Response and adaptation of photosynthesis, respiration, and antioxidant systems to elevated CO2 with environmental stress in plants

1The major components of climate change include elevated atmospheric carbon dioxide concentrations (elevated CO2), warming, and altered precipitation patterns, as well as their interactions within and with other environmental factors. Based on updated information, with increases in global atmospheric CO2 concentrations of 43% from the pre-industrial level of 280 μmol mol-1 in 1750 to the present level of 400 μmol mol-1 (an annual increase of 1.35%), the global CO2 concentration has increased by about 1.55 ppm CO2 per year over the past 55 years. It continues to be elevated at an unprecedented pace of ∼1.0 μmol mol-1 per year, as a result of the further increase in the cumulative emissions of CO2 to the atmosphere during the 21st century (400 μmol mol-1 in 2011 vs. 936 μmol mol-1 in 2100;; ). Meanwhile, the global mean surface temperature is expected to increase by 2.6–4.8°C by the end of the 21st century (2081–2100), relative to the 1986–2005 level under RCP8.5, based on a more undisciplined management scenario with higher greenhouse gas emissions. The climate changes, such as elevated CO2, rising temperature, and altered precipitation, have resulted in drastic impacts on the natural ecosystems, such as in vegetation function, sustainable food production, and crop yields, leading to more profound impacts when the climate changes are combined with other environmental constraints, such as air pollution, nutrition limitation, and their interactions.

2Herein, we focus on the critical biological processes of plants with regard to climate change, including (mainly) photosynthesis, respiration, the antioxidant system, and the related metabolic activities. Photosynthesis and respiration are two fundamental physiological processes of plants, because the former involves initial carbon fixation, light energy transfer, and oxygen release, and the latter works on carbon efflux, energy production, and the relevant substrate metabolisms, such as those providing the carbon skeleton. They play a critical role in balancing the carbon budget and maintaining the carbon sink in terrestrial ecosystems, as well as in the response and feedback to climate change. The excessive accumulation of reactive oxygen species (ROS) often occurs in plants grown under abiotic stress, while an enzymatic and non-enzymatic antioxidant defense system may work to protect plants against oxidative stress-induced damage, which can be affected by climate change (such as elevated CO2, drought, and heat waves;;;;; ).

3Rising CO2 has affected almost all crucial biological processes, including photosynthesis, respiration, and antioxidant systems, as well as other key secondary metabolisms in plants. All other effects of elevated CO2 on individual plants and ecosystems may be partly derived from these fundamental biological responses. Genetic variations relative to the biological processes’ traits might also be impacted by elevated CO2, closely linking to these responses in various spatiotemporal aspects, from molecular, biochemical, and physiological, through individual levels and ecosystems, up to the entire Earth’s life system, interacting with multiple environmental factors (both biotic and abiotic) as well as human-driven disturbances at different temporal scales.

4As stated above, plant responses to climate change have become a hot topic in botanical research across various scales in the recent decades. Many reports have reviewed the biological responses to CO2 enrichment, and their interactions with environmental change, including photosynthesis and stomatal behavior (e.g.,;; ). Our earlier review by examined plant growth, carbon and nitrogen (N) allocations, gas exchange responses to elevated CO2 with drought and high temperature. Although this review discussed the changes in growth and photosynthesis, and water use efficiency (WUE) in higher plants exposed to CO2 enrichment with abiotic variables, the various underlying mechanisms of the critical biological processes that are affected, modulated, and controlled by elevated CO2 with other abiotic environmental variables were not fully covered, particular at the molecular, organelle, cell, biochemical, physiological, organ, individual, and ecosystem scales. Actually, no systematic synthesis of these has been well reviewed, thus far. Therefore, in this review, based on correcting and synthesizing any new progress of the relevant research concerning plant biology and climatic change, we attempted to systematically summarize the considerable study results that have reported the responses of photosynthesis, respiration, and the antioxidant systems, as well as the key substrate metabolisms to elevated CO2 with other environmental variables. Particularly, we reviewed the underlying mechanisms and the response pathways, as well as their interrelationships. Finally, the future perspectives for this study related to the possible implications are briefly presented and discussed. Thus, the present review may be of current interest in terms of its interdisciplinary and systematic synthesis, providing comprehensive information on the important historical and new experimental results, relative theoretical analysis, underlying mechanisms, and potential applications to promote further research.

5The responses of photosynthesis to elevated CO2 concentrations have been reviewed in many reports [e.g., most for enclosure results;;;; for free-air CO2 enrichment (FACE)]. The stimulation of the light-saturated photosynthetic CO2 assimilation rate (Asat) is a general response to CO2 enrichment, with an average of 31% in the FACE experiments, and 23–58% in the potted plant experiments from earlier reports. The magnitude of the stimulation by CO2 enrichment varies with the different plant functional types (PFTs), with a maximum for trees and C3 grasses; moderate for shrubs, C3 and C4 crops, and legumes; and minimum for C4 grass (even with a negative response;;; ). Therefore, there is greater variation in the stimulation by elevated CO2, depending on the plant species, PFTs, and their surroundings, specifically environmental conditions like nutrition and water resource availability. For instance, elevated CO2 leads to an increase in the Asat of Arabidopsis thaliana leaves by 82%, since the N availability is ample. However, a recent study of soybean plants indicated that elevated CO2 did not produce significant effects on midday net photosynthetic rate (Anet), either in the FACE or open-top chamber (OTC) studies, suggesting that the Anet at high photosynthetic photon flux density (PPFD) might be limited by a low ribulose 1, 5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation capacity. Actually, other abiotic and biotic factors such as high temperature (e.g., ), drought, N deficit, genetic variation, and leaf senescence may also diminish the photosynthetic response to elevated CO2.

6The same results appeared in Lolium perenne and Medicago lupulina plants in controlled chambers. Moreover, C4 plants may have no response to elevated CO2, because their CO2 concentrating mechanism (CCM) may concentrate the CO2 12–20 times at the site of Rubisco, which is relatively higher than in C3 species. Case studies confirmed this theoretical conclusion under well-watered conditions in either enclosure (e.g., ) or FACE experiments (e.g.,; ). However, under a water deficit, the stimulation of the C4 Asat by elevated CO2 still appears, because the drought-induced impairment of C4 photosynthesis might be ameliorated by elevated CO2. Moreover, C4 plants can avoid photorespiration to promote CO2 fixation with higher light use efficiency. On the other hand, the down-regulation of the photosynthesis capacity is also more profound in C3 species than in C4 species, due in part to the N dilution, possibly because C3 plants need to invest more N from the leaf into Rubisco, relative to the C4 species, so that the former may easily undergo more severe N dilution under CO2 enrichment, with no CCM.

7In addition to N limitation, photosynthetic acclimation under higher CO2 levels may result from high stomatal and internal resistances, higher starch levels, and diluted chlorophyll concentrations. Under elevated CO2, carbohydrate accumulations, such as starch size and number of chloroplasts, can be enhanced, partially due to the carbon substrate increase. However, the excessive carbohydrate accumulation may cause feedback inhibition or physical damage at the chloroplast level, reducing the photosynthetic capacity. More importantly, the Rubisco response, excessive sugar feedback, and the related gene expression may, together; play crucial roles in plants’ photosynthetic acclimation under higher CO2 concentrations, particularly for long-term CO2 enrichment under a nitrogen availability deficit (see details below).

8The stimulation of photosynthesis in C3 species by short-term elevated CO2 has been well established, and confirmed under almost all experimental conditions, particularly with FACE (e.g.,;; ). However, with long-term exposure to elevated CO2 or other limitations, photosynthetic acclimation or the down-regulation of the photosynthetic capacity may occur, depending on the species, plant developmental stage, and environmental conditions.

9Rubisco has been identified as a controlling rate enzyme for carbon fixation. Here, we succinctly summarize the five major mechanisms that might explain the response to elevated CO2, involving Rubisco: (1) under current CO2 concentration levels, although the value of the Rubisco Michaelis–Menten constant (Km) for CO2 is close to the current intercellular CO2 concentration (Ci) (c. 190 μmol mol-1) at the site of carboxylation. CO2, as a substrate of photosynthesis, does not have to reach saturation; therefore, the rising CO2 can lead to an immediate increase in the Rubisco carboxylation velocity, due to an increase in the carbon substrate availability. (2) The Rubisco catalyzing function has two intrinsic side features: carboxylation and oxygenation. The carboxylation rate is ∼2.2 fold greater than the oxygenation rate at 25°C in C3 plants; that is, about one-third of the ribulose-1,5-bisphosphate (RuBP) may be consumed in the oxygenation reaction. Thus, elevated CO2, as a competing substrate, can competitively inhibit the oxygenation of RuBP (light-dependent photorespiration) through the down-regulation of Rubisco’s affinity for O2, while competitively promoting the carboxylation of RuBP via the up-regulation of Rubisco’s affinity for CO2. Consequently, this leads to the stimulation of photosynthesis, which may be compromised by heat and drought due to the enhancement of Rubisco’s affinity for O2 (Figure 1). On the other hand, (3) with continually increasing CO2, the ATP products may not meet enough of the demand for RuBP regeneration, and a reduction in Rubisco’s activation state may occur, usually accompanied by a decrease in the capacity for RuBP regeneration, as well as in the RuBP pool, as indicated by a decline in the ATP:ADP ratio in the chloroplast. (4) A reduction in the Rubisco content via N dilution, particularly under long-term elevated CO2, may finally contribute to the reduction of carboxylation at the Rubisco active site. In addition, the nitrogen use efficiency (NUE) might be increased due to the optimization of the resource use. Because the leaf N of C3 species can be more invested in Rubisco (more than 25% vs. 10–15% of the leaf N in C3 and C4 plants, respectively), the former may be affected more profoundly by N dilution.

10Finally, (5) Hexokinase (HXK), as a sensor of excessive photosynthate, may be involved in the downregulation of the Rubisco content (;; see below). In summary, with respect to the Rubisco response, parts (1) and (2) above may explain the stimulation of photosynthesis by elevated CO2, while the last three points may provide a mechanism for understanding the downregulation of the photosynthetic capacity under relatively long-term elevated CO2 or other resource deficit conditions, such as a scarcity of N.

11Under higher CO2 concentrations, a prevailing explanation of the downregulation of photosynthesis may be ascribed to the sugar feedback inhibition hypothesis: certain reactive bioprocess activities within the Calvin–Benson–Basshan (CBB) cycle may be inhibited by elevated CO2, due to the overload of the chemical reaction substrates. The hypothesis of the sugar feedback mechanism suggests that excessive photosynthate in chloroplasts under elevated CO2 may trigger the sugar signal network (HXK acting as a flux sensor) to down-regulate the Rubisco contents through the gene expression processes, affecting the subunit of Rubisco.

12As noted above, HXK acting as a flux sensor in mesophyll cells may involve the down-regulation of the Rubisco content associated with genetic expressions under elevated CO2; however, plants may prefer to reduce the Rubisco activity relative to the RuBP regeneration capacity. For instance, found a decline in wheat Rubisco and its activase protein content accompanying a photosynthetic down-regulation. In addition, the effects of the source-sink balance in response to CO2 enrichment may play an important role in the regulation of the photosynthetic capacity.

13Based on the “source-sink” hypothesis, some plants with a strong sink can encounter photosynthetic down-regulation, to some extent, under higher CO2, which can generally be repressed by other limitations, such as intrinsic genetic constraints or the specific plant developmental stage (such as the flowering stage;; ). Moreover, when enhanced carbohydrate availability exceeds the plants’ ability to fully utilize carbohydrates, due to nutrient or inherent internal growth limitations, the feedback may lead to a lower level of photosynthesis, which may lead to an imbalance in the carbon sink:source ratio. For instance, wheat plants exposed to high atmospheric CO2 are incapable of excessive accumulation of leaf photoassimilate, due to the lack of an increase in the carbon sink strength.

14Furthermore, the respiratory ATP may be consumed more under elevated CO2; for example, the rate of the carbohydrate/sugar export (i.e., the cost related to the carbohydrate export) is higher under elevated CO2 than under normal CO2, which may cause a negative feedback effect on photosynthesis. This also highlights the close link between the photosynthetic and respiratory bioprocesses, between both the CBB and tricarboxylic acid (TCA) cycle, under climate change.

15Photorespiration enables the photosynthetic process to recycle the phosphoglycolate produced by the oxygenase reaction of Rubisco, consequently avoiding more carbon loss, with some protective regulation functions for plants, such as in the oxidative defense mechanism. However, as reported in the review by, at room temperature (25°C), photorespiration can lead to a loss of 23–30% of the carbon fixed by photosynthesis with the rising temperature, whereas the CO2 fixation may be increased by ∼53% if only the carboxylation reaction occurs, without the oxygenation reaction.

16Widely accepted results show that photorespiration can be restricted when C3 plants are grown under high CO2 concentrations, because in C3 plants, the carboxylation capacity of Rubisco, with a low catalytic activity (operating below its Km for CO2), is easily promoted by high CO2. Meanwhile, an increase in the CO2 concentration, leading to a high CO2:O2 ratio, may reduce its oxygenation reaction capacity, inhibiting photorespiration (;; see above). For example, based on the earlier report by, the photorespiration rate should fall by ∼50% when the CO2 level is doubled. In A. thaliana plants grown under elevated CO2, although the accumulations of several major amino acids (including glutamate, aspartate, asparagine, and alanine) were enhanced, a lower level of glycine (Gly), an intermediate of photorespiration, was observed in the plants, leading to a decline in the Gly:Ser ratio, indicating a lower photorespiration activity (; Figure 1).

17Enhanced photoperoxidation in chloroplasts can induce a destruction of the chlorophyll and a disassembly of the chloroplast membranes, leading to a decline in photosynthesis. Conversely, the constraints of photorespiration by elevated CO2 may also reduce the H2O2 products, weakening oxidation stress, possibly protecting the photosynthetic apparatus. Based on the fact that photorespiration has a protective function against photo-oxidation, possibly via the up-regulation of glutamine synthetase (GS2) to recycle ammonia, diminishing photo-oxidation and photo-inhibition. This brings with it another dilemma: a decline in photorespiration under rising CO2 levels may cancel the protective role, leading to a higher level of photo-oxidation than the higher rate of carboxylation stimulated by elevated CO2 can maintain. In order to solve this dilemma, further research is required to cope with climate change, possibly by manipulating the modulated photorespiration bioprocess.

18Mitochondrial respiration involves the carbon balance in the whole plant, with 20–80% of the carbon fixed in photosynthesis being released again through the respiration process. The respiration of the leaves in both the light and dark can account for ∼50% of the whole-plant respiratory CO2. The response of dark leaf respiration (Rd) to elevated CO2 remains debatable, with a decrease in the major reports, while increasing or remaining stable in a number of experiments (e.g.,;;;;;; ). For instance, there is a 15–18% range in the reduction of foliar respiration when plants are grown under a doubled CO2 concentration, relative to the ambient CO2 level from one review (; references). However, no significant response to the leaf Rd was observed in L. perenne plants exposed to high CO2; although, the leaves grown in elevated CO2 had a relative lower Rd. A small response in the leaf respiration rate to a short-term CO2 elevation (a 1.5% decrease) was obtained from the deciduous tree species used in an earlier experiment by, with similar evidence found in soybean plants from a recent report by. Thus, for the plants grown under elevated CO2, the Rd decrease response is general, not universal.

19Correspondingly, the underlying mechanism has also been proposed in two contrasting hypotheses: elevated CO2 may enhance the Rd due to the great increase in the respiratory substrates, such as sugar; whereas the N dilution induced by elevated CO2 might reduce the demand on dark respiration to support the protein turnover, leading to a decline in the Rd. A recent report showed that CO2 enrichment can accelerate the accumulation of the relevant carbohydrates, such as sugar, starch, and respiratory glycolysis intermediates like hexose-P, phosphoglycerate (PGA), and phosphoenolpyruvate (PEP) in A. thaliana plants, which may enhance the respiration potential. Recent evidence has also indicated that the promotion to greater photo-assimilation availability at elevated CO2 leads to a great transcriptional up-regulation of the genes, in association with the respiratory pathway, supporting the first hypothesis. However, this may depend on the availability of the nutritional components, including nitrogen in the plants and/or the soil. For instance, based on a recent report by, widely and greatly adaptive responses of the expression of the respiratory genes were obtained when the plants were exposed to elevated CO2. However, the transcriptional reprogramming with the stimulation of leaf respiration by elevated CO2 can be suppressed by limited nitrogen availability.

20The ROS in plants, including superoxide radicals, hydrogen peroxide (H2O2), the hydroxyl radical (OH⋅), and the perhydroxy radical, often accumulate when plants are subjected to abiotic stress, while the antioxidant defense system with enzymatic and non-enzymatic machinery may work to protect the plants against damage due to oxidative stress. This occurs particularly in the face of stressful environmental changes, such as adverse climatic changes like droughts and heat waves (Figure 2). Generally, when plants become senesced, with some antioxidants increasing and others decreasing, the ROS may accumulate in a large amount, and the antioxidant system does not work well. This is often indicated by enhanced lipid peroxidation and decreased levels of antioxidant enzymes, such as superoxide dismutase (SOD) and catalase (CAT), leading to programmed cell death (PCD), particularly under severe abiotic stress.

21Elevated CO2 may increase the levels of antioxidants, including polyphenols, ascorbate (ASC), alkaloids, and some antioxidant enzyme activities (such as CAT and SOD), with a significant enhancement in the antioxidant capacity, leading to declines in the ROS levels. For example, when the plants were exposed to elevated CO2, increases in the ASC and phenol levels were obtained in Beta vulgaris, and increases in the ASC, glutathione (GSH), and ASC/GSH, as well as in their redox status, were found in L. perenne and M. lupulina. Ascorbate synthesis can be triggered and enhanced by excessive carbohydrate production due to elevated CO2, which is closely linked to carbon metabolism, and together improve the plant-antioxidant defense system. Moreover, with a delay in the onset of senescence and/or severe stress under elevated CO2 conditions, it is suggested that the antioxidant profiles, such as the accumulation of antioxidant compounds and antioxidant enzyme activity, may show better performance in dealing with the biological process of senescence. For instance, a reduction in the oxidative stress under elevated CO2 was found in Zingiber officinale, Catharanthus roseus, a temperate grassland shrub, Caragana microphylla, a bean, Vigna radiate, and A. thaliana plants.

22However, with the elevated CO2-alleviated oxidation stress evidence coming from a number of reports (e.g.,; ), these results have not been confirmed in some species, such as in Spinacia oleracea leaves. reported that CO2 enrichment could exacerbate lipid peroxidation in M. lupulina, but not in L. perenne plants, with no rising-CO2 responses in the ascorbate peroxidase (APX) and peroxidase (POX) in M. lupulina. No significant responses in the antioxidant enzyme activity, including APX, glutathione reductase (GR), POX, CAT, and SOD, were found when B. vulgaris plants were exposed to high levels of O3 with elevated CO2, except in the inhibition of APX.

23Instead, in Quercus pubescens and Q. ilex plants grown under elevated CO2, down-regulation of the protective systems was observed. According to the findings of, although the GR in oak leaves remains stable, the activities of SOD, CAT, POX, and APX, as well as the sum of dehydroascorbate and ASC, were reduced in CO2-elevated environments. Base on a recent report, the activities of the SOD, CAT, and APX declined, but the GR and POX were stimulated, finally leading to a significant reduction in the, H2O2, and malondialdehyde (MDA) contents in C. roseus plants grown under elevated CO2. Recently, marked decreases in the ROS levels (, H2O2) and reductions in some antioxidant enzymes, such as CAT and SOD, were observed simultaneously in mung bean plants exposed to elevated CO2, suggesting that a lower level of ROS might match the lower activity of antioxidant enzymes.

24Based on a report by, who used C3 grasses (L. perenne, Poa pratensis) and C3 legumes (M. lupulina, Lotus corniculatus) as experimental materials, elevated CO2 can reduce the H2O2 level, lipid peroxidation, and lipoxygenase (LOX) activities, while it decreased the SOD, CAT, glutathione peroxidase (GPX), and GR levels, but did not affect the ASC-GSH cycle. Thus, the predominant form of the enzymatic antioxidant defense may strongly depend on the species and the abiotic stress.

25The activities and gene transcription expression levels of ROS scavenging enzymes in A. thaliana at elevated CO2 remained unchanged, particularly under well-watered conditions. However, the excessive gene transcriptional response related to antioxidant metabolism due to O3 pollution was partly repressed by elevated CO2 in soybean (Glycine max) plants under field conditions, again arguing the protective role of elevated CO2. Lipid peroxidation, indicated by MDA accumulation, would be lessened by CO2 enrichment, especially under other severe abiotic stress conditions such as drought, heat wave stress, O3 pollution, and salinity, implying that oxidative stress induced by severe environmental constraints may be mitigated, generally or at least partly, by CO2 fertilization. It is again highlighted that the positive vs. negative roles of elevated CO2 concentrations in antioxidant enzyme regulation under severe stressful abiotic environments may depend considerably on different species.

26Moreover, based on a recent report using wheat plants, with increasing sugar levels via CO2 enrichment, sugar-derived reactive carbonyls (RCs; aggressive by-products of oxidative stress), including methylglyoxal (MG), were provoked by elevated CO2, which can negate the functions of multiple proteins, and impair the biological membrane, suggesting that plant diabetes may be inducible, supporting an earlier study by. Thus, whether and how much elevated CO2 affects antioxidant systems in plant tissues depends on the plant species, crop variety, developmental stage, abiotic factors, and the combination of these (e.g.,;;;; ). This is a debatable issue, requiring further research.

27The metabolism changes of certain important metabolites, and the related genetic variations induced by elevated CO2 have been found in a number of research reports. For instance, an accumulation of carbon compounds under elevated CO2 occurs in wheat leaves accompanied by an up-regulation of phosphoglycerate mutase (PGAM) involving carbohydrate transport, but a down-regulation of the adenosine diphosphate glucose pyrophosphatase protein for synthesizing starch; thus affecting the carbon flux within the plants’ tissues, and the balance between the carbon sink and source. Changes in the major chemical components induced by elevated CO2 have also been reported in many studies. Generally, under elevated CO2, there may be a decrease in the total N and organic N compounds, which define the elevated CO2-induced dilution effectiveness. However, there is an increase in the total non-structural carbohydrates (TNC), including starch and sugar (e.g., glucose, fructose, sucrose;;;; ), with a mostly stable level in the total structural carbohydrates (cellulose plus hemicellulose), lignin, and lipids (review by; ). Nitrogen assimilation may be enhanced by elevated CO2, and a recent report indicated that elevated CO2 may promote N assimilation and transamination-related enzyme activities, such as glutamate oxoglutarate aminotransferase (GOGAT) and glutamate oxalate transaminase (GOT), and lead to an increase in the phloem amino acid content in M. truncatula.

28Elevated CO2 can change not only the primary metabolic processes, but also the secondary metabolic composition in plant tissues. Here, we mainly address the secondary metabolite responses, because there are fewer studies related to the key metabolites. Plant secondary metabolites often indicate that these compounds have no primary functions in the maintenance of life processes in plants; however, they are involved in the biological processes of plants dealing with environmental stress, with regard to adaptation and defense. Changes in the secondary metabolites with rising CO2 have been reported in several relevant studies. For example, an alteration in the carbon allocation under elevated CO2 has revised the carbon-nutrient balance (CNB) hypothesis, increasing the C:N ratio in plant tissues (e.g.,;; ), while increasing the levels of the C-based secondary compounds due to easier synthesis, in plants with excess carbon relative to the other nutrients (such as N;; ). However, a contradiction may arise when elevated CO2-induced N dilution limits the carbohydrate reserves, leading instead to a reduction in some secondary substances. However, with rising CO2, a large accumulation of some secondary metabolites, including phenylpropanoids, tannins, triterpenoids, phenolic acids, and alkaloids, was observed, despite the effectiveness of the dilution. For example, in tobacco leaves there was a large accumulation of phenylpropanoids, including the major carbon-rich compound chlorogenic acid (CGA), and the scopolin and scopoletin coumarins. In the flavonoid response, although both the kaempferol and fisetin were increased by elevated CO2 in ginger (Zingiber officinale Roscoe; ), whether there was a decrease or increase in birch plants grown in elevated CO2 depended on the genetic type or environmental conditions. The glucosinolate accumulation was enhanced in Brassica plants exposed to elevated CO2, possibly changing the feeding behavior of specialized herbivores. In other metabolites, including lignin, cell wall polysaccharides, and terpenes, no obvious response was found, depending on the compound composition, species, genotype, nutrient status (such as N availability), and other environmental factors. For example, the levels of the condensed tannins, most flavonols, and phenolic acids in birch plants can be stimulated by elevated CO2 and elevated UVB, but this effect disappeared at high temperatures.

29Isoprene is a volatile hydrocarbon molecule, generally emitted by certain vegetation types, particularly tree species, protecting plants against damage from abiotic stress, and playing an important role in tropospheric chemistry and climate change due to its highly reactive molecular properties, especially in the formation processes of ozone and secondary organic aerosols. However, it has been confirmed that isoprene may have an active function in protecting the photosynthetic apparatus against oxidative stress from abiotic stress (such as heat), by quenching the ROS via the promotion of oxidative defense machinery. In a number of related reports, elevated CO2 has produced various effects on plant-derived isoprene emissions, including increases, remaining unchanged, and, most often, showing decreases (e.g.,;; ). The reason for the decreasing isoprene emission may be that the available reducing power captured by light may cause a large consumption, due to carbon fixation rather than isoprene synthesis, in CO2 enrichment conditions, resulting in a reduction in isoprene emissions. A reduction in the isoprene emission capacity may be attributable to a decrease in both the isoprene synthase activity and pool size of dimethylallyldiphosphate (DMADP), an immediate isoprene precursor. Actually, DMADP synthesis is involved in the primary photosynthetic product of glyceraldehyde-3-phosphate (GAP), linked to a leaf’s photosynthetic carbon metabolism. Reduced ATP induced by elevated CO2 may also diminish DMADP synthesis. Thus, the isoprene emission capacity may be determined by the status of the balance between the primary metabolites, such as sugar, and the secondary metabolites, such as isoprene, again highlighting the importance of the primary-secondary metabolite balance (abbreviated by PSMB) with CO2 enrichment. Based on the response model suggested by, Figure 3 succinctly describes a pathway involved in the downregulation of isoprene biosynthesis in response to elevated CO2. Under elevated CO2, more electron flux may be used in the CBB cycle for photosynthesis, whereas less electrons may flow into the photorespiration cycle, xanthophyll cycle, and the methylerythritol 4-phosphate (MEP, 5) pathway to synthesize isoprene, as well as other redox reactions, such as quenching ROS (e.g., GR reaction demands of NADPH; ). It is worth noting that the isoprene biosynthesis and emission, in and from plants, may be tightly associated with photosynthesis, photorespiration, the xanthophyll cycle, and oxidative defense systems in response to CO2 enrichment, with abiotic environmental changes (;;; Figure 3).

30The role of hormone pathways in regulating the growth and metabolic responses to elevated CO2 is not well known, despite there being a few reports (e.g.,;; ). Elevated CO2 can promote an accumulation in the salicylic acid (SA, ) and brassinosteroids (BR; ), while reducing Jasmonates (JA) and ethylene concentrations. Elevated CO2-induced genes were associated with the metabolic processes of the BR regulator in plant tissues, which can alleviate the heat-induced inhibition of photosynthesis, by increasing the carboxylation efficiency and enhancing the antioxidant systems in Lycopersicon esculentum. In one recent report, BRs were found to enhance the stimulation of plant growth and photosynthetic potential under elevated CO2. An increase in the indole-3-acetic acid (IAA), isopentenyl-adenosine (iPA), and dihydrozeatin riboside (DHZR) was found, while a decrease in the ABA and unchangeable zeatin riboside (ZR) occurred in Pinus tabulaeformis plants exposed to elevated CO2, which can encounter O3 exposure effects to alleviate damage. The iPA, DHZR, and ZR are recognized as the most commonly active cytokinins (CTKs) in plants. The results of the experiment by indicated that elevated CO2 may play a role similar to gibberellin (GA) in the integration of carbohydrate and nitrogen metabolisms underlying the optimal biomass determination. When the Arabidopsis plants exhibited the inhibition of growth via the GA biosynthesis inhibitor (low-GA regime), the activities of the enzymes involved in photosynthesis, including the CBB cycle enzymes [phosphoglycerate kinase (PGK) and transketolase (TK)], were enhanced by elevated CO2, whereas the activities of the enzymes related to organic acid metabolism, such as the NAD-dependent malate dehydrogenase (MDH), were inhibited. Moreover, nitrate reductase (NR) can be stimulated by elevated CO2 (by 31%) in plants with a low-GA content, indicating that rising CO2 may mediate inorganic N metabolism in association with GA. This clearly indicates that elevated CO2 can substitute for the relevant metabolic bioprocesses in the low-GA species, which may have a marked potential application for plants, particularly staple crops, to cope with future climate change in a high CO2 concentration environment.

31The genes expressed differently between ambient and elevated CO2 might encode great changes in their metabolic functions, including increases in the expression of a subset of genes encoding stress-related functions, and decreases in the expression of genes encoding chloroplast functions and other processes of photosynthesis. The decline in the gene expression may partly lead to so-called photosynthetic acclimation to long-term elevated CO2, particularly under limited environmental conditions or in carbon sink limited species. For example, found the overexpression of Rubisco activase in rice leaves grown under elevated CO2, possibly leading to a decrease in the photosynthetic capacity. However, the gene expression in response to CO2 fumigation may depend on different developmental stages at the time of sampling, and different physiological conditions of the ecotypes of A. thaliana. Moreover, limited N, a typical example of a nutrition resource deficit, may lower the stimulation of photosynthesis by elevated CO2, due to excess photoassimilate availability, triggering sugar-signaling feedback. This reduces the expression of the photosynthetic genes, especially in Rubisco, leading to the allocation of photosynthetic N into sinks that are more necessary for relative biosynthesis. Based on a report by, the enhanced expression of the limited CO2-induced gene HLA3 may increase the HCO3- transport and photosynthetic Ci affinity, which may counter the down-regulation of the photosynthetic capacity under CO2 enrichment, if the gene can be transported to higher plants. This demonstrates the potential modified gene applications in the improvement of photosynthetic regulation traits in high CO2 climates.

32In a rice cultivar, gene expression for D1 protein (a protein of PSII gene) was down-regulated by 20% at heat stress under elevated CO2, but this change did not occur in another cultivar, indicated that elevated CO2 may enhance the damage of D1 protein, depending on genotypic variation. Based on a recent study in poplar plants, only eight significantly changed key genes involved in crucial metabolisms in response to elevated CO2 were identified by a qRT-PCR test. During wheat plant senescence, up-regulation of genes related to nitrogen remobilization, and down-regulation of genes related to carbon remobilization were observed under elevated CO2, reflecting greater grain N-sink strength of developing grains. Based on a microarray analysis, the A. thaliana photosynthetic gene expression can be most adversely affected by abiotic stress, such as heat and drought, where almost all of genes were down-regulated. However, the greatest down-regulation in gene expression can be diminished by elevated CO2. From the genome-wide expression profiling of the mRNA in A. thaliana leaves, 3643 differentially expressed genes appeared between plants exposed to climate extremes and ambient CO2, whereas only 2841 genes were obtained when grown under elevated CO2. Specifically, both the up-regulated and down-regulated genes were remarkably lower in plants exposed to elevated CO2, than in ambient CO2. For example, under stressful conditions such as heat and drought, the down-regulations of the genes involved in the light reactions (photosystem I and II, light-harvesting complex II), pigment synthesis, and the Calvin cycle can be dampened by elevated CO2, being consistent with changes in photosynthetic rates. It is indicated that elevated CO2 may repress the impact of climate extremes on gene expression in rosette leaves. It is worth noting that we only presented a general description here, and a detailed list of the gene expression differences may be found in the report by.

33Elevated CO2 often can induce a marked decline in photorespiration (see above), suggesting that there may be an involvement of the expression of the genes related to photorespiration pathway including both transcripts and metabolite levels. A BOUT DE SOUFFLE (BOU) gene encoding a mitochondrial carrier may be involved in photorespiration in Arabidopsis because of the knockout mutant bou-2 can arrest growth at ambient CO2, but not at high CO2 concentration, implying BOU gene linking glycine decarboxylase (GDC) activity, may regulate the response to CO2 concentration changes. Plants defective (glyk1 mutants), the gene encoding glycerate kinase (GLYK), cannot grow in normal CO2 level but fully recover at elevated CO2, which the reasonable reason why the mutant requires a high CO2 concentration is unknown. The transcript levels of some photorespiratory genes up-regulated such as plastid chaperonin proteins (CPN60B), and those down-regulated such as GDC under heat and drought stresses, were largely repressed under elevated CO2, but that is not universal for all genes. Furthermore, according to a study by, the transcript levels of photorespiratory genes in Arabidopsis were almost unchanged at high CO2 concentration except a decline in transcript levels of glycine decarboxylase H-protein (GDCH1) that functions in photorespiratory carbon recovery. Thus, whether and how the photorespiratory gene expression play a major role in responses to atmospheric CO2 concentration changes are mostly unknown, which needs to be tested further.

34Because antioxidant defense systems would be enhanced by elevated CO2, the gene expression levels of antioxidant enzymes may be also promoted accordingly. In A. thaliana plants, CO2 enrichment can up-regulate the gene transcriptional expression of an antioxidant enzyme, dehydroascorbate reductase (DHAR), but down-regulate that of CAT, particularly under stressful environments. However, the gene expression changes in others such as APX, GR, GPX, POX, and SOD to elevated CO2 were not significant. Additionally, a high transcript abundance for the majority of the genes coding antioxidant recycling enzymes enhanced by high O3 concentration was also not affected by elevated CO2. Elevated CO2 did not modify the up-regulation of transcripts of oxidative-stress-related genes induced by herbivory or elevated O3 in soybean plants. indicating that CO2 enrichment cannot alleviate harmful effects from O3 pollution based on a gene expression test in birch plants. Thus, the authors could not conclude that CO2 enrichment can up-regulate the gene transcriptional expression levels of the antioxidant enzymes under stressful environment. Further studies are needed urgently to elucidate the molecular responses in the diverse antioxidant systems in responses to elevated CO2 with the key environmental factors including drought, heat, and ozone.

35One recent research study described the results of a gene bioinformatics analysis of hardy winter wheat (Triticum aestivum), with different low temperature adaptive capacities in response to elevated CO2. The genes induced by elevated CO2 was three times higher in the non-acclimated (NA) relative to cold-acclimated (CA) conditions (1,022 vs. 372). The greatest down-regulation of genes appeared in the plant defense responses in the NA plants. On the other hand, CA can reverse this down-regulation, due to the cold-induced genes involved in the plant’s resistance to pathogenesis, and cellular and chloroplast protection, suggesting that cold-adapted hardy winter plants may be less affected by elevated CO2. Conversely, the plants that are more sensitive to cold weather may be regulated both easily and drastically via CO2 enrichment. Of note is the down-regulative interaction of high CO2 levels with low temperature adaptations, which requires further investigation.

36Another recent microarray study describes the expression of the respiratory genes in A. thaliana plants exposed to elevated CO2, with both limited and sufficient N availabilities. This analysis showed that 4439 transcripts were significantly different between the ambient and elevated CO2. Particularly, the transcriptional response of the genes related to protein synthesis was greatest during the day, due to elevated CO2 induction. These genes included those related to the components of glycolysis, the TCA cycle, the mitochondrial electron transport chain (ETC), and the mitochondrial protein import complexes. The evidence of the up-regulation of the transcription of the genes, with relation to respiration under elevated CO2 levels, has also been obtained from rice and soybean plants. Furthermore, 1,708 transcripts differed significantly in abundance between the limited N and ample N availabilities, while 258 transcripts differed significantly due to the interactions of the CO2 level and N availability, again indicating that the expression of the genes related to the key physiological bioprocesses in response to elevated CO2 may be markedly affected by other environmental factors, such as N limitation and day length. It is worth pointing out that the systematicness and complexity of the underlying molecular mechanisms may coexist in the plant response to elevated CO2, and its interaction with other multiple abiotic factors including nutrition condition.

37In addition to the relevant studies concerning the specific gene manipulation and genome-wide transcriptional analysis, with strong selective pressure due to the novel CO2 level, the evolutionary adaption to an atmospheric CO2 concentration change has been found in many reports of the stomatal developmental response. Moreover, because the previous studies concerning the response to CO2 enrichment are often limited to one generation of the plant life-cycle, to further understand the genetic variations in the plants exposed to long-term elevated CO2, found that the maternal genetic effects of elevated CO2 cannot be retrieved in their offspring after undergoing 15 generations of A. thaliana grown in a long-term elevated CO2 atmosphere, indicating the lack of genetic variation and specific adaptations for CO2-enriched responsiveness. It is suggested that selective pressure from elevated CO2 may be not enough to produce a genetic modification to adapt to new environmental changes. This issue should be investigated further, with long-term exposure to elevated CO2.

38There have been several review reports concerning the interactions between elevated CO2 and other abiotic factors, such as temperature or drought, on plant growth and physiological processes (e.g.,;;; ). However, the underlying mechanism concerning the responses to CO2 enrichment with multiple factors has rarely been systematically reviewed. Although the related descriptions have been presented in the appropriate places above, here, we present a succinct statement, particularly for the underlying mechanisms in physiological responses to elevated CO2, in combination with several abiotic factors, such as drought and heat waves.

39Water deficits and heat waves are considered to be the most critical stress factors with markedly potential effects on plant growth, crop yield, vegetation productivity, photosynthetic capacity, promotion of ROS accumulation (such as H2O2), and the oxidative enhancement of functional molecules, such as active proteins and DNA (e.g.,;;;,,; ). With the exception of a few reports (e.g.,; ), most of the relevant studies have concluded that CO2 fertilization may mitigate the adverse impacts of environmental stresses, such as heat, drought, O3 pollution, and their combinations. These aspects of the mitigation of CO2 enrichment include relatively increased individual growth (e.g., ) and community production, enhanced photosynthesis, elevated WUE and NUE, optimized chlorophyll fluorescence, up-regulated antioxidant defense metabolism via increased lipophilic antioxidants and membrane-protecting enzymes, and decreased photorespiration with low H2O2 production.

40Elevated CO2 may help the leaf tissues of a dominant grass in Northern China to partly escape the negative effects of heat and drought stresses on plant growth, canopy structure, leaf development, photosynthetic potential, and antioxidant systems. For A. thaliana plants, the combination of the heat and drought-induced inhibition of photosynthesis was 62% under ambient CO2, but the reduction in photosynthesis was only 40% with elevated CO2. Furthermore, the protein carbonyl content, a marker of protein oxidation, increased significantly during a heat wave and drought, in which the effects were repressed by increased CO2. The dramatic differences between the altered transcriptional expression of A. thaliana plants subjected to a combination of heat and drought stresses were demonstrated in the presence and absence of elevated CO2, with less down-regulation of the genes involved in the light reactions (photosystem I and II, light-harvesting complex II), pigment synthesis, and the CBB under elevated CO2. Additionally, there was less limitation to the photosynthetic parameters, such as Anet, maximum photochemical efficiency (Fv/Fm), gs, and chlorophyll content, possibly due to the effectiveness of the mitigation of the CO2 enrichment. Moreover, following cancelation of the extreme heat and drought stresses, the plant growth and physiological activities related to the positive responses to growth may partly resume at high CO2 concentrations, and the oxidative stress can be greatly alleviated, although they cannot reach the control levels. Again, this implies that CO2 fertilization may alleviate the damage of extreme climatic events, such as snap heat waves and droughts, compromising part of the loss and accelerating recovery in case of the elimination of severe abiotic stress. However, a recent report indicated that high temperature, with no elevated CO2, provokes the drought sensitivity of the leaf to gas exchange, while the latter did not affect the Eucalyptus radiata seedling response to drought, and cannot alleviate the negative effects of rising temperature on drought stress. From another point of view, drought, warming, air pollution, and, particularly, their combination may substantially negate the elevated-CO2 stimulation in photosynthesis, plant growth, and productivity, which is worth noting.

41A hot issue has arisen, in which the photosynthetic responses to elevated CO2 and its combination with climatic change may differ completely between plant species within their photosynthetic pathways. Because of the C4 specific photosynthetic pathway with a CO2-concentrating pump, they cannot benefit from elevated CO2 relative to C3 plants. However, there is a practical and explicable positive response of growth and photosynthesis to elevated CO2, with drought and heat stress in C4 plants. (1) Although no obvious response to CO2 enrichment occurs under ample water availability, great stimulation in the growth and photosynthetic capacity may be obtained under water deficits, due to the marked alleviations of drought stress via gs reduction and WUE elevation, and oxidative stress mitigation under elevated CO2 (e.g.,; ). (2) The higher temperature might benefit the C4 species that originate from, and currently grow under warming conditions, and because the photorespiration of C3 plants increases with rising temperature, leading to a reduction in the Anet. The C4 plants lack photorespiration pathways, with no effect on photosynthesis. Actually, both hypotheses have been well tested in several reports. This positive response to a combination of CO2 enrichment and warming, as well as water deficits, highlights the fact that C4 plants may have a great potential advantage in future climatic change. A higher CO2 concentration with warming and drought suggests that C4 plants may prosper in these vulnerable ecosystems in arid and semiarid regions in the future. However, elevated CO2 induced an electron transfer rate (ETR) enhancement in one C3 species, Halimione portulacoides, and one C4 species, Spartina maritime, but with lower photosynthetic efficiency in the C4 plants due to an increase in the dissipated energy flux, indicated by higher non-photochemical quenching (NPQ), suggesting that the abundance of C3 species may increase in Mediterranean halophyte vegetation. Thus, the future climatic change may induce a rapid shift in some terrestrial vegetation, because of the different responses between the species with the specific photosynthetic pathways, such as C3 and C4 plants, depending on the combination of multiple climatic factors.

42We briefly summarize several key points. (1) Elevated CO2 generally increases the Anet, in which the positive responses strongly depend on the plant functional groups and species, with the expected stimulation from rising CO2, for almost all of the C3 species, but only for C4 plants under water deficit conditions (due to the CCM). The performance of Rubisco in fixing carbon is promoted by CO2 enrichment, because of its dual character. However, a downregulation in the photosynthetic capacity may occur because of the decreased ATP:ADP ratio, diluted N, and excessive photosynthate accumulation under continually rising CO2, particularly under N and/or carbon sink limitations. (2) An elevated CO2-induced suppression of photorespiration has been tested using a lower Gly:Ser ratio as an indicator, while a general negative response in mitochondrial respiration varies, depending on the species. The balance between the increased respiratory substrate and diluted N may play a key role in the rising CO2-induced response, with evidence from the expression up-regulation of the genes related to the respiratory pathway. (3) Plants may run an antioxidant defense system with both the enzymatic and non-enzymatic machinery protected from the damage of oxidative stress due to the generation of ROS under abiotic stresses (such as drought and heat), while elevated CO2 may partly promote the accumulation of antioxidants like polyphenols and ascorbate, and enhance some antioxidant enzyme activities to diminish the oxidative stress from abiotic factors, alone or combination, depending on the genetic variations and plant developmental stage. (4) Elevated CO2 leads to a lower N level and higher content of the total non-structural carbohydrates (TNC), including starch and sugars, while remaining mostly stable in the totals of the structural carbohydrates, lignin, and lipids. However, some secondary metabolites, such as phenylpropanoids, tannins, and phenolic acids, are enhanced by CO2 enrichment. Isoprene emissions may be weakened by elevated CO2, because biosynthesis may need to balance the ATP and NADPH with photosynthetic metabolism. (5) Elevated CO2 might mitigate the adverse effects of abiotic stresses via relatively increased individual growth, enhanced photosynthesis, increased resource use efficiency, promoted antioxidant defense metabolism, and decreased photorespiration under multiple environmental stresses. In terms of the photosynthetic pathway, CO2 enrichment did not affect C4 plants under ample environmental conditions, but promoted it when exposed to drought, warming and their combination, predicting a great potential advantage in future climatic change scenarios for the C4 species, particularly in arid and semiarid areas.

43In the future, we may focus on several crucial research aspects: (1) to further elucidate the underlying mechanisms of the response to CO2 enrichment in key biological processes, including photosynthesis, antioxidant machinery, and other related critical metabolic bioprocesses, such as hormone-involved regulation, as well as the relevant biochemical signal cascades; (2) to disentangle and compare the diverse responses from different species and PFTs to elevated CO2 or its combination with other abiotic factors; (3) to integrate various spatial-temporal scales from molecular, cellular, biochemical, physiological, individual, ecosystem, and global vegetation levels, and from instantaneous to annual or longer time-scales to elucidate the underlying genetic mechanisms in association with key biological processes under the effects of global environmental factors, including elevated CO2, warming, drought, and air pollution; (4) to strengthen the linkages to other relevant research subjects, including ecological, biogeoscience, environmental, climatic, and social-economic aspects, to find appropriate synthetic solutions to urgent practical issues like environmental contamination, ecosystem damage, and global warming impacts.

44Future climate change may impact key biological metabolic processes and their feedback. For example, environmental stresses may provoke the generation of ROS in chloroplasts, the site of photosynthesis, while future high CO2 levels may alleviate the limitations of these stresses. We might also use biotechnological tools such as the protection function against ROS to deal with future climatic change. In addition, Rubisco properties may be improved by regulating the transgenic expression of Rubisco activase in crops such as rice, possibly enhancing the photosynthetic capacity under rising CO2, while the high CO2-induced downregulation of the photosynthetic capacity might induce the modification of the photosynthetic pathway. Furthermore, the modified genetic capacity for the high utilization of photosynthate to strengthen sink storage may make plants capable of sustaining increased photosynthesis when the plants are grown in elevated atmospheric CO2, while additional thermo-tolerant transgenic crops may be required to cope simultaneously with climatic warming. Finally, research should be conducted to strengthen the feasible applications from the relevant research results in response to CO2 enrichment, and its combination with multiple environmental factors for ecological management, climate change mitigation, sustainable development, and related policy decisions, but not at the expense of environment.

45YJ is co-first author, ZX and YJ collected and analyzed the data, ZX, YJ, and GZ wrote the manuscript.

46The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.