of C(3) plants. Although overproduction of a single C(4) enzyme can alter the carbon metabolism of C(3) plants, it does not show any positive effects on photosynthesis. Transgenic C(3) plants overproducing multiple enzymes are now being produced for improving the photosynthetic performance of C(3) plants.     

Comparison of the photosynthetic characteristics of three submersed aquatic plants

Light- and CO(2)-saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum, and Myriophyllum spicatum were 50 to 60 mumol O(2)/mg Chl.hr at 30 C. At air levels of CO(2), the rates were less than 5% of those achieved by terrestrial C(3) plants. The low photosynthetic rates correlated with low activities of the carboxylation enzymes. In each species, ribulose 1,5-diphosphate carboxylase was the predominant carboxylation enzyme. The apparent K(m)(CO(2)) values for photosynthesis were 150 to 170 mum at pH 4, and 75 to 95 mum at pH 8. The K(m)(CO(2)) of Hydrilla ribulose 1,5-diphosphate carboxylase was 45 mum at pH 8. Optimum temperatures for the photosynthesis of Hydrilla, Myriophyllum, and Ceratophyllum were 36.5, 35.0, and 28.5 C, respectively. The apparent ability of each species to use HCO(3) (-) ions for photosynthesis was similar, but at saturating free CO(2) levels, there was no indication of HCO(3) (-) use. Increasing the pH from 3.1 to 9.2 affected the photosynthetic rate indirectly, by decreasing the free CO(2). With saturating free CO(2) (0.5 mm), the maximum photosynthetic rates were similar at pH 4 and 8. Carbonic anhydrase activity, although much lower than in terrestrial C(3) plants, was still in excess of that required to support HCO(3) (-) utilization.Hydrilla and Ceratophyllum had CO(2) compensation points of 44 and 41 mul/l, respectively, whereas the value for Myriophyllum was 19. Relatively high CO(2) compensation points under 1% O(2) indicated that some "dark" respiration occurred in the light. The inhibition of photosynthesis by O(2) was less than with terrestrial C(3) plants. Glycolate oxidase activity was 12.3 to 27.5 mumol O(2)/mg Chl.hr, as compared to 78.4 for spinach. Light saturation of photosynthesis occurred at 600 to 700 mueinsteins/m(2).sec in each species grown under full sunlight. Hydrilla had the lowest light compensation point, and required the least irradiance to achieve the half-maximal photosynthetic rate.Field measurements in a Hydrilla mat indicated that in the afternoon, free CO(2) dropped to zero, and O(2) rose to over 200% air saturation. Most photosynthetic activity occurred in the morning when the free CO(2) was highest and O(2) and solar radiation lowest. The low light requirement of Hydrilla probably provides a competitive advantage under these field conditions.     

Mixotrophy everywhere on land and in water: the grand écart hypothesis

There is increasing awareness that many terrestrial and aquatic organisms are not strictly heterotrophic or autotrophic but rather mixotrophic. Mixotrophy is an intermediate nutritional strategy, merging autotrophy and heterotrophy to acquire organic carbon and/or other elements, mainly N, P or Fe. We show that both terrestrial and aquatic mixotrophs fall into three categories, namely necrotrophic (where autotrophs prey on other organisms), biotrophic (where heterotrophs gain autotrophy by symbiosis) and absorbotrophic (where autotrophs take up environmental organic molecules). Here we discuss their physiological and ecological relevance since mixotrophy is found in virtually every ecosystem and occurs across the whole eukaryotic phylogeny, suggesting an evolutionary pressure towards mixotrophy. Ecosystem dynamics tend to separate light from non‐carbon nutrients (N and P resources): the biological pump and water stratification in aquatic ecosystems deplete non‐carbon nutrients from the photic zone, while terrestrial plant successions create a canopy layer with light but devoid of non‐carbon soil nutrients. In both aquatic and terrestrial environments organisms face a grand écart (dancer's splits, i.e., the need to reconcile two opposing needs) between optimal conditions for photosynthesis vs. gain of non‐carbon elements. We suggest that mixotrophy allows adaptation of organisms to such ubiquist environmental gradients, ultimately explaining why mixotrophic strategies are widespread.     

Crassulacean acid metabolism photosynthesis in Bromeliaceae: an evolutionary key innovation

Crassulacean acid metabolism (CAM) is a photosynthetic pathway that significantly increases water use efficiency in plants. It has been proposed that CAM photosynthesis, which evolved from the ancestral C3 pathway, has played a role in the diversification of some prominent plant groups because it may have allowed them to colonize and successfully spread into arid or semi‐arid environments. However, the hypothesis that CAM photosynthesis constitutes an evolutionary key innovation, thereby enhancing diversification rates of the clades possessing it, has not been evaluated quantitatively. We tested whether CAM photosynthesis is a key innovation in the Bromeliaceae, a large and highly diversified plant family that has successfully colonized arid environments. We identified five pairs of sister groups with and without the CAM feature, including 31 genera and over 2000 species. In all five cases, the clades with CAM photosynthesis were more diverse than their C3 counterparts. We provide quantitative evidence that the evolution of CAM photosynthesis is significantly associated with increased diversification in Bromeliaceae and thus constitutes an evolutionary key innovation. We also found preliminary evidence of an association between the CAM pathway and growth habit in bromeliads, with terrestrial species being more likely to show CAM photosynthesis than epiphytic species. To our knowledge, this is the first case of a physiological attribute shown to be a key innovation in plants. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 480–486.     

Le vie fotosintetiche C3, C4 e CAM nel contesto ecologico

Abstract

Ecological aspects of C₃, C₄ and CAM photosynthetic pathways. - Three different photosynthetic CO₂ fixation pathways are known to occur in higher plants. However all three pathways ultimately depend on the Calvin-Benson cycle for carbon reduction. The oxygenase activity of RuBP carboxilase is responsible for photorespiratory CO₂ release. Both C₄ and CAM pathways behave as a CO₂ concentrating mechanism which prevent photorespiration. The CO₂-concentrating mechanism in C₄ plants is based on intracellular symplastic transport of C₄ dicarboxylic acids from mesophyll-cells to the adjacent bundle-sheath cells. On the contrary in CAM plants the CO₂-concentrating mechanism is based on the intracellular transport of malic acid into and out of the vacuole.

The C₄ photosynthetic pathway as compared to the C₃ pathway permits higher rates of CO₂ fixation in high light and high temperature environments at low costs in terms of water loss, given the stability of the photosynthetic apparatus under such conditions.

CAM is interpreted as an adaptation to arid environments because it enables carbon assimilation to take place at very low water costs during the night when the evaporative demand is low. Nevertheless many aquatic species of Isoetes and some relatives are CAM, suggesting the adaptive role of CAM to environments which become depleted in CO₂.

The photosynthetic carbon fixation pathway certainly contributes to the ecological success of plants in different environments. However the distribution of plants may also reflect their biological history. On the other hand plants with different photosynthetic pathways coexist in many communities and tend to share resources in time. In any case some generalizations are possible: C₄ plants enjoy an ecological advantage in hot, moist, high light regions while the majority of species in desert environments are C₃; CAM plants are more frequent in semiarid regions with seasonal rainfall, coastal fog deserts, and in epiphytic habitats in tropical rain forests.     

Underwater photosynthesis in flooded terrestrial plants: a matter of leaf plasticity

BACKGROUND: Flooding causes substantial stress for terrestrial plants, particularly if the floodwater completely submerges the shoot. The main problems during submergence are shortage of oxygen due to the slow diffusion rates of gases in water, and depletion of carbohydrates, which is the substrate for respiration. These two factors together lead to loss of biomass and eventually death of the submerged plants. Although conditions under water are unfavourable with respect to light and carbon dioxide supply, photosynthesis may provide both oxygen and carbohydrates, resulting in continuation of aerobic respiration. SCOPE: This review focuses on evidence in the literature that photosynthesis contributes to survival of terrestrial plants during complete submergence. Furthermore, we discuss relevant morphological and physiological responses of the shoot of terrestrial plant species that enable the positive effects of light on underwater plant performance. CONCLUSIONS: Light increases the survival of terrestrial plants under water, indicating that photosynthesis commonly occurs under these submerged conditions. Such underwater photosynthesis increases both internal oxygen concentrations and carbohydrate contents, compared with plants submerged in the dark, and thereby alleviates the adverse effects of flooding. Additionally, several terrestrial species show high plasticity with respect to their leaf development. In a number of species, leaf morphology changes in response to submergence, probably to facilitate underwater gas exchange. Such increased gas exchange may result in higher assimilation rates, and lower carbon dioxide compensation points under water, which is particularly important at the low carbon dioxide concentrations observed in the field. As a result of higher internal carbon dioxide concentrations in submergence-acclimated plants, underwater photorespiration rates are expected to be lower than in non-acclimated plants. Furthermore, the regulatory mechanisms that induce the switch from terrestrial to submergence-acclimated leaves may be controlled by the same pathways as described for heterophyllous aquatic plants.     

Does the different photosynthetic pathway of plants affect soil respiration in a subtropical wetland?

Plants with different photosynthetic pathways could produce different amounts and types of root exudates and debris which may affect soil respiration rates. Therefore, wetland vegetation succession between plants with different photosynthetic pathways may ultimately influence the wetland carbon budget. The middle and lower reaches of the Yangtze River has the largest floodplain wetland group in China. Tian'e Zhou wetland reserve (29°48'N, 112°33′E) is located in Shishou city, Hubei province and covers about 77.5 square kilometers. Hemathria altissima (C4) was found gradually being replaced by Carex argyi (C3) for several years in this place. An in situ experiment was conducted in Tian'e Zhou wetland to determine the change of soil respiration as the succession proceeds. Soil respiration, substrate‐induced respiration, and bacterial respiration of the C4 species was greater than those of the C3 species, but below‐ground biomass and fungal respiration of the C4 species was less than that of the C3 species. There were no significant differences in above‐ground biomass between the two species. Due to the higher photosynthesis capability, higher soil respiration and lower total plant biomass, we inferred that the C4 species, H. altissima, may transport more photosynthate below‐ground as a substrate for respiration. The photosynthetic pathway of plants might therefore play an important role in regulating soil respiration. As C. argyi replaces H. altissima, the larger plant biomass and lower soil respiration would indicate that the wetland in this area could fix more carbon in the soil than before.     

A critical review on the improvement of photosynthetic carbon assimilation in C3 plants using genetic engineering

Global warming is one of the most serious challenges facing us today. It may be linked to the increase in atmospheric CO2 and other greenhouse gases (GHGs), leading to a rise in sea level, notable shifts in ecosystems, and in the frequency and intensity of wild fires. There is a strong interest in stabilizing the atmospheric concentration of CO2 and other GHGs by decreasing carbon emission and/or increasing carbon sequestration. Biotic sequestration is an important and effective strategy to mitigate the effects of rising atmospheric CO2 concentrations by increasing carbon sequestration and storage capacity of ecosystems using plant photosynthesis and by decreasing carbon emission using biofuel rather than fossil fuel. Improvement of photosynthetic carbon assimilation, using transgenic engineering, potentially provides a set of available and effective tools for enhancing plant carbon sequestration. In this review, firstly different biological methods of CO2 assimilation in C3, C4 and CAM plants are introduced and three types of C4 pathways which have high photosynthetic performance and have evolved as CO2 pumps are briefly summarized. Then (i) the improvement of photosynthetic carbon assimilation of C3 plants by transgenic engineering using non-C4 genes, and (ii) the overexpression of individual or multiple C4 cycle photosynthetic genes (PEPC, PPDK, PCK, NADP-ME and NADP-MDH) in transgenic C3 plants (e.g. tobacco, potato, rice and Arabidopsis) are highlighted. Some transgenic C3 plants (e.g. tobacco, rice and Arabidopsis) overexpressing the FBP/SBPase, ictB and cytochrome c6 genes showed positive effects on photosynthetic efficiency and growth characteristics. However, over the last 28 years, efforts to overexpress individual, double or multiple C4 enzymes in C3 plants like tobacco, potato, rice, and Arabidopsis have produced mixed results that do not confirm or eliminate the possibility of improving photosynthesis of C3 plants by this approach. Finally, a prospect is provided on the challenges of enhancing carbon assimilation of C3 plants using transgenic engineering in the face of global warming, and the trends of the most promising approaches to improving the photosynthetic performance of C3 plants.     

Acclimation of a terrestrial plant to submergence facilitates gas exchange under water

Flooding imposes stress upon terrestrial plants since it severely hampers gas exchange rates between the shoot and the environment. The resulting oxygen deficiency is considered to be the major problem for submerged plants. Oxygen microelectrode studies have, however, shown that aquatic plants maintain relatively high internal oxygen pressures under water, and even may release oxygen via the roots into the sediment, also in dark. Based on these results, we challenge the dogma that oxygen pressures in submerged terrestrial plants immediately drop to levels at which aerobic respiration is impaired. The present study demonstrates that the internal oxygen pressure in the petioles of Rumex palustris plants under water is indeed well above the critical oxygen pressure for aerobic respiration, provided that the air‐saturated water is not completely stagnant. The beneficial effect of shoot acclimation of this terrestrial plant species to submergence for gas exchange capacity is also shown. Shoot acclimation to submergence involved a reduction of the diffusion resistance to gases, which was not only functional by increasing diffusion of oxygen into the plant, but also by increasing influx of CO₂, which enhances underwater photosynthesis.