Ambient Atmospheric CO2 Concentrations Research Articles

Rare earth elements as a tool to study the foliar nutrient uptake phenomenon under ambient and elevated atmospheric CO2 concentration

The ability of plants to uptake nutrients from mineral dust lying on their foliage may prove to be an important mechanism by which plants will cope with increasing CO2 levels in the atmosphere. This mechanism had only recently been reported and was shown to compensate for the projected dilution in plants ionome. However, this phenomenon has yet to be thoroughly studied, particularly in terms of the expected trends under different dust types and varying atmospheric CO2 concentrations, as projected by the IPCC. We treated plants grown under ambient (415 ppm) and elevated CO2 (850 ppm) conditions with either desert dust, volcanic ash, and fire ash analogues by applying it solely on plant foliage and studied their Rare Earth Elements concentrations and patterns. The Rare Earth Elements compositions of the treated plants originated from the dust application, and their incorporation into the plants led to a significant increase in plants vitality, evident in increased photosynthetic activity and biomass. Two trends in the foliar nutrient uptake mechanism were revealed by the Rare Earth Elements, one is that different treatments affected the plant in decreasing order volcanic ash > desert dust > fire ash. The second trend is that foliar intake becomes more significant under elevated CO2, an observation not previously seen. This testifies that the use of Rare Earth Elements in the study of foliar nutrient uptake, and other biological mechanisms is fundamental, and that foliar pathways of nutrient uptake will indeed become more dominant with increasing CO2 under expected atmospheric changes.

Effects of elevated CO2 concentration on cell structure and stress resistance physiology of Setaria italica under drought stress.

The frequency of drought will increase under further warming. The increase in atmospheric CO2 concentration, along with more frequent drought, will affect crop growth. We examined the changes of cell structure, photosynthetic physiology, antioxidant enzymes, osmotic regulatory substances, and yield of foxtail millet (Setaria ita-lica) leaves under different CO2 concentrations (ambient air CO2 concentration and ambient atmospheric CO2 concentration + 200 μmol·mol-1) and water treatment (soil moisture content maintained at 45%-55%, and 70%-80% of field capacity, representing mild drought and normal water condition, respectively). The results showed that elevated CO2 concentration increased the number of starch grains, the area of single starch grains, and the total area of starch grains in the chloroplast of millet mesophyll cells. Under mild drought condition, elevated CO2 concentration increased net photosynthetic rate of millet leaves at the booting stage by 37.9%, but did not affect water use efficiency at this stage. Elevated CO2 concentration increased net photosynthetic rate and water use efficiency of millet leaves under mild drought condition at the filling stage by 15.0% and 44.2%, respectively. Under mild drought condition, elevated CO2 concentration increased the content of peroxidase (POD) and soluble sugar in millet leaves at the booting stage by 39.3% and 8.0%, respectively, but decreased proline content by 31.5%. It increased the content of POD in millet leaves at the filling stage by 26.5% but decreased the content of MDA and proline by 37.2% and 39.3%, respectively. Under mild drought condition, elevated CO2 concentration significantly increased the number of grain spikes by 44.7% and yield by 52.3% in both years compared with normal water condition. The effect of elevated CO2 concentration on grain yield under mild drought conditions was higher than that under normal water condition. Under mild drought conditions, elevated CO2 concentration increased leaf thickness, vascular bundle sheath cross-sectional area, net photosynthetic rate, and water use efficiency of millet, improved the antioxidant oxidase activity, and changed the concentration of osmotic regulatory substances, alleviated the nega-tive effect of drought on foxtail millet, and finally increased the number of grains per ear and yield of foxtail millet. This study would provide a theoretical basis for millet production and sustainable agricultural development in arid areas under future climate change.

Meeting the heavy-metal safety requirements for food crops by using biochar: An investigation using sunflower as a representative plant under different atmospheric CO2 concentrations

Global warming impacts on plant growth and food safety are emerging topics of concern, while biochar as a soil additive benefits plants. This study investigates (1) sunflower plant growth at various biochar concentrations in a soil-compost growing substrate under both ambient (420 ppm) and elevated (740 ppm) atmospheric CO2 concentrations, and (2) concentrations of heavy metals in the growing substrates and organs of the plants. The elevated CO2 concentration benefits the vegetative parts but harms the reproductive parts of the plants. Additionally, the elevated CO2 concentration inhibits the beneficial effects that biochar confers on the plants at the ambient concentration. The optimum biochar concentration at both CO2 levels was found to be 15%. At the time of harvest, most of the heavy-metal concentrations in the growing substrate increased. It was demonstrated that biochar can reduce the amount of heavy metals that accumulate in the roots and seeds whose heavy-metal concentrations complied with Singapore food safety regulations, while those for the biochar met the proposed Singapore biochar standard's thresholds. Our results show that the proposed Singapore biochar standard is practical and sound.

Elevated CO2 enhanced water use efficiency of wheat to progressive drought stress but not on maize.

Global rising atmospheric CO2 concentration ([CO2]) and drought stress exert profound influences on crop growth and yield. The objective of the present study was to investigate the responses of leaf gas exchange and plant water use efficiency (WUE) of wheat (C3) and maize (C4) plants to progressive drought stress under ambient (a[CO2], 400 ppm) and elevated (e[CO2], 800 ppm) atmospheric CO2 concentrations. The fraction of transpirable soil water (FTSW) was used to evaluate soil water status in the pots. Under non-drought stress, e[CO2] increased the net photosynthetic rate (An) solely in wheat, and dry matter accumulation (DMA), whereas it decreased stomatal conductance (g s) and water consumption (WC), resulting in enhanced WUE by 27.82% for maize and 49.86% for wheat. After onset of progressive soil drying, maize plants in e[CO2] showed lower FTSW thresholds than wheat, at which e.g. gs (0.31 vs 0.40) and leaf relative water content (0.21 vs 0.43) starts to decrease, indicating e[CO2] conferred a greater drought resistance in maize. Under the combination of e[CO2] and drought stress, enhanced WUE was solely found in wheat, which is mainly associated with increased DMA and unaffected WC. These varied responses of leaf gas exchange and WUE between the two species to combined drought and e[CO2] suggest that specific water management strategies should be developed to optimize crop WUE for different species in a future drier and CO2-enriched environment.

Elevated Atmospheric CO2 Concentration Influences the Rooting Habits of Winter-Wheat (Triticum aestivum L.) Varieties

The intensity and the frequency of extreme drought are increasing worldwide. An elevated atmospheric CO2 concentration could counterbalance the negative impacts of water shortage; however, wheat genotypes show high variability in terms of CO2 reactions. The development of the root system is a key parameter of abiotic stress resistance. In our study, biomass and grain production, as well as the root growth of three winter-wheat varieties were examined under optimum watering and simulated drought stress in a combination with ambient and elevated atmospheric CO2 concentrations. The root growth was monitored by a CI-600 in situ root imager and the photos were analyzed by RootSnap software. As a result of the water shortage, the yield-related parameters decreased, but the most substantial yield reduction was first detected in Mv Karizma. The water shortage influenced the depth of the intensive root development, while under water-limited conditions, the root formation occurred in the deeper soil layers. The most intensive root development was observed until the heading, and the maximum root length was recorded at the beginning of the heading. The period of root development took longer under elevated CO2 concentration. The elevated CO2 concentration induced an accelerated root development in almost every soil layer, but generally, the CO2 fertilization induced in the root length of all genotypes and under each treatment.

Modification of storage proteins in the barley grain increases endosperm zinc and iron under both normal and elevated atmospheric CO2.

Increasing atmospheric CO2 concentration is expected to enhance the grain yield of C3 cereal plants, while at the same time reducing the concentrations of minerals and proteins. This will lead to a lower nutritional quality and increase global problems associated with micronutrient malnutrition. Among the barley grain storage proteins, the C‐hordein fraction has the lowest abundance of sulfur (S) containing amino acids and is poorest in binding of zinc (Zn). In the present study, C‐hordein‐suppressed barley lines with reduced C‐hordein content, obtained by use of antisense or RNAi technology, were investigated under ambient and elevated atmospheric CO2 concentration. Grains of the C‐hordein‐suppressed lines showed 50% increase in the concentrations of Zn and iron (Fe) in the core endosperm relative to the wild‐type under both ambient and elevated atmospheric CO2. Element distribution images obtained using laser ablation‐inductively coupled plasma‐mass spectrometry confirmed the enrichment of Fe and Zn in the core endosperm of the lines with modified storage protein composition. We conclude that modification of grain storage proteins may improve the nutritional value of cereal grain with respect to Zn and Fe under both normal and future conditions of elevated atmospheric CO2.

Plant productivity is a key driver of soil respiration response to climate change in a nutrient-limited soil.

Despite knowledge of the interaction between climate change factors significant uncertainty exists concerning the individual and interactive effects of elevated carbon dioxide (eCO2) and elevated temperature (eT) on the soil microbiome and function. Here we examine the individual and interactive effects of eCO2 and eT on tree growth, soil respiration (Rsoil), biomass, structural and functional composition of microbial community, nitrogen (N) mineralisation and N availability in a whole tree chamber experiment. Eucalyptus globulus plants were grown from seedling to ca. 10 m tall for 15 months in a nutrient-poor sandy soil under ambient and elevated (+ 240 ppm) atmospheric CO2 concentrations combined with ambient or elevated temperatures (+ 3 °C) in a full factorial design. Plant growth was strongly stimulated under eCO2, but eT had little impact on any measured plant property. In contrast, Rsoil was not consistently affected by eCO2 or eT, but correlated strongly with root and leaf biomass. The response of N-mineralisation and nutrient availability to eCO2 and eT varied across time, and available N correlated strongly with plant height. Further, the C:N ratio of the microbial biomass and leaves were both higher under eCeT treatment. However, these functional measures were not significantly linked to either structural or functional diversity of the soil microbiome. Taken together, these results suggest that in this low-nutrient soil, belowground processes are principally driven by aboveground productivity. Our work provides novel insight into mechanisms underlying above- and belowground response to climate change, and the potential to sequester C in a low-nutrient status soil under future climatic conditions may be limited .

Woody tissue photosynthesis increases radial stem growth of young poplar trees under ambient atmospheric CO2 but its contribution ceases under elevated CO2.

Woody tissue photosynthesis (Pwt) contributes to the tree carbon (C) budget and generally stimulates radial stem growth under ambient atmospheric CO2 concentration (aCO2). Moreover, Pwt has potential to enhance tree survival under changing climates by delaying negative effects of drought stress on tree hydraulic functioning. However, the relevance of Pwt on tree performance under elevated atmospheric CO2 concentration (eCO2) remains unexplored. To fill this knowledge gap, 1-year-old Populus tremula L. seedlings were grown in two treatment chambers at aCO2 and eCO2 (400 and 660ppm, respectively), and woody tissues of half of the seedlings in each treatment chamber were light-excluded to prevent Pwt. Radial stem growth, sap flow, leaf photosynthesis and stomatal and canopy conductance were measured throughout the growing season, and the concentration of non-structural carbohydrates (NSC) in stem tissues was determined at the end of the experiment. Fuelled by eCO2, an increase in stem growth of 18 and 50% was observed in control and light-excluded trees, respectively. Woody tissue photosynthesis increased radial stem growth by 39% under aCO2, while, surprisingly, no impact of Pwt on stem growth was observed under eCO2. By the end of the growing season, eCO2 and Pwt had little effect on stem growth, leaf photosynthesis acclimated to eCO2, but stomatal conductance did not, and homeostatic stem NSC pools were observed among combined treatments. Our results highlight that eCO2 potentially fulfils plant C requirements, limiting the contribution of Pwt to stem growth as atmospheric [CO2] rises, and that radial stem growth in young developing trees was C (source) limited during early phenological stages but transitioned towards sink-driven control at the end of the growing season.

Special issue in honour of Prof. Reto J. Strasser - Seasonal variation and trade-off between frost resistance and photosynthetic performance in woody species

We tested the seasonal variability of frost resistance and photosynthesis characteristics, namely net photosynthetic rate at saturating irradiance and ambient atmospheric CO2 concentrations, maximum carboxylation rate, and measurements based on the JIP-test, namely maximum quantum yield of PSII and the performance index. Additionally, we measured leaf functional traits, namely specific leaf area, leaf dry matter content, stomatal pore area index, and chlorophyll content. We assessed all traits weekly on two evergreen species Berberis vulgaris and Quercus × hispanica as well as the two summer green species Betula pendula and Quercus pubescens. We found that there was a high seasonal variability in all traits except stomatal pore area index. In a multivariate analysis, we could show trade-offs between growth rates and leaf thickness on the first axis, as well as resistance and performance on the second axis. The findings help understand ecophysiological processes and fine-tune biosphere models.

凋落物多样性及组成对凋落物分解及土壤微生物群落的影响——二氧化碳倍增条件下

凋落物多样性及组成对凋落物分解及土壤微生物群落的影响——二氧化碳倍增条件下

No evidence for triose phosphate limitation of light-saturated leaf photosynthesis under current atmospheric CO2 concentration.

The triose phosphate utilization (TPU) rate has been identified as one of the processes that can limit terrestrial plant photosynthesis. However, we lack a robust quantitative assessment of TPU limitation of photosynthesis at the global scale. As a result, TPU, and its potential limitation of photosynthesis, is poorly represented in terrestrial biosphere models (TBMs). In this study, we utilized a global data set of photosynthetic CO2 response curves representing 141 species from tropical rainforests to Arctic tundra. We quantified TPU by fitting the standard biochemical model of C3 photosynthesis to measured photosynthetic CO2 response curves and characterized its instantaneous temperature response. Our results demonstrate that TPU does not limit leaf photosynthesis at the current ambient atmospheric CO2 concentration. Furthermore, our results showed that the light-saturated photosynthetic rates of plants growing in cold environments are not more often limited by TPU than those of plants growing in warmer environments. In addition, our study showed that the instantaneous temperature response of TPU is distinct from temperature response of the maximum rate of Rubisco carboxylation. The new formulations of the temperature response of TPU derived in this study may prove useful in quantifying the biochemical limits to terrestrial plant photosynthesis and improve the representation of plant photosynthesis in TBMs.

CO2 fertilization does not affect biomass production and nutritive value of a C4 tropical grass in short timeframe

AbstractIncreased atmospheric carbon dioxide (CO2) is a consequence of recent anthropogenic environmental changes, and few studies have evaluated its effects on tropical grasses used in Brazilian pastures, the main feed source for major part of ruminant livestock. This study evaluated forage production, chemical composition, in vitro total gas production and organic matter degradability of Brachiaria brizantha under contrasting CO2 atmospheric conditions in a free air carbon dioxide enrichment (FACE) facility. The forage plants were sown in each of the 12 octagonal rings of the FACE facility: six under ambient atmospheric CO2 concentration of approximately 390 μmol/mol, hereafter referred to as control (CON) plots, and other six rings enriched with pure CO2 flux to achieve a target CO2 concentration of 550 μmol/mol, hereafter called elevated CO2 (eCO2) plots. Soil samples were collected to determine carbon and nitrogen concentrations. After seventy days of sowing, a standardization cutting was performed and then at regular intervals of 21 days the forage was harvested (ten harvest dates) and forwarded to laboratorial analyses. Forage above‐ground biomass production (dry matter (DM): 6,143 vs. 6,554 kg/ha), as well as morphological characteristics (leaves: 71% vs. 68%; stem: 28% vs. 31%), chemical composition (crude protein: 162.9 vs. 161.8; neutral detergent fibre: 663.8 vs. 664.3; acid detergent fibre: 369.5 vs. 381; lignin: 60.1 vs. 64.1 g/kg DM; total C: 45.9 vs. 45.9; total N: 2.8 vs. 2.8; total S: 0.2% vs. 0.2%), organic matter in vitro degradability (573.5 vs. 585.3 g/kg), methane (5.7 vs. 4.3 ml/g DM) and total gas (128.3 vs. 94.5 ml/g DM) production did not differ significantly between CON and eCO2 treatments (p > .05). The results indicated that at least under short‐term enrichment, B. brizantha was not affected by eCO2.

Data from the Braunschweig FACE (free-air CO2 enrichment) experiments on sugar beet at adequate and low levels of nitrogen supply

Sugar beet was grown within a crop rotation over two rotation cycles (2001, 2004) at ambient and elevated atmospheric CO2 concentration (375 and 550 ppm) with practical (126, 156 kg N ha-1) and low levels (63, 78 kg N ha-1) of nitrogen supply. In the second year another cultivar was used to prevent infestation by rhizomania, which occured on one half of the field plots at the end of the season of 2001. In 2004, shading was included as an additional treatment. The objectives were to investigate the growth response of sugar beet to elevated CO2 concentration at high and low nitrogen availability. Data set includes data on management, soil condition, weather, below and above ground growth (individual leaves, leaf area index, total biomass, beet yield and composition, water soluble carbohydrates, root biomass).

Impact of different climates on the resistance of concrete to natural carbonation

Abstract This paper describes a unique international inter-laboratory study on the carbonation resistance of concrete prepared with different supplementary cementing materials. Concrete specimens – from 45 different concrete mixtures – prepared centrally in Lafarge Centre de Recherche (France) were shipped in a sealed condition to 4 other academia research laboratories (located in USA, Canada, India and China). The specimens were exposed to the ambient environments and atmospheric CO2 concentrations in the five locations, including Lafarge Centre de Recherche in France, in both sheltered and unsheltered condition for a period of 5 years. Measurements of carbonation depth were performed at periodic intervals, and the data was analyzed to assess the influence of climatic conditions on the resistance to carbonation. The results indicate that the general trend of carbonation is not much different irrespective of the macroclimate. Further, the number of rainy days seems to have a more significant influence on the progress of carbonation than the total rainfall in the region.

Plant-PET to investigate phloem vulnerability to drought in Populus tremula under changing climate regimes.

Phloem transport is of great importance in trees to distribute assimilated carbon across the entire tree. Nevertheless, knowledge of phloem is incomplete, because of the complexity of measuring its transport and characteristics. Only few studies have addressed how phloem transport might alter under climatic changes, with most data originating from theoretical studies. We measured phloem characteristics in leaves of young Populus tremula L. trees grown during 5 months under ambient (TA, 404 ppm ± 5) and elevated (TE, 659 ppm ± 3) atmospheric CO2 concentration ([CO2]) using a combination of positron emission tomography (PET) and compartmental modelling. Short-term phloem dynamics were measured in vivo and non-invasively using the short-lived isotope of carbon, 11C (half-life 20.4 min). Trees were scanned in well-watered and dry conditions to assess changes in phloem characteristics induced by drought. Reliability of the PET-derived results was verified with reported observations in the literature. Phloem speed was highest in well-watered TE trees and strongly reduced by 81% under drought, whereas phloem speed reduced by 61% in TA trees at the same level of drought. These findings led us to speculate that phloem transport in TE trees might be more vulnerable to drought. We discuss how a higher phloem vulnerability to drought in a changing climate could impact tree hydraulic functioning. Taken together our results suggest that trees grown for 5 months under elevated [CO2] seem to be less well-acclimated to face projected hotter droughts in a changing climate.

CO2 elevation modulates the response of leaf gas exchange to progressive soil drying in tomato plants

The objective of this study was to investigate the response of leaf gas exchange of tomato plant to progressive drought stress under ambient (a[CO2], 400 ppm) and elevated (e[CO2], 800 ppm) atmospheric CO2 concentration. The fraction of transpirable soil water (FTSW) was used to evaluate soil water status in the pots. The results showed that stomatal conductance (gs) and transpiration rate (Tr) were significantly lower while the net photosynthetic rate (An) was significantly higher in plants grown under e[CO2] than those under a[CO2] at onset of drought stress. Along with soil drying, the FTSW thresholds at which gs and An started to decrease were significantly lower in plants grown under e[CO2] as compared to plants grown under a[CO2]. The intrinsic water use efficiency and instantaneous water use efficiency of plants grown under e[CO2] was significantly higher than those under a[CO2]. Under e[CO2], the drought-stressed plants had greater leaf area, dry matter and water use efficiency than those grown under a[CO2]. e[CO2] notably enhanced shoot C concentration while decreased shoot N concentration hereby increased the C:N ratio. With the decrease of FTSW, the concentration of abscisic acid in leaf ([ABA]leaf) and xylem sap ([ABA]xylem) increased exponentially. When FTSW > 0.2, under both CO2 environments, gs decreased linearly with increasing [ABA]leaf and [ABA]xylem; and similar slopes but different intercepts were noticed for the regression lines, indicating that the responsiveness of gs to ABA was unaffected by CO2. In conclusion, CO2 elevation retarded the response of leaf gas exchange to progressive soil drying in tomato plants. This result provides novel knowledge for more precise prediction of plant response to drought stress in a future CO2-enriched environment.

Dissolved greenhouse gas concentrations in 40 lakes in the Alpine area

While it has been recently recognized that freshwater ecosystems may significantly offset the terrestrial carbon sink through emissions of carbon dioxide (CO2) and methane (CH4), empirical data on the magnitude of these sources are still scarce, in particular in temperate regions. In this study, we measured the near-surface dissolved concentrations of CH4 and CO2 from 40 lakes in the Alpine area to estimate their potential for greenhouse gas (GHG) emissions. We hypothesized (1) a temperature-driven gradient of dissolved gas concentrations in terms of elevation and latitude of the lakes and (2) that lower concentrations would be measured in man-made reservoirs compared to natural lakes. Average CH4 and CO2 surface dissolved concentrations amounted to 1.10 ± 1.30 and 36.23 ± 31.15 µmol L−1, respectively. All the lakes, except for one, were supersaturated, exceeding ambient atmospheric CH4 and CO2 concentrations by a factor of 400 ± 424 and 2.43 ± 2.29, respectively. Consistent with our hypothesis, we found lower surface dissolved GHG concentrations in man-made reservoirs compared to natural lakes, which was shown to be related to their greater depth. Even though temperature is known to affect multiple physico-chemical and biological processes governing the strength of the uptake, release and conversion of CH4 and CO2, and temperature is inversely related to elevation, no relationship between dissolved GHG concentrations and elevation could be determined. This is believed to be the result of the overriding importance of lake depth for near-surface CH4 concentrations and the lack of explanatory variables related to lake carbon cycling. Overall, this study suggests that lakes in the Alpine region act as sources of CO2 and CH4 to the atmosphere and that further research should be carried out to quantify the actual GHG emissions from Alpine freshwater bodies and how these are affected by ongoing changes in climate and land use.

Chlorophyll fluorescence and gas exchange measurements in field research: an ecological case study

We tested whether cheap and quick chlorophyll (Chl) fluorescence can be used in ecophysiological field studies as proxies for gas-exchange measurements. We measured net photosynthetic rate at saturating irradiance and ambient atmospheric CO2 concentrations (PNsat), maximum carboxylation rate (Vcmax), maximum quantum yield of PSII (Fv/Fm), the performance index (PIabs), leaf nitrogen (Narea), and carbon isotope discrimination (Δ13C) within four herbaceous species along two elevational gradients. We analysed the relationship between Chl fluorescence and gas-exchange parameters and their link to indirect assessment of plant performance via ecophysiological traits. Fv/Fm showed no relationship to PNsat and only weak relationships to Vcmax. PIabs was positively related to PNsat and Vcmax. PIabs, PNsat, and Vcmax were positively associated with Narea and negatively to Δ13C, whereas Fv/Fm showed no relationship to Narea and a positive to Δ13C. Thus, PIabs might be suitable to characterize the photosynthetic activity when aiming on large numbers of samples.

Assessing the potential of using biochar in mine rehabilitation under elevated atmospheric CO2 concentration

Re-establishment of soil nitrogen (N) capital is a priority in mine rehabilitation. We aimed to evaluate the effects of biochar addition on improving mine spoil N pools and the influence of elevated CO2 concentration on mine rehabilitation. We assessed the effects of pinewood biochar, produced at three temperatures (650, 750 and 850 °C, referred as B650, B750 and B850, respectively), on mine spoil total N concentrations with five different plant species, including a tree species (Eucalyptus crebra), N-fixing shrubs (Acacia floribunda and Allocasuarina littoralis) and C3 and C4 grasses (Austrodanthonia tenuior and Themeda australis) incubated at ambient (400 μL L−1) and elevated (700 μL L−1) atmospheric CO2 concentrations, as well as the effects of elevated CO2 on mine rehabilitation. Soil total N significantly improved following biochar incorporation under all plant species (P < 0.05) except for T. Australis. E. crebra had the highest soil total N (0.197%, 0.198% and 0.212% for B650, B750 and B850, respectively). Different from the negligible influence of elevated CO2 on soil properties under the grasses and the N-fixing shrubs, elevated CO2 significantly increased soil water and hot water extractable organic C (WEOC and HWEOC, respectively) and decreased total C under E. crebra, indicating that the nutrient demands were not met. Biochar addition showed the potential in mine rehabilitation in terms of improving soil N pool, especially with E. crebra. However, it would be more difficulty to rehabilitate mine spoils in future with the rising atmospheric CO2 concentration.

ABA signaling and stomatal control in tomato plants exposure to progressive soil drying under ambient and elevated atmospheric CO2 concentration

The objective of this study was to investigate the relative significance of hydraulic and chemical signals in controlling stomatal conductance (gs) of drought-stressed tomato plants grown under ambient (a[CO2], 400ppm) and elevated atmospheric CO2 concentration (e[CO2], 800ppm). The fraction of transpirable soil water (FTSW) was lower and decreased much faster in plants grown at e[CO2] than those grown at a[CO2]. During soil drying, midday leaf water potential (Ψl), osmotic potential (Ψπ), and turgor pressure (Ψp) were kept constant until FTSW reached ca. 0.2; while the plants grown under e[CO2] had significantly lower Ψπ but significantly higher Ψp than those grown under a[CO2]. The An was significantly greater in plants grown under e[CO2] than those grown under a[CO2] during soil drying. The gs was significant higher for plants grown under a[CO2] at onset of drought but decreased linearly with decreasing FTSW, whereas it was kept almost unchanged from onset of drought until FTSW reached ca. 0.2 in plants grown under e[CO2]. The intrinsic water use efficiency (WUEi) increased linearly with decreasing FTSW to ca. 0.2 in plants grown under a[CO2], while a decline of WUEi was observed for plants grown under e[CO2]. The xylem sap ABA concentration ([ABA]xylem) increased exponentially with decreasing FTSW, and the increase was more pronounced in plants grown under e[CO2]. When FTSW>0.3,gs decreased linearly with increasing [ABA]xylem for plants grown under a[CO2]; while for plants grown under e[CO2], gs decreased linearly with decreasing Ψp, indicating that at moderate drought stress the gs of tomato leaves was mainly regulated by the xylem-borne ABA signaling when grown at a[CO2]; whereas under e[CO2] the gs was insensitive to [ABA]xylem and controlled predominantly by Ψp. The results provide some novel insights into the mechanisms of plant response to drought stress in future CO2 enriched environment.