Mesophyll Chloroplast Investment in C3, C4 and C2 Species of the Genus Flaveria.

The mesophyll (M) cells of C4 plants contain fewer chloroplasts than observed in related C3 plants; however, it is uncertain where along the evolutionary transition from C3 to C4 that the reduction in M chloroplast number occurs. Using 18 species in the genus Flaveria, which contains C3, C4 and a range of C3-C4 intermediate species, we examined changes in chloroplast number and size per M cell, and positioning of chloroplasts relative to the M cell periphery. Chloroplast number and coverage of the M cell periphery declined in proportion to increasing strength of C4 metabolism in Flaveria, while chloroplast size increased with increasing C4 cycle strength. These changes increase cytosolic exposure to the cell periphery which could enhance diffusion of inorganic carbon to phosphenolpyruvate carboxylase (PEPC), a cytosolic enzyme. Analysis of the transcriptome from juvenile leaves of nine Flaveria species showed that the transcript abundance of four genes involved in plastid biogenesis-FtsZ1, FtsZ2, DRP5B and PARC6-was negatively correlated with variation in C4 cycle strength and positively correlated with M chloroplast number per planar cell area. Chloroplast size was negatively correlated with abundance of FtsZ1, FtsZ2 and PARC6 transcripts. These results indicate that natural selection targeted the proteins of the contractile ring assembly to effect the reduction in chloroplast numbers in the M cells of C4 Flaveria species. If so, efforts to engineer the C4 pathway into C3 plants might evaluate whether inducing transcriptome changes similar to those observed in Flaveria could reduce M chloroplast numbers, and thus introduce a trait that appears essential for efficient C4 function.

Promotion of Cyclic Electron Transport Around Photosystem I with the Development of C4 Photosynthesis.

C4 photosynthesis is present in approximately 7,500 species classified into 19 families, including monocots and eudicots. In the majority of documented cases, a two-celled CO2-concentrating system that uses a metabolic cycle of four-carbon compounds is employed. C4 photosynthesis repeatedly evolved from C3 photosynthesis, possibly driven by the survival advantages it bestows in the hot, often dry, and nutrient-poor soils of the tropics and subtropics. The development of the C4 metabolic cycle greatly increased the ATP demand in chloroplasts during the evolution of malic enzyme-type C4 photosynthesis, and the additional ATP required for C4 metabolism may be produced by the cyclic electron transport around PSI. Recent studies have revealed the nature of cyclic electron transport and the elevation of its components during C4 evolution. In this review, we discuss the energy requirements of C3 and C4 photosynthesis, the current model of cyclic electron transport around PSI and how cyclic electron transport is promoted during C4 evolution using studies on the genus Flaveria, which contains a number of closely related C3, C4 and C3-C4 intermediate species.

Carbon isotope discrimination as a diagnostic tool for C4 photosynthesis in C3-C4 intermediate species.

The presence and activity of the C4 cycle in C3-C4 intermediate species have proven difficult to analyze, especially when such activity is low. This study proposes a strategy to detect C4 activity and estimate its contribution to overall photosynthesis in intermediate plants, by using tunable diode laser absorption spectroscopy (TDLAS) coupled to gas exchange systems to simultaneously measure the CO2 responses of CO2 assimilation (A) and carbon isotope discrimination (Δ) under low O2 partial pressure. Mathematical models of C3-C4 photosynthesis and Δ are then fitted concurrently to both responses using the same set of constants. This strategy was applied to the intermediate species Flaveria floridana and F. brownii, and to F. pringlei and F. bidentis as C3 and C4 controls, respectively. Our results support the presence of a functional C4 cycle in F. floridana, that can fix 12-21% of carbon. In F. brownii, 75-100% of carbon is fixed via the C4 cycle, and the contribution of mesophyll Rubisco to overall carbon assimilation increases with CO2 partial pressure in both intermediate plants. Combined gas exchange and Δ measurement and modeling is a powerful diagnostic tool for C4 photosynthesis.

Tracking the evolutionary rise of C4 metabolism.

Upregulation of the C4 metabolic cycle is a major step in the evolution of C4 photosynthesis. Why this happened remains unclear, in part because of difficulties measuring the C4 cycle in situ in C3-C4 intermediate species. Now, Alonso-Cantabrana and von Caemmerer (2016) have described a new approach for quantifying C4 cycle activity, thereby providing the means to analyze its upregulation in an evolutionary context.

RNA-Seq based phylogeny recapitulates previous phylogeny of the genus Flaveria (Asteraceae) with some modifications.

BACKGROUND: The genus Flaveria has been extensively used as a model to study the evolution of C4 photosynthesis as it contains C3 and C4 species as well as a number of species that exhibit intermediate types of photosynthesis. The current phylogenetic tree of the genus Flaveria contains 21 of the 23 known Flaveria species and has been previously constructed using a combination of morphological data and three non-coding DNA sequences (nuclear encoded ETS, ITS and chloroplast encoded trnL-F). RESULTS: Here we developed a new strategy to update the phylogenetic tree of 16 Flaveria species based on RNA-Seq data. The updated phylogeny is largely congruent with the previously published tree but with some modifications. We propose that the data collection method provided in this study can be used as a generic method for phylogenetic tree reconstruction if the target species has no genomic information. We also showed that a "F. pringlei" genotype recently used in a number of labs may be a hybrid between F. pringlei (C3) and F. angustifolia (C3-C4). CONCLUSIONS: We propose that the new strategy of obtaining phylogenetic sequences outlined in this study can be used to construct robust trees in a larger number of taxa. The updated Flaveria phylogenetic tree also supports a hypothesis of stepwise and parallel evolution of C4 photosynthesis in the Flavaria clade.

Increasing water use efficiency along the C3 to C4 evolutionary pathway: a stomatal optimization perspective.

C4 photosynthesis evolved independently numerous times, probably in response to declining atmospheric CO2 concentrations, but also to high temperatures and aridity, which enhance water losses through transpiration. Here, the environmental factors controlling stomatal behaviour of leaf-level carbon and water exchange were examined across the evolutionary continuum from C3 to C4 photosynthesis at current (400 µmol mol(-1)) and low (280 µmol mol(-1)) atmospheric CO2 conditions. To this aim, a stomatal optimization model was further developed to describe the evolutionary continuum from C3 to C4 species within a unified framework. Data on C3, three categories of C3-C4 intermediates, and C4 Flaveria species were used to parameterize the stomatal model, including parameters for the marginal water use efficiency and the efficiency of the CO2-concentrating mechanism (or C4 pump); these two parameters are interpreted as traits reflecting the stomatal and photosynthetic adjustments during the C3 to C4 transformation. Neither the marginal water use efficiency nor the C4 pump strength changed significantly from C3 to early C3-C4 intermediate stages, but both traits significantly increased between early C3-C4 intermediates and the C4-like intermediates with an operational C4 cycle. At low CO2, net photosynthetic rates showed continuous increases from a C3 state, across the intermediates and towards C4 photosynthesis, but only C4-like intermediates and C4 species (with an operational C4 cycle) had higher water use efficiencies than C3 Flaveria. The results demonstrate that both the marginal water use efficiency and the C4 pump strength increase in C4 Flaveria to improve their photosynthesis and water use efficiency compared with C3 species. These findings emphasize that the advantage of the early intermediate stages is predominantly carbon based, not water related.

C2 photosynthesis generates about 3-fold elevated leaf CO2 levels in the C3-C4 intermediate species Flaveria pubescens.

Formation of a photorespiration-based CO2-concentrating mechanism in C3-C4 intermediate plants is seen as a prerequisite for the evolution of C4 photosynthesis, but it is not known how efficient this mechanism is. Here, using in vivo Rubisco carboxylation-to-oxygenation ratios as a proxy to assess relative intraplastidial CO2 levels is suggested. Such ratios were determined for the C3-C4 intermediate species Flaveria pubescens compared with the closely related C3 plant F. cronquistii and the C4 plant F. trinervia. To this end, a model was developed to describe the major carbon fluxes and metabolite pools involved in photosynthetic-photorespiratory carbon metabolism and used quantitatively to evaluate the labelling kinetics during short-term (14)CO2 incorporation. Our data suggest that the photorespiratory CO2 pump elevates the intraplastidial CO2 concentration about 3-fold in leaves of the C3-C4 intermediate species F. pubescens relative to the C3 species F. cronquistii.

Promotion of cyclic electron transport around photosystem I during the evolution of NADP-malic enzyme-type C4 photosynthesis in the genus Flaveria.

C4 plants display higher cyclic electron transport activity than C3 plants. This activity is suggested to be important for the production of ATPs required for C4 metabolism. To understand the process by which photosystem I (PSI) cyclic electron transport was promoted during C4 evolution, we conducted comparative analyses of the functionality of PSI cyclic electron transport among members of the genus Flaveria, which contains several C3, C3-C4 intermediate, C4-like and C4 species. The abundance of NDH-H, a subunit of NADH dehydrogenase-like complex, increased markedly in bundle sheath cells with the activity of the C4 cycle. By contrast, PROTON GRADIENT REGULATION5 (PGR5) and PGR5-LIKE1 increased in both mesophyll and bundle sheath cells in C4-like Flaveria palmeri and C4 species. Grana stacks were drastically reduced in bundle sheath chloroplasts of C4-like F. palmeri and C4 species; these species showed a marked increase in PSI cyclic electron transport activity. These results suggest that both the expression of proteins involved in PSI cyclic electron transport and changes in thylakoid structure contribute to the high activity of cyclic electron flow in NADP-malic enzyme-type C4 photosynthesis. We propose that these changes were important for the establishment of C4 photosynthesis from C3-C4 intermediate photosynthesis in Flaveria.

Antisense reduction of NADP-malic enzyme in Flaveria bidentis reduces flow of CO2 through the C4 cycle.

An antisense construct targeting the C(4) isoform of NADP-malic enzyme (ME), the primary enzyme decarboxylating malate in bundle sheath cells to supply CO(2) to Rubisco, was used to transform the dicot Flaveria bidentis. Transgenic plants (α-NADP-ME) exhibited a 34% to 75% reduction in NADP-ME activity relative to the wild type with no visible growth phenotype. We characterized the effect of reducing NADP-ME on photosynthesis by measuring in vitro photosynthetic enzyme activity, gas exchange, and real-time carbon isotope discrimination (Δ). In α-NADP-ME plants with less than 40% of wild-type NADP-ME activity, CO(2) assimilation rates at high intercellular CO(2) were significantly reduced, whereas the in vitro activities of both phosphoenolpyruvate carboxylase and Rubisco were increased. Δ measured concurrently with gas exchange in these plants showed a lower Δ and thus a lower calculated leakiness of CO(2) (the ratio of CO(2) leak rate from the bundle sheath to the rate of CO(2) supply). Comparative measurements on antisense Rubisco small subunit F. bidentis plants showed the opposite effect of increased Δ and leakiness. We use these measurements to estimate the C(4) cycle rate, bundle sheath leak rate, and bundle sheath CO(2) concentration. The comparison of α-NADP-ME and antisense Rubisco small subunit demonstrates that the coordination of the C(3) and C(4) cycles that exist during environmental perturbations by light and CO(2) can be disrupted through transgenic manipulations. Furthermore, our results suggest that the efficiency of the C(4) pathway could potentially be improved through a reduction in C(4) cycle activity or increased C(3) cycle activity.

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

The efficiency of C(4) photosynthesis in Zea mays, Miscanthus x giganteus and Flaveria bidentis in response to light was determined using measurements of gas exchange, (13) CO(2) photosynthetic discrimination, metabolite pools and spectroscopic assays, with models of C(4) photosynthesis and leaf (13) CO(2) discrimination. Spectroscopic and metabolite assays suggested constant energy partitioning between the C(4) and C(3) cycles across photosynthetically active radiation (PAR). Leakiness (φ), modelled using C(4) 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 [CO(2)] 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 C(4) cycle and low bundle-sheath cells CO(2 ) conductance (g(bs)). While F. bidentis had larger g(bs) but lower respiration rates and M. giganteus had less C(4) cycle capacity but low g(bs), which resulted in similar φ. This demonstrates that low g(bs) is important for efficient C(4) photosynthesis but it is not the only factor determining φ. Additionally, these C(4) species are able to optimize photosynthesis and minimize φ over a range of PARs, including low light.

Effects of low atmospheric CO2 and elevated temperature during growth on the gas exchange responses of C3, C3-C4 intermediate, and C4 species from three evolutionary lineages of C4 photosynthesis.

This study evaluates acclimation of photosynthesis and stomatal conductance in three evolutionary lineages of C(3), C(3)-C(4) intermediate, and C(4) species grown in the low CO(2) and hot conditions proposed to favo r the evolution of C(4) photosynthesis. Closely related C(3), C(3)-C(4), and C(4) species in the genera Flaveria, Heliotropium, and Alternanthera were grown near 380 and 180 µmol CO(2) mol(-1) air and day/night temperatures of 37/29°C. Growth CO(2) had no effect on photosynthetic capacity or nitrogen allocation to Rubisco and electron transport in any of the species. There was also no effect of growth CO(2) on photosynthetic and stomatal responses to intercellular CO(2) concentration. These results demonstrate little ability to acclimate to low CO(2) growth conditions in closely related C(3) and C(3)-C(4) species, indicating that, during past episodes of low CO(2), individual C(3) plants had little ability to adjust their photosynthetic physiology to compensate for carbon starvation. This deficiency could have favored selection for more efficient modes of carbon assimilation, such as C(3)-C(4) intermediacy. The C(3)-C(4) species had approximately 50% greater rates of net CO(2) assimilation than the C(3) species when measured at the growth conditions of 180 µmol mol(-1) and 37°C, demonstrating the superiority of the C(3)-C(4) pathway in low atmospheric CO(2) and hot climates of recent geological time.

Potential errors in electron transport rates calculated from chlorophyll fluorescence as revealed by a multilayer leaf model.

Increasingly, photosynthetic electron transport rate is being calculated from chlorophyll fluorescence measurements. The fluorescence signal is a complex mixture of contributions from different depths within the mesophyll. One condition required for electron transport calculated from fluorescence to represent the rate accurately is that the ratio of photosynthetic capacity to light absorbed be constant throughout the leaf. In order to explore the fluorescence properties of leaves where this assumption is not true, a new approximation for phiPSII is used to generate F'm and Fs values throughout the leaf. Fs is assumed to be proportional to the amount of light absorbed from the fluorescence measuring beam and constant, i.e. independent of the actinic irradiance or CO2 concentration. This assumption is validated by measurements from Eucalyptus maculata, Flaveria bidentis and Triticum aestivum, with two different types of fluorometer, where irradiance or CO2 response curves were measured with normal or inverted leaf orientations. The new approach enables fluorescence values to be generated at each layer in a multilayer model. Two applications using this approach are presented. First, the model is used to show that when quantum yield varies through a leaf, then fluorescence will lead to an incorrect estimate of electron transport rate. Secondly, since chlorophyll fluorescence is also used to calculate the CO2 concentration at the sites of carboxylation within chloroplasts, Cc, the model is also used to show that Cc may vary with depth. Significant variation in Cc through the mesophyll could lead to an apparent dependence of internal conductance on irradiance or CO2.

Photosynthetic pathway influences xylem structure and function in Flaveria (Asteraceae).

Higher water use efficiency (WUE) in C(4) plants may allow for greater xylem safety because transpiration rates are reduced. To evaluate this hypothesis, stem hydraulics and anatomy were compared in 16 C(3), C(3)-C(4) intermediate, C(4)-like and C(4) species in the genus Flaveria. The C(3) species had the highest leaf-specific conductivity (K(L)) compared with intermediate and C(4) species, with the perennial C(4) and C(4)-like species having the lowest K(L) values. Xylem-specific conductivity (K(S)) was generally highest in the C(3) species and lower in intermediate and C(4) species. Xylem vessels were shorter, narrower and more frequent in C(3)-C(4) intermediate, C(4)-like and C(4) species compared with C(3) species. WUE values were approximately double in the C(4)-like and C(4) species relative to the C(3)-C(4) and C(3) species. C(4)-like photosynthesis arose independently at least twice in Flaveria, and the trends in WUE and K(L) were consistent in both lineages. These correlated changes in WUE and K(L) indicate WUE increase promoted K(L) decline during C(4) evolution; however, any involvement of WUE comes late in the evolutionary sequence. C(3)-C(4) species exhibited reduced K(L) but little change in WUE compared to C(3) species, indicating that some reduction in hydraulic efficiency preceded increases in WUE.

Floral Visitors of Three Asteraceae Species in a Xeric Environment in Central Mexico.

We describe the spatial variation in the structure and composition of the communities of insects visiting the inflorescences of Flaveria ramosissima Klatt, Florestina pedata (Cav.) Cass., and Parthenium bipinnatifidum (Ort.) Rollins (Asteraceae) in a xeric environment in Central Mexico. Inflorescences of the three Asteraceae were visited by a total of 96 species of Hymenoptera, Diptera, Lepidoptera, Coleoptera, and Hemiptera. Total species richness of floral visitors to the three Asteraceae and total abundance of insects of Fl. pedata and P. bipinnatifidum did not differ between low and high vegetation cover sites. Total abundance of insects visiting the inflorescences of F. ramosissima and abundance of Hymenoptera in all three Asteraceae were higher at the low vegetation coverage (LVC) site than at the high vegetation coverage (HVC) one. Diversity of insects of Fl. pedata and P. bipinnatifidum was higher at the HVC site. However, in F. ramosissima diversity was higher at the LVC site. The communities of insects of each Asteraceae were dissimilar between sites. These differences can be attributed to variation in the abundance of Lepidophora (Diptera: Bombyliidae), Miridae (Hemiptera), Melyridae (Coleoptera), Tiphiidae (Hymenoptera), Myrmecocystus mexicanus Wesmael, and Dorymyrmex grandulus (Forel) (Hymenoptera: Formicidae). The first three insect groups were sensitive to LVC, high temperature, and low humidity, whereas the last three tolerated those same environmental conditions. Changes in temperature, humidity, and resources associated with vegetation coverage seem to differentially affect each species of floral visitors of the three Asteraceae species studied.

[Effects of shade and competition of Chenopodium album on photosynthesis, fluorescence and growth characteristics of Flaveria bidentis].

It is necessary to elucidate its growth mechanism in order to prevent and control the further spread of Flaveria bidentis, an invasive plant in China. The effects of shading (shading rate of 0, 50% and 80%, respectively) and planting pattern (single cropping of F. bidentis, single cropping of Chenopodium album and their intercropping) on germination rate, fluorescence characteristics and growth characteristics of the two plants were investigated. The results showed that moderate shading contributed to emergence rate, but emergence rate of F. bidentis was not uniform, which was one of important factors as a stronger invader. With the increasing light intensity, net photosynthetic rate (Pn), photochemical quenching (qP), electron transport rate of PS II (ETR), quantum yield of PS II (Y), non-photochemical quenching (qN), water use efficiency (WUE), shoot bio-mass rate (SMR), crown width (CW) and dry biomass (DM) increased, specific leaf area (SLA) decreased, LMR of F. bidentis significantly increased, LMR of C. album changed insignificantly, and the increment of DM of F. bidentis was higher than that of C. album. In 80% shade treatment, Pn and DM of F. bidentis were lower than those of C. album. In natural light treatment, Pn, qN, WUE and relative competitive index (RCI) were the highest, CW and DM of intercropped F. bidentis and Pn, Y of C. album were significantly lower than that of the respective single treatment. F. bidentis had higher light saturation point (LSP) and light compensation point (LCP). In conclusion, the shade-tolerant ability of F. bidentis was weaker than that of C. album, but it was reversed in natural light treatment. The two plants adapted to the weak light in 80% shade treatment by increasing SLA and decreasing LMR. F. bidentis improved competition under natural light by increasing SMR and decreasing CW.

Strategies of ROS regulation and antioxidant defense during transition from C3 to C4 photosynthesis in the genus Flaveria under PEG-induced osmotic stress.

In the present study, we aimed to elucidate how strategies of reactive oxygen species (ROS) regulation and the antioxidant defense system changed during transition from C3 to C4 photosynthesis, by using the model genus Flaveria, which contains species belonging to different steps in C4 evolution. For this reason, four Flaveria species that have different carboxylation mechanisms, Flaveria robusta (C3), Flaveria anomala (C3-C4), Flaveria brownii (C4-like) and Flaveria bidentis (C4), were used. Physiological (growth, relative water content (RWC), osmotic potential), and photosynthetical parameters (stomatal conductance (g(s)), assimilation rate (A), electron transport rate (ETR)), antioxidant defense enzymes (superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), glutathione reductases(GR)) and their isoenzymes, non-enzymatic antioxidant contents (ascorbate, glutathione), NADPH oxidase (NOX) activity, hydrogen peroxide (H2O2) content and lipid peroxidation levels (TBARS) were measured comparatively under polyethylene glycol (PEG 6000) induced osmotic stress. Under non-stressed conditions, there was a correlation only between CAT (decreasing), APX and GR (both increasing) and the type of carboxylation pathways through C3 to C4 in Flaveria species. However, they responded differently to PEG-induced osmotic stress in regards to antioxidant defense. The greatest increase in H2O2 and TBARS content was observed in C3 F. robusta, while the least substantial increase was detected in C4-like F. brownii and C4 F. bidentis, suggesting that oxidative stress is more effectively countered in C4-like and C4 species. This was achieved by a better induced enzymatic defense in F. bidentis (increased SOD, CAT, POX, and APX activity) and non-enzymatic antioxidants in F. brownii. As a response to PEG-induced oxidative stress, changes in activities of isoenzymes and also isoenzymatic patterns were observed in all Flaveria species, which might be related to ROS produced in different compartments of cells.

The role of photorespiration during the evolution of C4 photosynthesis in the genus Flaveria.

C4 photosynthesis represents a most remarkable case of convergent evolution of a complex trait, which includes the reprogramming of the expression patterns of thousands of genes. Anatomical, physiological, and phylogenetic and analyses as well as computational modeling indicate that the establishment of a photorespiratory carbon pump (termed C2 photosynthesis) is a prerequisite for the evolution of C4. However, a mechanistic model explaining the tight connection between the evolution of C4 and C2 photosynthesis is currently lacking. Here we address this question through comparative transcriptomic and biochemical analyses of closely related C3, C3-C4, and C4 species, combined with Flux Balance Analysis constrained through a mechanistic model of carbon fixation. We show that C2 photosynthesis creates a misbalance in nitrogen metabolism between bundle sheath and mesophyll cells. Rebalancing nitrogen metabolism requires anaplerotic reactions that resemble at least parts of a basic C4 cycle. Our findings thus show how C2 photosynthesis represents a pre-adaptation for the C4 system, where the evolution of the C2 system establishes important C4 components as a side effect.

[Prediction of potential suitable distribution area of Flaveria bidentis in China based on niche models].

Based on the distribution records of Flaveria bidentis in China, and by using five ecological niche models (GARP, Maxent, ENFA, Bioclim, and Domain), 32 eco-geographical variables were chosen to simulate the potential suitable distribution area of F. bidentis in the country, and the simulation precision of the models was assessed by the method of Receiver Operating Characteristic (ROC) curve analysis. Among the models adopted, Maxent model had the best simulation precision. Its prediction showed that the potential suitable distribution area of F. bidenti in this country accounted for 7. 5% of the total, with the central and southern Hebei, Beijing, Tianjin, Henan, Shandong, Anhui, and Jiangsu having high potential invasion risk.

[Potential distribution areas of alien invasive plant Flaveria bidentis (Asteraceae) in China].

Flaveria bidentis (Asteraceae), a potential exotic invasive weed to agro-ecosystem and rangeland ecosystem, has recently invaded Tianjin City and Hebei Province (Hengshui and Langfang) in North China, and is spreading further. Based on its current geographical distribution in the world, the potential distribution areas of this weed in China were predicted by using CLIMEX software, aimed to assess the potential risks of this invasive weed. Following provinces in China could be the potential areas being invaded by F. bidentis, i. e., Guangdong, Guangxi, Yunnan, Hainan, Fujian, Taiwan, Jiangxi, Hunan, Guizhou, Sichuan, Chongqing, Hubei, Anhui, Jiangsu, and Shanghai, among which, Guangdong, Guangxi, Taiwan, Hainan, Fujian, Yunnan, Sichuan, Guizhou, Chongqing, and part of Xizang would be at high risk.

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.