Rising atmospheric carbon dioxide: plants FACE the future

The relationship between transpiration and nutrient uptake in wheat changes under elevated atmospheric CO2

The impact of elevated [CO₂ ] (e[CO₂ ]) on crops often includes a decrease in their nutrient concentrations where reduced transpiration-driven mass flow of nutrients has been suggested to play a role. We used two independent approaches, a free-air CO₂ enrichment (FACE) experiment in the South Eastern wheat belt of Australia and a simulation study employing the agricultural production systems simulator (APSIM), to show that transpiration (mm) and nutrient uptake (g m^-2 ) of nitrogen (N), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg) and manganese (Mn) in wheat are correlated under e[CO₂ ], but that nutrient uptake per unit water transpired is higher under e[CO₂ ] than under ambient [CO₂ ] (a[CO₂ ]). This result suggests that transpiration-driven mass flow of nutrients contributes to decreases in nutrient concentrations under e[CO₂ ], but cannot solely explain the overall decline.

Do atmospheric CO2 concentration increase, climate and forest management affect iWUE of common beech? Evidences from carbon isotope analyses in tree rings

Beech is one of the most important forest tree species in Europe, hence possible adverse factors affecting its physiology and productivity can have strong ecological and economic impacts. In this context, four beech forests along a latitudinal gradient from southern Apennines to middle European lowlands were selected for chronological determinations of carbon isotope composition (δ13C) in tree-ring cellulose. The main objectives of this study were to assess (i) the effect of climate on the carbon signature of tree-ring cellulose (δ13C); (ii) the physiological response to recent CO2 concentration increment and to climatic variation; and (iii) the relationship between intrinsic water-use efficiency (iWUE, here the average long-term ratio of net photosynthesis to stomatal conductance) and growth of trees in different sites since 1950. Our results demonstrated that site climatic conditions peculiarly affect δ13C. In northern sites, a climatic control of summer precipitation and temperature on stomatal conductance was demonstrated by their opposite correlations with δ13C, negative and positive, respectively. Furthermore, an 'earliness effect' was suggested by a significant relationship between spring temperature and δ13C in the coldest sites and by a positive one between winter temperature and δ13C in the warmest ones. In all the study sites, during the maturity phase, a positive correlation between the increment of CO2 and iWUE was observed, due to an active response of trees to CO2 increment. This increment of CO2 was the main driver of the long term increasing trend of iWUE, resulting by an active response of trees to CO2 fertilization. Moreover, precipitation mostly influences positively and negatively the inter-annual variations of iWUE of the southernmost and northernmost sites, respectively. Overall, we observed a mean increment of 40% of iWUE. Moreover, the sensitivity of iWUE to the increase of CO2 was different between the northernmost and southernmost sites. Increasing iWUE was correlated to growth in the two sites during the release phase and we hypothesize a positive effect of silvicultural treatments.

Elevated [CO2] mitigates the effect of surface drought by stimulating root growth to access sub-soil water

Through stimulation of root growth, increasing atmospheric CO2 concentration ([CO2]) may facilitate access of crops to sub-soil water, which could potentially prolong physiological activity in dryland environments, particularly because crops are more water use efficient under elevated [CO2] (e[CO2]). This study investigated the effect of drought in shallow soil versus sub-soil on agronomic and physiological responses of wheat to e[CO2] in a glasshouse experiment. Wheat (Triticum aestivum L. cv. Yitpi) was grown in split-columns with the top (0-30 cm) and bottom (31-60 cm; 'sub-soil') soil layer hydraulically separated by a wax-coated, root-penetrable layer under ambient [CO2] (a[CO2], ∼400 μmol mol-1) or e[CO2] (∼700 μmol mol-1) [CO2]. Drought was imposed from stem-elongation in either the top or bottom soil layer or both by withholding 33% of the irrigation, resulting in four water treatments (WW, WD, DW, DD; D = drought, W = well-watered, letters denote water treatment in top and bottom soil layer, respectively). Leaf gas exchange was measured weekly from stem-elongation until anthesis. Above-and belowground biomass, grain yield and yield components were evaluated at three developmental stages (stem-elongation, anthesis and maturity). Compared with a[CO2], net assimilation rate was higher and stomatal conductance was lower under e[CO2], resulting in greater intrinsic water use efficiency. Elevated [CO2] stimulated both above- and belowground biomass as well as grain yield, however, this stimulation was greater under well-watered (WW) than drought (DD) throughout the whole soil profile. Imposition of drought in either or both soil layers decreased aboveground biomass and grain yield under both [CO2] compared to the well-watered treatment. However, the greatest 'CO2 fertilisation effect' was observed when drought was imposed in the top soil layer only (DW), and this was associated with e[CO2]-stimulation of root growth especially in the well-watered bottom layer. We suggest that stimulation of belowground biomass under e[CO2] will allow better access to sub-soil water during grain filling period, when additional water is converted into additional yield with high efficiency in Mediterranean-type dryland agro-ecosystems. If sufficient water is available in the sub-soil, e[CO2] may help mitigating the effect of drying surface soil.

Elevated CO2 induces alteration in lignin accumulation in celery (Apium graveolens L.)

Carbon dioxide (CO₂) is an important regulator of plant growth and development, and its proportion in the atmosphere continues to rise now. Lignin is one of the major secondary products in plants with vital biological functions. However, the relationship between CO₂ level and xylogenesis in celery is still unknown. In order to investigate the effects of increasing CO₂ concentration on lignin accumulation in celery, 'Jinnanshiqin' were exposed to two CO₂ applications, 400 (e₀) and 1000 μmol mol^-1 (e₁), respectively. Plant morphology and lignin distribution in celery plants treated with elevated CO₂ did not change significantly. There was an upward trend on lignin content in celery leaves, and the transcript abundance of 12 genes involved in lignin metabolism has altered in response to elevated CO₂. The effects of high level of CO₂ on different tissues were different. Our works confirmed that CO₂ may play an important role in lignin accumulation in celery leaves. The current study will offer new evidence to understand the regulation mechanism of lignin biosynthesis under elevated CO₂ and provide a reference to improve celery quality by adjusting the growth environment.

Determination of leaf respiration in the light: comparison between an isotopic disequilibrium method and the Laisk method

Quantification of leaf respiration is important for understanding plant physiology and ecosystem biogeochemical processes. Leaf respiration continues in the light (R_L ) but supposedly at a lower rate than in the dark (R_Dk ). However, there is no method for direct measurement of R_L and the available methods require nonphysiological measurement conditions. A method based on isotopic disequilibrium quantified R_L (R_L13C ) and mesophyll conductance of young and old fully expanded leaves of six species. R_L13C was compared to R_L determined by the Laisk method (R_L Laisk ) on the very same leaves with a minimum time lag. R_L 13C and R_L Laisk were generally lower than R_Dk , and were not significantly affected by leaf ageing. R_L Laisk and R_L 13C were positively correlated (r²  = 0.35), and both were positively correlated with R_Dk (r²  ≥ 0.6). R_L Laisk was systematically lower than R_L 13C by 0.4 μmol m^-2  s^-1 . Using A/C_c instead of A/C_i curves, a higher photocompensation point Γ* (by 5 μmol mol^-1 ) was found but no influence on R_L Laisk estimates was observed. The results imply that the Laisk method underestimates actual R_L significantly, probably related to the measurement condition of low CO₂ and irradiance. The isotopic disequilibrium method is useful for assessing responses of R_L to irradiance and CO₂ , improving our mechanistic understanding of R_L .

Dynamic analysis of the impact of free-air CO2 enrichment (FACE) on biomass and N uptake in two contrasting genotypes of rice

Elevated CO2 concentrations ([CO2]) in the atmosphere often increase photosynthetic rates and crop yields. However, the degree of the CO2 enhancement varies substantially among cultivars and with growth stage. Here, we examined the responses of two rice cultivars, Wuyunjing23 (WYJ) and IIyou084 (IIY), to two [CO2] (~400 vs ~600) and two nitrogen (N) provision conditions at five growth stages. In general, both seed yield and aboveground biomass were more responsive to elevated [CO2] in IIY than WYJ. However, the responses significantly changed at different N levels and growth stages. At the low N input, yield response to elevated [CO2] was negligible in both cultivars while, at the normal input, yield in IIY was 18.8% higher under elevated [CO2] than ambient [CO2]. Also, responses to elevated [CO2] significantly differed among various growth stages. Elevated [CO2] tended to increase aboveground plant biomass in both cultivars at the panicle initiation (PI) and the heading stages, but this effect was significant only in IIY by the mid-ripening and the grain maturity stages. In contrast, CO2 enhancement of root biomass only occurred in IIY. Elevated [CO2] increased both total N uptake and seed N in IIY but only increased seed N in WYJ, indicating that it enhanced N translocation to seeds in both cultivars but promoted plant N acquisition only in IIY. Root C accumulation and N uptake also exhibited stronger responses in IIY than in WYJ, particularly at the heading stage, which may play a pivotal role in seed filling and seed yield. Our results showed that the more effective use of CO2 in IIY compared with WYJ results in a strong response in root growth, nitrogen uptake, and in yield. These findings suggest that selection of [CO2]-responsive rice cultivars may help optimise the rice yield under future [CO2] scenarios.

Impact of priming on global soil carbon stocks

Fresh carbon input (above and belowground) contributes to soil carbon sequestration, but also accelerates decomposition of soil organic matter through biological priming mechanisms. Currently, poor understanding precludes the incorporation of these priming mechanisms into the global carbon models used for future projections. Here, we show that priming can be incorporated based on a simple equation calibrated from incubation and verified against independent litter manipulation experiments in the global land surface model, ORCHIDEE. When incorporated into ORCHIDEE, priming improved the model's representation of global soil carbon stocks and decreased soil carbon sequestration by 51% (12 ± 3 Pg C) during the period 1901-2010. Future projections with the same model across the range of CO₂ and climate changes defined by the IPCC-RCP scenarios reveal that priming buffers the projected changes in soil carbon stocks - both the increases due to enhanced productivity and new input to the soil, and the decreases due to warming-induced accelerated decomposition. Including priming in Earth system models leads to different projections of soil carbon changes, which are challenging to verify at large spatial scales.

Structural and functional changes in coffee trees after 4 years under free air CO2 enrichment

BACKGROUND AND AIMS

Climate forecasts suggest that [CO2] in the atmosphere will continue to increase. Structural and ecophysiological responses to elevated air [CO2] (e[CO2]) in tree species are contradictory due to species-dependent responses and relatively short-term experiments. It was hypothesized that long-term exposure (4 year) to e[CO2] would change canopy structure and function of Coffea arabica trees.

METHODS

Coffee plants were grown in a FACE (free air CO2 enrichment) facility under two air [CO2]: actual and elevated (actual + approx. 200 μL CO2 L-1). Plants were codified following the VPlants methodology to obtain coffee mock-ups. Plant canopies were separated into three 50 cm thick layers over a vertical profile to evaluate their structure and photosynthesis, using functional-structural plant modelling.

KEY RESULTS

Leaf area was strongly reduced on the bottom and upper canopy layers, and increased soil carbon concentration suggested changes in carbon partitioning of coffee trees under e[CO2]. Increased air [CO2] stimulated stomatal conductance and leaf photosynthesis at the middle and upper canopy layers, increasing water-use efficiency. Under e[CO2], plants showed reduced diameter of the second-order axes and higher investment in the youngest third to fifth-order axes.

CONCLUSIONS

The responses of Arabica coffee grown under long-term exposure to e[CO2] integrated structural and functional modifications, which balanced leaf area loss through improvements in leaf and whole-plant photosynthesis.

Local plant responses to global problems: Dactylis glomerata responses to different traffic pollutants on roadsides

The growing number of road vehicles is a major source of regional and global atmospheric pollution increasing concentrations of CO₂ in the air, and levels of metals in air and soil. Nevertheless, the effects of these pollutants on plants growing at roadsides are poorly documented. We carried out an observational study of unmanipulated plants growing by the road, to identify the morpho-physiological responses in a perennial grass Dactylis glomerata. Firstly, we wanted to know the general effect of traffic intensity and ambient CO₂ and its interactions on different plant traits. Accordingly, we analyzed the photosynthetic response by field A/Ci Response Curves, SLA, pigment pools, foliar nitrogen, carbohydrates and morphological traits in plants at three distances to the road. Secondly, we wanted to know if Dactylis glomerata plants can accumulate metals present on the roadside (Pb, Zn, Cu, and Sr) in their tissues and rhizosphere, and the effect of these metals on morphological traits. The MANCOVA whole model results shown: 1) a significant effect of road ambient CO₂ concentration on morphological traits (not affected by traffic intensity, P _{interaction CO2 x traffic intensity}>0.05), that was mainly driven by a significant negative relationship between the inflorescence number and ambient CO₂; 2) a positive and significant relationship between ambient CO₂ and the starch content in leaves (unaffected by traffic intensity); 3) a reduction in J_max (electron transport rate) at high traffic intensity. These lines of evidences suggest a decreased photosynthetic capacity due to high traffic intensity and high levels of ambient CO₂. In addition, Pb, Cu, Zn and Sr were detected in Dactylis glomerata tissues, and Cu accumulated in roots. Finally, we observed that Dactylis glomerata individuals growing at the roadside under high levels of CO₂ and in the presence of metal pollutants, reduced their production of inflorescences.

Dynamics of metabolic responses to periods of combined heat and drought in Arabidopsis thaliana under ambient and elevated atmospheric CO2

As a consequence of global change processes, plants will increasingly be challenged by extreme climatic events, against a background of elevated atmospheric CO2. We analysed responses of Arabidopsis thaliana to periods of a combination of elevated heat and water deficit at ambient and elevated CO2 in order to gain mechanistic insights regarding changes in primary metabolism. Metabolic changes induced by extremes of climate are dynamic and specific to different classes of molecules. Concentrations of soluble sugars and amino acids increased transiently after short (4-d) exposure to heat and drought, and readjusted to control levels under prolonged (8-d) stress. In contrast, fatty acids showed persistent changes during the stress period. Elevated CO2 reduced the impact of stress on sugar and amino acid metabolism, but not on fatty acids. Integrating metabolite data with transcriptome results revealed that some of the metabolic changes were regulated at the transcriptional level. Multivariate analyses grouped metabolites on the basis of stress exposure time, indicating specificity in metabolic responses to short and prolonged stress. Taken together, the results indicate that dynamic metabolic reprograming plays an important role in plant acclimation to climatic extremes. The extent of such metabolic adjustments is less under high CO2, further pointing towards the role of high CO2 in stress mitigation.

Do all leaf photosynthesis parameters of rice acclimate to elevated CO2 , elevated temperature, and their combination, in FACE environments?

Leaf photosynthesis of crops acclimates to elevated CO₂ and temperature, but studies quantifying responses of leaf photosynthetic parameters to combined CO₂ and temperature increases under field conditions are scarce. We measured leaf photosynthesis of rice cultivars Changyou 5 and Nanjing 9108 grown in two free-air CO₂ enrichment (FACE) systems, respectively, installed in paddy fields. Each FACE system had four combinations of two levels of CO₂ (ambient and enriched) and two levels of canopy temperature (no warming and warmed by 1.0-2.0°C). Parameters of the C₃ photosynthesis model of Farquhar, von Caemmerer and Berry (the FvCB model), and of a stomatal conductance (g_s ) model were estimated for the four conditions. Most photosynthetic parameters acclimated to elevated CO₂ , elevated temperature, and their combination. The combination of elevated CO₂ and temperature changed the functional relationships between biochemical parameters and leaf nitrogen content for Changyou 5. The g_s model significantly underestimated g_s under the combination of elevated CO₂ and temperature by 19% for Changyou 5 and by 10% for Nanjing 9108 if no acclimation was assumed. However, our further analysis applying the coupled g_s -FvCB model to an independent, previously published FACE experiment showed that including such an acclimation response of g_s hardly improved prediction of leaf photosynthesis under the four combinations of CO₂ and temperature. Therefore, the typical procedure that crop models using the FvCB and g_s models are parameterized from plants grown under current ambient conditions may not result in critical errors in projecting productivity of paddy rice under future global change.

Effects of Elevated Atmospheric CO2 on Respiratory Rates in Mature Leaves of Two Rice Cultivars Grown at a Free-Air CO2 Enrichment Site and Analyses of the Underlying Mechanisms

Respiratory CO2 efflux and O2 uptake rates in leaves change in response to the growth CO2 concentration ([CO2]). The degrees of change vary depending on the responses of cellular processes such as nitrogen (N) assimilation and accumulation of organic acids to growth [CO2]. However, the underlying mechanisms remain unclear. Here, we examined the respiratory characteristics of mature leaves of two rice varieties with different yield capacities at different growth stages under ambient and elevated [CO2] conditions at a free-air CO2 enrichment site. We also examined the effect of increased water temperature on leaf respiration. We measured the rates of CO2 efflux and O2 uptake, and determined N contents, primary metabolite contents and maximal activities of respiratory enzymes. The leaf CO2 efflux rates decreased in plants grown at elevated [CO2] in both varieties, and were higher in high-yielding Takanari than in Koshihikari. The leaf O2 uptake rates showed little change with respect to growth [CO2] and variety. The increased water temperature did not significantly affect the CO2 efflux and O2 uptake rates. The N and amino acid contents were significantly higher in Takanari than in Koshihikari. The enhanced N assimilation in Takanari may have consumed more respiratory NADH, leading to higher CO2 efflux rates. In Koshihikari, the ratio of tricarboxylic acid (TCA) cycle intermediates changed and maximal activities of enzymes in the TCA cycle decreased at elevated [CO2]. Therefore, the decreased rates of CO2 efflux in Koshihikari may be due to the decreased activities of TCA cycle enzymes at elevated [CO2].

The optimal CO2 concentrations for the growth of three perennial grass species

BACKGROUND

Grasslands are one of the most representative vegetation types accounting for about 20% of the global land area and thus the response of grasslands to climate change plays a pivotal role in terrestrial carbon balance. However, many current climate change models, based on earlier results of the doubling-CO2 experiments, may overestimate the CO2 fertilization effect, and as a result underestimate the potentially effects of future climate change on global grasslands when the atmospheric CO2 concentration goes beyond the optimal level. Here, we examined the optimal atmospheric CO2 concentration effect on CO2 fertilization and further on the growth of three perennial grasses in growth chambers with the CO2 concentration at 400, 600, 800, 1000, and 1200 ppm, respectively.

RESULTS

All three perennial grasses featured an apparent optimal CO2 concentration for growth. Initial increases in atmospheric CO2 concentration substantially enhanced the plant biomass of the three perennial grasses through the CO2 fertilization effect, but this CO2 fertilization effect was dramatically compromised with further rising atmospheric CO2 concentration beyond the optimum. The optimal CO2 concentration for the growth of tall fescue was lower than those of perennial ryegrass and Kentucky bluegrass, and thus the CO2 fertilization effect on tall fescue disappeared earlier than the other two species. By contrast, the weaker CO2 fertilization effect on the growth of perennial ryegrass and Kentucky bluegrass was sustained for a longer period due to their higher optimal CO2 concentrations than tall fescue. The limiting effects of excessively high CO2 concentrations may not only associate with changes in the biochemical and photochemical processes of photosynthesis, but also attribute to the declines in stomatal conductance and nitrogen availability.

CONCLUSIONS

In this study, we found apparent differences in the optimal CO2 concentrations for the growth of three grasses. These results suggest that the growth of different types of grasses may respond differently to future elevated CO2 concentrations through the CO2 fertilization effect, and thus potentially alter the community composition and structure of grasslands. Meanwhile, our results may also be helpful for improving current process-based ecological models to more accurately predict the structure and function of grassland ecosystems under future rising atmospheric CO2 concentration and climate change scenarios.

Physiological acclimation dampens initial effects of elevated temperature and atmospheric CO2 concentration in mature boreal Norway spruce

Physiological processes of terrestrial plants regulate the land-atmosphere exchange of carbon, water, and energy, yet few studies have explored the acclimation responses of mature boreal conifer trees to climate change. Here we explored the acclimation responses of photosynthesis, respiration, and stomatal conductance to elevated temperature and/or CO₂ concentration ([CO₂ ]) in a 3-year field experiment with mature boreal Norway spruce. We found that elevated [CO₂ ] decreased photosynthetic carboxylation capacity (-23% at 25 °C) and increased shoot respiration (+64% at 15 °C), while warming had no significant effects. Shoot respiration, but not photosynthetic capacity, exhibited seasonal acclimation. Stomatal conductance at light saturation and a vapour pressure deficit of 1 kPa was unaffected by elevated [CO₂ ] but significantly decreased (-27%) by warming, and the ratio of intercellular to ambient [CO₂ ] was enhanced (+17%) by elevated [CO₂ ] and decreased (-12%) by warming. Many of these responses differ from those typically observed in temperate tree species. Our results show that long-term physiological acclimation dampens the initial stimulation of plant net carbon assimilation to elevated [CO₂ ], and of plant water use to warming. Models that do not account for these responses may thus overestimate the impacts of climate change on future boreal vegetation-atmosphere interactions.

Responses of Intrinsic Water-use Efficiency and Tree Growth to Climate Change in Semi-Arid Areas of North China

Tree-level intrinsic water-use efficiency (iWUE) is derived from the tree-ring 13C isotope composition (δ13C) and is an important indicator of the adaptability for trees to climate change. However, there is still uncertainty regarding the relationship between long-term forest ecosystem carbon sequestration capacity and iWUE. To determine whether elevated atmospheric CO2 concentration (Ca) increase iWUE and tree growth (basal area increment, BAI), dendrochronological methods and stable isotope analyses were used to examine annual changes in the tree-ring width and carbon isotope composition (δ13C) of Platycladus orientalis in northern China. The iWUE derived from δ13C has increased significantly (p < 0.01). Long-term iWUE trend was largely and positively driven by the elevated atmospheric CO2 concentration and temperature. We observed a general increase in averaged BAI, which had significant positive correlation with iWUE (R2 = 0.3186, p < 0.01). Increases in iWUE indeed translated into enhanced P. orientalis growth in semi-arid areas of northern China. Elevated atmospheric CO2 concentration significantly (p < 0.01) stimulated P. orientalis biomass accumulation when Ca was less than approximately 320 ppm in the early phase; however, this effect was not pronounced when Ca exceeded 320 ppm.

Interactive Effects of CO2 Concentration and Water Regime on Stable Isotope Signatures, Nitrogen Assimilation and Growth in Sweet Pepper

Sweet pepper is among the most widely cultivated horticultural crops in the Mediterranean basin, being frequently grown hydroponically under cover in combination with CO2 fertilization and water conditions ranging from optimal to suboptimal. The aim of this study is to develop a simple model, based on the analysis of plant stable isotopes in their natural abundance, gas exchange traits and N concentration, to assess sweet pepper growth. Plants were grown in a growth chamber for near 6 weeks. Two [CO2] (400 and 800 μmol mol-1), three water regimes (control and mild and moderate water stress) and four genotypes were assayed. For each combination of genotype, [CO2] and water regime five plants were evaluated. Water stress applied caused significant decreases in water potential, net assimilation, stomatal conductance, intercellular to atmospheric [CO2], and significant increases in water use efficiency, leaf chlorophyll content and carbon isotope composition, while the relative water content, the osmotic potential and the content of anthocyanins did change not under stress compared to control conditions support this statement. Nevertheless, water regime affects plant growth via nitrogen assimilation, which is associated with the transpiration stream, particularly at high [CO2], while the lower N concentration caused by rising [CO2] is not associated with stomatal closure. The stable isotope composition of carbon, oxygen, and nitrogen (δ13C, δ18O, and δ15N) in plant matter are affected not only by water regime but also by rising [CO2]. Thus, δ18O increased probably as response to decreases in transpiration, while the increase in δ15N may reflect not only a lower stomatal conductance but a higher nitrogen demand in leaves or shifts in nitrogen metabolism associated with decreases in photorespiration. The way that δ13C explains differences in plant growth across water regimes within a given [CO2], seems to be mediated through its direct relationship with N accumulation in leaves. The changes in the profile and amount of amino acids caused by water stress and high [CO2] support this conclusion. However, the results do not support the use of δ18O as an indicator of the effect of water regime on plant growth.

Methods for Elucidation of Plant Senescence in Response to C/N-Nutrient Balance

Carbon (C) and nitrogen (N) are essential elements for metabolism, and the ratio of C to N availability is called the C/N balance. C/N balance is very important for plant growth, but little is known about the detailed mechanisms of plant C/N responses. Previously a method of treating Arabidopsis plants with sugar-supplemented medium for studying C/N responses at early post-germinative growth stages has been developed. This method, however, cannot be used to determine physiological C/N effects in plants of mature growth stages, including senescence. Here we present two methods of analyzing responses to C/N treatments in senescing plants: transient C/N treatment with liquid medium and long-term C/N treatment with elevated atmospheric CO₂.

Decadal analysis of impact of future climate on wheat production in dry Mediterranean environment: A case of Jordan

Different aspects of climate change, such as increased temperature, changed rainfall and higher atmospheric CO₂ concentration, all have different effects on crop yields. Process-based crop models are the most widely used tools for estimating future crop yield responses to climate change. We applied APSIM crop simulation model in a dry Mediterranean climate with Jordan as sentinel site to assess impact of climate change on wheat production at decadal level considering two climate change scenarios of representative concentration pathways (RCP) viz., RCP4.5 and RCP8.5. Impact of climatic variables alone was negative on grain yield but this adverse effect was negated when elevated atmospheric CO₂ concentrations were also considered in the simulations. Crop cycle of wheat was reduced by a fortnight for RCP4.5 scenario and by a month for RCP8.5 scenario at the approach of end of the century. On an average, a grain yield increase of 5 to 11% in near future i.e., 2010s-2030s decades, 12 to 16% in mid future i.e., 2040s-2060s decades and 9 to 16% in end of century period can be expected for moderate climate change scenario (RCP4.5) and 6 to 15% in near future, 13 to 19% in mid future and 7 to 20% increase in end of century period for a drastic climate change scenario (RCP8.5) based on different soils. Positive impact of elevated CO₂ is more pronounced in soils with lower water holding capacity with moderate increase in temperatures. Elevated CO₂ had greater positive effect on transpiration use efficiency (TUE) than negative effect of elevated mean temperatures. The change in TUE was in near perfect direct relationship with elevated CO₂ levels (R²>0.99) and every 100-ppm atmospheric CO₂ increase resulted in TUE increase by 2kgha^-1mm^-1. Thereby, in this environment yield gains are expected in future and farmers can benefit from growing wheat.

Climate Change and the Impact of Greenhouse Gasses: CO2 and NO, Friends and Foes of Plant Oxidative Stress

Here, we review information on how plants face redox imbalance caused by climate change, and focus on the role of nitric oxide (NO) in this response. Life on Earth is possible thanks to greenhouse effect. Without it, temperature on Earth's surface would be around -19°C, instead of the current average of 14°C. Greenhouse effect is produced by greenhouse gasses (GHG) like water vapor, carbon dioxide (CO₂), methane (CH₄), nitrous oxides (N_xO) and ozone (O₃). GHG have natural and anthropogenic origin. However, increasing GHG provokes extreme climate changes such as floods, droughts and heat, which induce reactive oxygen species (ROS) and oxidative stress in plants. The main sources of ROS in stress conditions are: augmented photorespiration, NADPH oxidase (NOX) activity, β-oxidation of fatty acids and disorders in the electron transport chains of mitochondria and chloroplasts. Plants have developed an antioxidant machinery that includes the activity of ROS detoxifying enzymes [e.g., superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione peroxidase (GPX), and peroxiredoxin (PRX)], as well as antioxidant molecules such as ascorbic acid (ASC) and glutathione (GSH) that are present in almost all subcellular compartments. CO₂ and NO help to maintain the redox equilibrium. Higher CO₂ concentrations increase the photosynthesis through the CO₂-unsaturated Rubisco activity. But Rubisco photorespiration and NOX activities could also augment ROS production. NO regulate the ROS concentration preserving balance among ROS, GSH, GSNO, and ASC. When ROS are in huge concentration, NO induces transcription and activity of SOD, APX, and CAT. However, when ROS are necessary (e.g., for pathogen resistance), NO may inhibit APX, CAT, and NOX activity by the S-nitrosylation of cysteine residues, favoring cell death. NO also regulates GSH concentration in several ways. NO may react with GSH to form GSNO, the NO cell reservoir and main source of S-nitrosylation. GSNO could be decomposed by the GSNO reductase (GSNOR) to GSSG which, in turn, is reduced to GSH by glutathione reductase (GR). GSNOR may be also inhibited by S-nitrosylation and GR activated by NO. In conclusion, NO plays a central role in the tolerance of plants to climate change.

Elevated CO2 increases photosynthesis in fluctuating irradiance regardless of photosynthetic induction state

Leaves are often exposed to fluctuating irradiance, which limits assimilation. Elevated CO2 enhances dynamic photosynthesis (i.e. photosynthesis in fluctuating irradiance) beyond its effects on steady-state photosynthesis rates. Studying the role of CO2 in dynamic photosynthesis is important for understanding plant responses to changing atmospheric CO2 partial pressures. The rise of photosynthesis after a step-wise increase to 1000 μmol m-2 s-1, the loss of photosynthetic induction after irradiance decreases, and rates of photosynthesis during sinusoidal changes in irradiance were studied in tomato (Solanum lycopersicum L.) leaves, using three CO2 partial pressures (200, 400, and 800 µbar). Initial irradiance was set to 0, 50, 100, and 200 μmol m-2 s-1 to vary the initial induction state. Most responses at 200 µbar were not different from those at 400 µbar. In contrast, CO2 at 800 µbar increased the relative carbon gain by 12% after an increase in irradiance, decreased the loss of photosynthetic induction by 14%, and increased dynamic photosynthesis during sine waves by 17%, compared with 400 µbar. These effects were additional to steady-state effects of elevated CO2 on photosynthesis. The enhancement of dynamic photosynthesis rates by elevated CO2 may therefore additionally increase photosynthesis in a future, CO2-enriched climate.

Impacts of elevated CO2 on exogenous Bacillus thuringiensis toxins and transgene expression in transgenic rice under different levels of nitrogen

Recent studies have highlighted great challenges of transgene silencing for transgenic plants facing climate change. In order to understand the impacts of elevated CO2 on exogenous Bacillus thuringiensis (Bt) toxins and transgene expression in transgenic rice under different levels of N-fertilizer supply, we investigated the biomass, exogenous Bt toxins, Bt-transgene expression and methylation status in Bt rice exposed to two levels of CO2 concentrations and nitrogen (N) supply (1/8, 1/4, 1/2, 1 and 2 N). It is elucidated that the increased levels of global atmospheric CO2 concentration will trigger up-regulation of Bt toxin expression in transgenic rice, especially with appropriate increase of N fertilizer supply, while, to some extent, the exogenous Bt-transgene expression is reduced at sub-N levels (1/4 and 1/2N), even though the total protein of plant tissues is reduced and the plant growth is restricted. The unpredictable and stochastic occurrence of transgene silencing and epigenetic alternations remains unresolved for most transgenic plants. It is expected that N fertilization supply may promote the expression of transgenic Bt toxin in transgenic Bt rice, particularly under elevated CO2.

Elevated carbon dioxide reduces emission of herbivore-induced volatiles in Zea mays

Terpene volatiles produced by sweet corn (Zea mays) upon infestation with pests such as beet armyworm (Spodoptera exigua) function as part of an indirect defence mechanism by attracting parasitoid wasps; yet little is known about the impact of climate change on this form of plant defence. To investigate how a central component of climate change affects indirect defence, we measured herbivore-induced volatile emissions in plants grown under elevated carbon dioxide (CO₂ ). We found that S. exigua infested or elicitor-treated Z. mays grown at elevated CO₂ had decreased emission of its major sesquiterpene, (E)-β-caryophyllene and two homoterpenes, (3E)-4,8-dimethyl-1,3,7-nonatriene and (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. In contrast, inside the leaves, elicitor-induced (E)-β-caryophyllene hyper-accumulated at elevated CO₂ , while levels of homoterpenes were unaffected. Furthermore, gene expression analysis revealed that the induction of terpene synthase genes following treatment was lower in plants grown at elevated CO₂ . Our data indicate that elevated CO₂ leads both to a repression of volatile synthesis at the transcriptional level and to limitation of volatile release through effects of CO₂ on stomatal conductance. These findings suggest that elevated CO₂ may alter the ability of Z. mays to utilize volatile terpenes to mediate indirect defenses.

Leaf and canopy scale drivers of genotypic variation in soybean response to elevated carbon dioxide concentration

The atmospheric [CO2 ] in which crops grow today is greater than at any point in their domestication history and represents an opportunity for positive effects on seed yield that can counteract the negative effects of greater heat and drought this century. In order to maximize yields under future atmospheric [CO2 ], we need to identify and study crop cultivars that respond most favorably to elevated [CO2 ] and understand the mechanisms contributing to their responsiveness. Soybean (Glycine max Merr.) is a widely grown oilseed crop and shows genetic variation in response to elevated [CO2 ]. However, few studies have studied the physiological basis for this variation. Here, we examined canopy light interception, photosynthesis, respiration and radiation use efficiency along with yield and yield parameters in two cultivars of soybean (Loda and HS93-4118) previously reported to have similar seed yield at ambient [CO2 ], but contrasting responses to elevated [CO2 ]. Seed yield increased by 26% at elevated [CO2 ] (600 μmol/mol) in the responsive cultivar Loda, but only by 11% in HS93-4118. Canopy light interception and leaf area index were greater in HS93-4118 in ambient [CO2 ], but increased more in response to elevated [CO2 ] in Loda. Radiation use efficiency and harvest index were also greater in Loda than HS93-4118 at both ambient and elevated [CO2 ]. Daily C assimilation was greater at elevated [CO2 ] in both cultivars, while stomatal conductance was lower. Electron transport capacity was also greater in Loda than HS93-4118, but there was no difference in the response of photosynthetic traits to elevated [CO2 ] in the two cultivars. Overall, this greater understanding of leaf- and canopy-level photosynthetic traits provides a strong conceptual basis for modeling genotypic variation in response to elevated [CO2 ].

Acclimation of light and dark respiration to experimental and seasonal warming are mediated by changes in leaf nitrogen in Eucalyptus globulus

Quantifying the adjustments of leaf respiration in response to seasonal temperature variation and climate warming is crucial because carbon loss from vegetation is a large but uncertain part of the global carbon cycle. We grew fast-growing Eucalyptus globulus Labill. trees exposed to +3 °C warming and elevated CO2 in 10-m tall whole-tree chambers and measured the temperature responses of leaf mitochondrial respiration, both in light (RLight) and in darkness (RDark), over a 20-40 °C temperature range and during two different seasons. RLight was assessed using the Laisk method. Respiration rates measured at a standard temperature (25 °C - R25) were higher in warm-grown trees and in the warm season, related to higher total leaf nitrogen (N) investment with higher temperatures (both experimental and seasonal), indicating that leaf N concentrations modulated the respiratory capacity to changes in temperature. Once differences in leaf N were accounted for, there were no differences in R25 but the Q10 (i.e., short-term temperature sensitivity) was higher in late summer compared with early spring. The variation in RLight between experimental treatments and seasons was positively correlated with carboxylation capacity and photorespiration. RLight was less responsive to short-term changes in temperature than RDark, as shown by a lower Q10 in RLight compared with RDark. The overall light inhibition of R was ∼40%. Our results highlight the dynamic nature of leaf respiration to temperature variation and that the responses of RLight do not simply mirror those of RDark. Therefore, it is important not to assume that RLight is the same as RDark in ecosystem models, as doing so may lead to large errors in predicting plant CO2 release and productivity.

Photo-protective mechanisms in reed canary grass to alleviate photo-inhibition of PSII on the Qinghai-Tibet Plateau

Due to its characteristic of high biomass yield potential, there is considerable interest in cultivating Phalaris arundinacea L. cv. 'chuancaoyin No.3' (reed canary grass) on the Qinghai-Tibet Plateau where there is an abundance of alpine steppe meadow and a potential large market for animal husbandry. In this study, we 1) investigate whether reed canary grass exhibits superior productive capacity to Elymus nutans 'Aba' (E. nutans), ordinary common pasture, during the long warm days of summer at high-altitude; and 2) compare the cold tolerance between reed canary grass and E. nutans, including photosynthesis, photo-inhibition, and photo-protection. The results suggest that reed canary grass exhibits higher photosynthetic capacity compared to E. nutans at latitudes of the cool temperate zone. Meanwhile, cold-induced photo-inhibition and photo-damage at high altitudes in reed canary grass were due to both stomatal and non-stomatal limitation, and the enhancement in photo-respiration, thermal dissipation, and Mehler reaction are important processes to minimize the negative effects of high elevation and a cold environment.

Improved responses to elevated CO2 in durum wheat at a low nitrate supply associated with the upregulation of photosynthetic genes and the activation of nitrate assimilation

Elevated CO₂ often leads to photosynthetic acclimation, and N availability may alter this response. We investigated whether the coordination of shoot-root N assimilation by elevated CO₂ may help to optimize the whole-plant N allocation and maximize photosynthesis in hydroponically-grown durum wheat at two NO₃^- supplies in interaction with plant development. Transcriptional and biochemical analyses were performed on flag leaves and roots. At anthesis, the improved photosynthetic acclimation response to elevated CO₂ at low N was associated with increased Rubisco, chlorophyll and amino acid contents, and upregulation of genes related to their biosynthesis, light reactions and Calvin-Benson cycle, while a decrease was recorded at high N. Despite the decrease in carbohydrates with elevated CO₂ at low N and the increase at high N, a stronger upward trend in leaf NR activity was found at low rather than high N. The induction of N recycling-related genes was accompanied by an amino acids decline at high N. At the grain-filling stage, the photosynthetic acclimation to elevated CO₂ at high N was associated with the downregulation of both N assimilation, mainly in roots, and photosynthetic genes. At low N, enhanced root N assimilation partly compensated for slower shoot N assimilation and maximized photosynthetic capacity.

Strong thermal acclimation of photosynthesis in tropical and temperate wet-forest tree species: the importance of altered Rubisco content

Understanding of the extent of acclimation of light-saturated net photosynthesis (A_n ) to temperature (T), and associated underlying mechanisms, remains limited. This is a key knowledge gap given the importance of thermal acclimation for plant functioning, both under current and future higher temperatures, limiting the accuracy and realism of Earth system model (ESM) predictions. Given this, we analysed and modelled T-dependent changes in photosynthetic capacity in 10 wet-forest tree species: six from temperate forests and four from tropical forests. Temperate and tropical species were each acclimated to three daytime growth temperatures (T_growth ): temperate - 15, 20 and 25 °C; tropical - 25, 30 and 35 °C. CO₂ response curves of A_n were used to model maximal rates of RuBP (ribulose-1,5-bisphosphate) carboxylation (V_cmax ) and electron transport (J_max ) at each treatment's respective T_growth and at a common measurement T (25 °C). SDS-PAGE gels were used to determine abundance of the CO₂ -fixing enzyme, Rubisco. Leaf chlorophyll, nitrogen (N) and mass per unit leaf area (LMA) were also determined. For all species and T_growth , A_n at current atmospheric CO₂ partial pressure was Rubisco-limited. Across all species, LMA decreased with increasing T_growth . Similarly, area-based rates of V_cmax at a measurement T of 25 °C (V_cmax²⁵ ) linearly declined with increasing T_growth , linked to a concomitant decline in total leaf protein per unit leaf area and Rubisco as a percentage of leaf N. The decline in Rubisco constrained V_cmax and A_n for leaves developed at higher T_growth and resulted in poor predictions of photosynthesis by currently widely used models that do not account for T_growth -mediated changes in Rubisco abundance that underpin the thermal acclimation response of photosynthesis in wet-forest tree species. A new model is proposed that accounts for the effect of T_growth -mediated declines in V_cmax²⁵ on A_n , complementing current photosynthetic thermal acclimation models that do not account for T sensitivity of V_cmax²⁵ .

Time dependent genetic analysis links field and controlled environment phenotypes in the model C4 grass Setaria

Vertical growth of plants is a dynamic process that is influenced by genetic and environmental factors and has a pronounced effect on overall plant architecture and biomass composition. We have performed six controlled growth trials of an interspecific Setaria italica x Setaria viridis recombinant inbred line population to assess how the genetic architecture of plant height is influenced by developmental queues, water availability and planting density. The non-destructive nature of plant height measurements has enabled us to monitor height throughout the plant life cycle in both field and controlled environments. We find that plant height is reduced under water limitation and high density planting and affected by growth environment (field vs. growth chamber). The results support a model where plant height is a heritable, polygenic trait and that the major genetic loci that influence plant height function independent of growth environment. The identity and contribution of loci that influence height changes dynamically throughout development and the reduction of growth observed in water limited environments is a consequence of delayed progression through the genetic program which establishes plant height in Setaria. In this population, alleles inherited from the weedy S. viridis parent act to increase plant height early, whereas a larger number of small effect alleles inherited from the domesticated S. italica parent collectively act to increase plant height later in development.

Growth is a dynamic process that responds to a changing environment. Most of the methods that we have for measuring are static and collecting information throughout an organisms lifecycle is labor and cost prohibitive. Advances in imaging and robotics technology have enabled novel approaches to understanding how plants adapt to the environment. Using the model grass Setaria and new methods for measuring parameters from images, we investigate the genetic architecture of plant height in response to water availability and planting density. Height is one of the most influential components of plant architecture, determining tradeoffs between competition and resource allocation and is an important trait for boosting yields. The non-destructive nature of plant height measurements has enabled us to monitor growth throughout the plant life cycle in both field and controlled environments. We identified several loci controlling height in a population derived from a wild strain of Setaria viridis and its domesticated relative Setaria italica, as well as the developmental time in which these loci act. In this population, alleles inherited from the wild parent act to increase plant height early, whereas a larger number of small effect alleles inherited from the domesticated parent collectively act to increase plant height later in development.

A synthesis of the effects of atmospheric carbon dioxide enrichment on plant hydraulics: implications for whole-plant water use efficiency and resistance to drought

Here, we summarize studies on the effects of elevated [CO₂ ] (CO₂^e ) on the structure and function of plant hydraulic architecture and explore the implications of those changes using a model. Changes in conduit diameter and hydraulic conductance due to CO₂^e vary among species. Ring-porous species tend towards an increase in conduit size and consequently conductivity. The effect in diffuse-porous species is much more limited. In conifers, the results are mixed, some species show minor changes in xylem structure, while other studies found increases in tracheid density and diameter. Non-woody plants generally exhibited the reverse pattern with narrower conduits and lower hydraulic conductivity under CO₂^e . Further, changes in drought-resistance traits suggest that non-woody plants were the most affected by CO₂^e , which may permit them to better resist drought-induced embolism under future conditions. Due to their complexity, acclimation in hydraulic traits in response to CO₂^e is difficult to interpret when relying solely on measurements. When we examined how the observed tissues-specific trends might alter plant function, our modelling results suggest that these hydraulic changes would lead to reduced conductance and more frequent drought stress in trees that develop under CO₂^e with a more pronounced effect in isohydric than in anisohydric species.

Differences in the photosynthetic plasticity of ferns and Ginkgo grown in experimentally controlled low [O2]:[CO2] atmospheres may explain their contrasting ecological fate across the Triassic-Jurassic mass extinction boundary

Background and Aims

Fluctuations in [CO 2 ] have been widely studied as a potential driver of plant evolution; however, the role of a fluctuating [O 2 ]:[CO 2 ] ratio is often overlooked. The present study aimed to investigate the inherent physiological plasticity of early diverging, extant species following acclimation to an atmosphere similar to that across the Triassic-Jurassic mass extinction interval (TJB, approx. 200 Mya), a time of major ecological change.

Methods

Mature plants from two angiosperm ( Drimys winteri and Chloranthus oldhamii ), two monilophyte ( Osmunda claytoniana and Cyathea australis ) and one gymnosperm ( Ginkgo biloba ) species were grown for 2 months in replicated walk-in Conviron BDW40 chambers running at TJB treatment conditions of 16 % [O 2 ]-1900 ppm [CO 2 ] and ambient conditions of 21 % [O 2 ]-400 ppm [CO 2 ], and their physiological plasticity was assessed using gas exchange and chlorophyll fluorescence methods.

Key Results

TJB acclimation caused significant reductions in the maximum rate of carboxylation ( V Cmax ) and the maximum electron flow supporting ribulose-1,5-bisphosphate regeneration ( J max ) in all species, yet this downregulation had little effect on their light-saturated photosynthetic rate ( A sat ). Ginkgo was found to photorespire heavily under ambient conditions, while growth in low [O 2 ]:[CO 2 ] resulted in increased heat dissipation per reaction centre ( DI o / RC ), severe photodamage, as revealed by the species' decreased maximum efficiency of primary photochemistry ( F v / F m ) and decreased in situ photosynthetic electron flow ( Jsitu ).

Conclusions

It is argued that the observed photodamage reflects the inability of Ginkgo to divert excess photosynthetic electron flow to sinks other than the downregulated C 3 and the diminished C 2 cycles under low [O 2 ]:[CO 2 ]. This finding, coupled with the remarkable physiological plasticity of the ferns, provides insights into the underlying mechanism of Ginkgoales' near extinction and ferns' proliferation as atmospheric [CO 2 ] increased to maximum levels across the TJB.

Quantitative trait loci for large sink capacity enhance rice grain yield under free-air CO2 enrichment conditions

The global atmospheric CO2 concentration has been increasing annually. To determine the trait that effectively increases rice (Oryza sativa L.) grain yield under increased atmospheric CO2 concentrations, as predicted in the near future, we grew a chromosome segment substitution line (CSSL) and a near-isogenic line (NIL) producing high spikelet numbers per panicle (CSSL-GN1 and NIL-APO1, respectively) under free-air CO2 enrichment (FACE) conditions and examined the effects of a large sink capacity on grain yield, its components, and growth-related traits under increased atmospheric CO2 concentrations. Under ambient conditions, CSSL-GN1 and NIL-APO1 exhibited a similar grain yield to Koshihikari, as a result of the trade-off between increased spikelet number and reduced grain filling. However, under FACE conditions, CSSL-GN1 and NIL-APO1 had an equal or a higher grain yield than Koshihikari because of the higher number of spikelets and lower reduction in grain filling. Thus, the improvement of source activity by increased atmospheric CO2 concentrations can lead to enhanced grain yield in rice lines that have a large sink capacity. Therefore, introducing alleles that increase sink capacity into conventional varieties represents a strategy that can be used to develop high-yielding varieties under increased atmospheric CO2 concentrations, such as those predicted in the near future.

Responses to atmospheric CO2 concentrations in crop simulation models: a review of current simple and semicomplex representations and options for model development

Elevated atmospheric CO₂ concentrations ([CO₂ ]) cause direct changes in crop physiological processes (e.g. photosynthesis and stomatal conductance). To represent these CO₂ responses, commonly used crop simulation models have been amended, using simple and semicomplex representations of the processes involved. Yet, there is no standard approach to and often poor documentation of these developments. This study used a bottom-up approach (starting with the APSIM framework as case study) to evaluate modelled responses in a consortium of commonly used crop models and illuminate whether variation in responses reflects true uncertainty in our understanding compared to arbitrary choices of model developers. Diversity in simulated CO₂ responses and limited validation were common among models, both within the APSIM framework and more generally. Whereas production responses show some consistency up to moderately high [CO₂ ] (around 700 ppm), transpiration and stomatal responses vary more widely in nature and magnitude (e.g. a decrease in stomatal conductance varying between 35% and 90% among models was found for [CO₂ ] doubling to 700 ppm). Most notably, nitrogen responses were found to be included in few crop models despite being commonly observed and critical for the simulation of photosynthetic acclimation, crop nutritional quality and carbon allocation. We suggest harmonization and consideration of more mechanistic concepts in particular subroutines, for example, for the simulation of N dynamics, as a way to improve our predictive understanding of CO₂ responses and capture secondary processes. Intercomparison studies could assist in this aim, provided that they go beyond simple output comparison and explicitly identify the representations and assumptions that are causal for intermodel differences. Additionally, validation and proper documentation of the representation of CO₂ responses within models should be prioritized.

Inorganic nitrogen form: a major player in wheat and Arabidopsis responses to elevated CO2

Critical for predicting the future of primary productivity is a better understanding of plant responses to rising atmospheric carbon dioxide (CO2) concentration. This review considers recent results on the role of the inorganic nitrogen (N) forms nitrate (NO3-) and ammonium (NH4+) in determining the responses of wheat and Arabidopsis to elevated atmospheric CO2 concentration. Here, we identify four key issues: (i) the possibility that different plant species respond similarly to elevated CO2 if one accounts for the N form that they are using; (ii) the major influence that plant-soil N interactions have on plant responses to elevated CO2; (iii) the observation that elevated CO2 may favor the uptake of one N form over others; and (iv) the finding that plants receiving NH4+ nutrition respond more positively to elevated CO2 than those receiving NO3- nutrition because elevated CO2 inhibits the assimilation of NO3- in shoots of C3 plants. We conclude that the form and amount of N available to plants from the rhizosphere and plant preferences for the different N forms are essential for predicting plant responses to elevated CO2.

Enhancing soybean photosynthetic CO2 assimilation using a cyanobacterial membrane protein, ictB

Soybean C3 photosynthesis can suffer a severe loss in efficiency due to photorespiration and the lack of a carbon concentrating mechanism (CCM) such as those present in other plant species or cyanobacteria. Transgenic soybean (Glycine max cv. Thorne) plants constitutively expressing cyanobacterial ictB (inorganic carbon transporter B) gene were generated using Agrobacterium-mediated transformation. Although more recent data suggest that ictB does not actively transport HCO3-/CO2, there is nevertheless mounting evidence that transformation with this gene can increase higher plant photosynthesis. The hypothesis that expression of the ictB gene would improve photosynthesis, biomass production and seed yield in soybean was tested, in two independent replicated greenhouse and field trials. Results showed significant increases in photosynthetic CO2 uptake (Anet) and dry mass in transgenic relative to wild type (WT) control plants in both the greenhouse and field trials. Transgenic plants also showed increased photosynthetic rates and biomass production during a drought mimic study. The findings presented herein demonstrate that ictB, as a single-gene, contributes to enhancement in various yield parameters in a major commodity crop and point to the significant role that biotechnological approaches to increasing photosynthetic efficiency can play in helping to meet increased global demands for food.

Informing climate models with rapid chamber measurements of forest carbon uptake

Models predicting ecosystem carbon dioxide (CO₂ ) exchange under future climate change rely on relatively few real-world tests of their assumptions and outputs. Here, we demonstrate a rapid and cost-effective method to estimate CO₂ exchange from intact vegetation patches under varying atmospheric CO₂ concentrations_. We find that net ecosystem CO₂ uptake (NEE) in a boreal forest rose linearly by 4.7 ± 0.2% of the current ambient rate for every 10 ppm CO₂ increase, with no detectable influence of foliar biomass, season, or nitrogen (N) fertilization. The lack of any clear short-term NEE response to fertilization in such an N-limited system is inconsistent with the instantaneous downregulation of photosynthesis formalized in many global models. Incorporating an alternative mechanism with considerable empirical support - diversion of excess carbon to storage compounds - into an existing earth system model brings the model output into closer agreement with our field measurements. A global simulation incorporating this modified model reduces a long-standing mismatch between the modeled and observed seasonal amplitude of atmospheric CO₂ . Wider application of this chamber approach would provide critical data needed to further improve modeled projections of biosphere-atmosphere CO₂ exchange in a changing climate.

Release of resource constraints allows greater carbon allocation to secondary metabolites and storage in winter wheat

The atmospheric CO₂ concentration ([CO₂ ]) is rapidly increasing, and this may have substantial impact on how plants allocate metabolic resources. A thorough understanding of allocation priorities can be achieved by modifying [CO₂ ] over a large gradient, including low [CO₂ ], thereby altering plant carbon (C) availability. Such information is of critical importance for understanding plant responses to global environmental change. We quantified the percentage of daytime whole-plant net assimilation (A) allocated to night-time respiration (R), structural growth (SG), nonstructural carbohydrates (NSC) and secondary metabolites (SMs) during 8 weeks of vegetative growth in winter wheat (Triticum aestivum) growing at low, ambient and elevated [CO₂ ] (170, 390 and 680 ppm). R/A remained relatively constant over a large gradient of [CO₂ ]. However, with increasing C availability, the fraction of assimilation allocated to biomass (SG + NSC + SMs), in particular NSC and SMs, increased. At low [CO₂ ], biomass and NSC increased in leaves but decreased in stems and roots, which may help plants achieve a functional equilibrium, that is, overcome the most severe resource limitation. These results reveal that increasing C availability from rising [CO₂ ] releases allocation constraints, thereby allowing greater investment into long-term survival in the form of NSC and SMs.

CO2-Induced Changes in Wheat Grain Composition: Meta-Analysis and Response Functions

Elevated carbon dioxide (eCO2) stimulates wheat grain yield, but simultaneously reduces protein/nitrogen (N) concentration. Also, other essential nutrients are subject to change. This study is a synthesis of wheat experiments with eCO2, estimating the effects on N, minerals (B, Ca, Cd, Fe, K, Mg, Mn, Na, P, S, Zn), and starch. The analysis was performed by (i) deriving response functions to assess the gradual change in element concentration with increasing CO2 concentration, (ii) meta-analysis to test the average magnitude and significance of observed effects, and (iii) relating CO2 effects on minerals to effects on N and grain yield. Responses ranged from zero to strong negative effects of eCO2 on mineral concentration, with the largest reductions for the nutritionally important elements of N, Fe, S, Zn, and Mg. Together with the positive but small and non-significant effect on starch concentration, the large variation in effects suggests that CO2-induced responses cannot be explained only by a simple dilution model. To explain the observed pattern, uptake and transport mechanisms may have to be considered, along with the link of different elements to N uptake. Our study shows that eCO2 has a significant effect on wheat grain stoichiometry, with implications for human nutrition in a world of rising CO2.

A new theory of plant-microbe nutrient competition resolves inconsistencies between observations and model predictions

Terrestrial plants assimilate anthropogenic CO₂ through photosynthesis and synthesizing new tissues. However, sustaining these processes requires plants to compete with microbes for soil nutrients, which therefore calls for an appropriate understanding and modeling of nutrient competition mechanisms in Earth System Models (ESMs). Here, we survey existing plant-microbe competition theories and their implementations in ESMs. We found no consensus regarding the representation of nutrient competition and that observational and theoretical support for current implementations are weak. To reconcile this situation, we applied the Equilibrium Chemistry Approximation (ECA) theory to plant-microbe nitrogen competition in a detailed grassland ¹⁵ N tracer study and found that competition theories in current ESMs fail to capture observed patterns and the ECA prediction simplifies the complex nature of nutrient competition and quantitatively matches the ¹⁵ N observations. Since plant carbon dynamics are strongly modulated by soil nutrient acquisition, we conclude that (1) predicted nutrient limitation effects on terrestrial carbon accumulation by existing ESMs may be biased and (2) our ECA-based approach may improve predictions by mechanistically representing plant-microbe nutrient competition.

Atmospheric CO2 mole fraction affects stand-scale carbon use efficiency of sunflower by stimulating respiration in light

Plant carbon-use-efficiency (CUE), a key parameter in carbon cycle and plant growth models, quantifies the fraction of fixed carbon that is converted into net primary production rather than respired. CUE has not been directly measured, partly because of the difficulty of measuring respiration in light. Here, we explore if CUE is affected by atmospheric CO₂ . Sunflower stands were grown at low (200 μmol mol^-1 ) or high CO₂ (1000 μmol mol^-1 ) in controlled environment mesocosms. CUE of stands was measured by dynamic stand-scale ¹³ C labelling and partitioning of photosynthesis and respiration. At the same plant age, growth at high CO₂ (compared with low CO₂ ) led to 91% higher rates of apparent photosynthesis, 97% higher respiration in the dark, yet 143% higher respiration in light. Thus, CUE was significantly lower at high (0.65) than at low CO₂ (0.71). Compartmental analysis of isotopic tracer kinetics demonstrated a greater commitment of carbon reserves in stand-scale respiratory metabolism at high CO₂ . Two main processes contributed to the reduction of CUE at high CO₂ : a reduced inhibition of leaf respiration by light and a diminished leaf mass ratio. This work highlights the relevance of measuring respiration in light and assessment of the CUE response to environment conditions.

Atmospheric CO2 enrichment and drought stress modify root exudation of barley

Rising CO₂ concentrations associated with drought stress is likely to influence not only aboveground growth, but also belowground plant processes. Little is known about root exudation being influenced by elements of climate change. Therefore, this study wanted to clarify whether barley root exudation responds to drought and CO₂ enrichment and whether this reaction differs between an old and a recently released malting barley cultivar. Barley plants were grown in pots filled with sand in controlled climate chambers at ambient (380 ppm) or elevated (550 ppm) atmospheric [CO₂ ] and a normal or reduced water supply. Root exudation patterns were examined at the stem elongation growth stage and when the inflorescences emerged. At both dates, root exudates were analyzed for different compounds such as total free amino acids, proline, potassium, and some phytohormones. Elevated [CO₂ ] decreased the concentrations in root exudates of some compounds such as total free amino acids, proline, and abscisic acid. Moreover, reduced water supply increased proline, potassium, electric conductivity, and hormone concentrations. In general, the modern cultivar showed higher concentrations of proline and abscisic acid than the old one, but the cultivars responded differentially under elevated CO₂ . Plant developmental stage had also an impact on the root exudation patterns of barley. Generally, we observed significant effects of CO₂ enrichment, watering levels, and, to a lesser extent, cultivar on root exudation. However, we did not find any mitigation of the adverse effects of drought by elevated CO₂ . Understanding the multitude of relationships within the rhizosphere is an important aspect that has to be taken into consideration in the context of crop performance and carbon balance under conditions of climate change.

Linking gene expression to productivity to unravel long- and short-term responses of seagrasses exposed to CO2 in volcanic vents

Ocean acidification is a major threat for marine life but seagrasses are expected to benefit from high CO₂. In situ (long-term) and transplanted (short-term) plant incubations of the seagrass Cymodocea nodosa were performed near and away the influence of volcanic CO₂ vents at Vulcano Island to test the hypothesis of beneficial effects of CO₂ on plant productivity. We relate, for the first time, the expression of photosynthetic, antioxidant and metal detoxification-related genes to net plant productivity (NPP). Results revealed a consistent pattern between gene expression and productivity indicating water origin as the main source of variability. However, the hypothesised beneficial effect of high CO₂ around vents was not supported. We observed a consistent long- and short-term pattern of gene down-regulation and 2.5-fold NPP decrease in plants incubated in water from the vents and a generalized up-regulation and NPP increase in plants from the vent site incubated with water from the Reference site. Contrastingly, NPP of specimens experimentally exposed to a CO₂ range significantly correlated with CO₂ availability. The down-regulation of metal-related genes in C. nodosa leaves exposed to water from the venting site suggests that other factors than heavy metals, may be at play at Vulcano confounding the CO₂ effects.