Acclimation of peanut (Arachis hypogaea L.) leaf photosynthesis to elevated growth CO2 and temperature

Peanut (Arachis hypogaea L. cv. Florunner) was grown from seed sowing to plant maturity under two daytime CO₂ concentrations ([CO₂]) of 360 μmol mol⁻¹ (ambient) and 720 μmol mol⁻¹ (elevated) and at two temperatures of 1.5 and 6.0 °C above ambient temperature. The objectives were to characterize peanut leaf photosynthesis responses to long-term elevated growth [CO₂] and temperature, and to assess whether elevated [CO₂] regulated peanut leaf photosynthetic capacity, in terms of activity and protein content of ribulose bisphosphate carboxylase-oxygenase (Rubisco), Rubisco photosynthetic efficiency, and carbohydrate metabolism. At both growth temperatures, leaves of plants grown under elevated [CO₂] had higher midday photosynthetic CO₂ exchange rate (CER), lower transpiration and stomatal conductance and higher water-use efficiency, compared to those of plants grown at ambient [CO₂]. Both activity and protein content of Rubisco, expressed on a leaf area basis, were reduced at elevated growth [CO₂]. Declines in Rubisco under elevated growth [CO₂] were 27–30% for initial activity, 5–12% for total activity, and 9–20% for protein content. Although Rubisco protein content and activity were down-regulated by elevated [CO₂], Rubisco photosynthetic efficiency, the ratio of midday light-saturated CER to Rubisco initial or total activity, of the elevated-[CO₂] plants was 1.3- to 1.9-fold greater than that of the ambient-[CO₂] plants at both growth temperatures. Leaf soluble sugars and starch of plants grown at elevated [CO₂] were 1.3- and 2-fold higher, respectively, than those of plants grown at ambient [CO₂]. Under elevated [CO₂], leaf soluble sugars and starch, however, were not affected by high growth temperature. In contrast, high temperature reduced leaf soluble sugars and starch of the ambient-[CO₂] plants. Activity of sucrose-P synthase, but not adenosine 5′-diphosphoglucose pyrophosphorylase, was up-regulated under elevated growth [CO₂]. Thus, in the absence of other environmental stresses, peanut leaf photosynthesis would perform well under rising atmospheric [CO₂] and temperature as predicted for this century.     

Pea plant responsiveness under elevated [CO2] is conditioned by the N source (N2 fixation versus NO3− fertilization)

The main goal of this study was to test the effect of [CO₂] on C and N management in different plant organs (shoots, roots and nodules) and its implication in the responsiveness of exclusively N₂-fixing and NO₃⁻-fed plants. For this purpose, exclusively N₂-fixing and NO₃⁻-fed (10 mM) pea (Pisum sativum L.) plants were exposed to elevated [CO₂] (1000 μmol mol⁻¹ versus 360 μmol mol⁻¹ CO₂). Gas exchange analyses, together with carbohydrate, nitrogen, total soluble proteins and amino acids were determined in leaves, roots and nodules. The data obtained revealed that although exposure to elevated [CO₂] increased total dry mass (DM) in both N treatments, photosynthetic activity was down-regulated in NO₃⁻-fed plants, whereas N₂-fixing plants were capable of maintaining enhanced photosynthetic rates under elevated [CO₂]. In the case of N₂-fixing plants, the enhanced C sink strength of nodules enabled the avoidance of harmful leaf carbohydrate build up. On the other hand, in NO₃⁻-fed plants, elevated [CO₂] caused a large increase in sucrose and starch. The increase in root DM did not contribute to stimulation of C sinks in these plants. Although N₂ fixation matched plant N requirements with the consequent increase in photosynthetic rates, in NO₃⁻-fed plants, exposure to elevated [CO₂] negatively affected N assimilation with the consequent photosynthetic down-regulation.     

Free-air CO2 enrichment in a wheat-weed assembly – effects on water relations

In a three-year free-air CO₂ enrichment study (Mini-FACE), spring wheat associated with typical arable weeds were grown under present and elevated atmospheric carbon dioxide concentrations [CO₂] (ambient air+150 μmol mol^?1). Analyses of plant stable carbon isotope ratios and in vivo measurements of leaf gas exchange were used to describe the CO₂ effects on water relations. For most species examined elevated [CO₂] significantly increased the intrinsic water-use efficiency (A/g_s) as derived from carbon isotope analyses. In some of the species, seasonal averages of the ratio between leaf internal to atmospheric CO₂ (c_i/c_a) were found to be significantly reduced by elevated [CO₂]. Periodic leaf gas exchange measurements confirmed the increased water-use efficiency, but significant CO₂ effects became evident only over the entire season by carbon isotope analysis. In both types of analysis conducted, spring wheat was found to react significantly different from all other species examined. The relation between A/g_s and biomass production was significantly influenced by elevated [CO₂] in all three years of the study. At the end of the drier growing seasons 2003 and 2004, the soil water content tended to be increased in the CO₂ enriched plots indicating a water saving effect. These observations demonstrate the impact of elevated [CO₂] on plant water relations with a likely positive feedback leading to higher soil water availability. Due to the differences in the CO₂ responses of spring wheat compared to the weeds we suggest that rising [CO₂] may cause shifts in the species composition of crop-weed communities.     

Increased feeding damage under elevated Co2: The case of the Russian wheat aphid

We report the results of a baseline study on the effects of Russian wheat aphid infestation on barley lines grown under ambient and elevated (450 and 550 μmol mol^− 1) CO₂ concentrations [CO₂]. Elevated CO₂ impacted on plant biomass, C:N ratios and leaf nitrogen concentrations. Visible manifestation of aphid feeding related damage was assessed by examining resultant chlorosis and leaf roll under ambient and two elevated [CO₂] levels using a control and three resistant barley host combinations. Elevated [CO₂] had a significant positive effect on the growth of the four barley lines that were not infested by the aphids. However under the same conditions aphid feeding under elevated CO₂ conditions caused very high biomass loss, which was more noticeable in experiments involving non-resistant PUMA than in the resistant barley lines. The results of this study demonstrate that CO₂ enrichment substantially increases aphid populations of RWASA1 and RWASA2 on the four barley lines investigated. Furthermore, aphid populations were higher on non-resistant PUMA than the three resistant lines and the RWASA2 biotype out-performed RWASA1 in each case. Under elevated [CO₂], aphid feeding, resulted in a significant increase in the leaf C:N ratios (as a percentage change) in most treatments, compared to levels recorded on uninfested plants. The resistant lines also showed a significant reduction in leaf nitrogen (~ 40% for PUMA and not less than 30% for the resistant STARS lines tested). C:N ratio changes and N loss correlated to [CO₂] and aphid biotype. By 28 days of infestation, most of the non-resistant PUMA line in particular showed significant irrecoverable levels of leaf chlorosis. At level 9 rating on the chlorosis scale (i.e. plant death when recovery was not possible), experiments were terminated. As aphid success is unlikely to be the sole product of [CO₂], but also of other limiting nutrients such as N, it may be worth further investigating the effect of plant quality and ultimately plant nutrition on the population growth of aphids.     

Verification of a threshold concept of ecologically effective precipitation pulse: From plant individuals to ecosystem

In water-limited ecosystems, an ecologically significant rainfall pulse was defined as a rainfall event that altered both soil water status and plant physiological activity. We developed a new threshold concept of an ecologically effective precipitation pulse (EEPP) applicable to both plant individual and ecosystem scales. The concept was tested in a typical steppe on Inner Mongolia plateau. Two EEPPs, single 3-mm rainfall and 5-mm rainfall, were applied to investigate their effects on soil and plant water status, CO₂ assimilation of five species (four C₃ plants and one C₄ plant), whole-plot soil respiration (R_s), and net ecosystem CO₂ exchange (NEE) on 1 June and 28 July 2009, respectively. Both EEPPs increased leaf water potential (Ψ_l) of all the species, which peaked 1–3 days after rainfall pulses. Soil water content (SWC) in two depths (5 cm and 20 cm) significantly increased after the two EEPPs for 1–3 days. Soil water potential (Ψ_s) within 20‐cm soil layer in EEPP treatments significantly differed (p < 0.05) from control. Net assimilation rates (A_net) of all C₃ plants had a slight increase at the next day after two EEPPs, in contrast to the C₄ species. R_s elevated and peaked 1–3 days later after water supply. Ecosystem net CO₂ absorption rate rose to maximum value 3 days after the 5-mm pulse on 28 July, higher than the response to 3-mm pulse on June 1. The grassland turned to net emission of CO₂ after 3-mm pulse on 28 July. The results supported that there was an ecosystem level threshold for EEPP, and the threshold was temporally variable. It also highlighted the necessity of considering the response threshold of EEPP in rainfall manipulative experiment. In addition, effective rainfall amount was more approriate than total rainfall amount in modeling ecosystem carbon balance.     

Gas exchange acclimation to elevated CO2 in upper-sunlit and lower-shaded canopy leaves in relation to nitrogen acquisition and partitioning in wheat grown in field chambers

Growth at elevated CO₂ often decreases photosynthetic capacity (acclimation) and leaf N concentrations. Lower-shaded canopy leaves may undergo both CO₂ and shade acclimation. The relationship of acclimatory responses of flag and lower-shaded canopy leaves of wheat (Triticum aestivum L.) to the N content, and possible factors affecting N gain and distribution within the plant were investigated in a wheat crop growing in field chambers set at ambient (360 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂, and with two amounts of N fertilizer (none and 70 kg ha⁻¹ applied on 30 April). Photosynthesis, stomatal conductance and transpiration at a common measurement CO₂, chlorophyll and Rubisco levels of upper-sunlit (flag) and lower-shaded canopy leaves were significantly lower in elevated relative to ambient CO₂-grown plants. Both whole shoot N and leaf N per unit area decreased at elevated CO₂, and leaf N declined with canopy position. Acclimatory responses to elevated CO₂ were enhanced in N-deficient plants. With N supply, the acclimatory responses were less pronounced in lower canopy leaves relative to the flag leaf. Additional N did not increase the fraction of shoot N allocated to the flag and penultimate leaves. The decrease in photosynthetic capacity in both upper-sunlit and lower-shaded leaves in elevated CO₂ was associated with a decrease in N contents in above-ground organs and with lower N partitioning to leaves. A single relationship of N per unit leaf area to the transpiration rate accounted for a significant fraction of the variation among sun-lit and shaded leaves, growth CO₂ level and N supply. We conclude that reduced stomatal conductance and transpiration can decrease plant N, leading to acclimation to CO₂ enrichment.     

Exposures to elevated CO2, elevated temperature and enhanced UV-B radiation modify activities of polyphenol oxidase and guaiacol peroxidase and concentrations of chlorophylls,polyamines and soluble proteins in the leaves of Betula pendula seedlings

The activity of polyphenol oxidase (PPO) and guaiacol peroxidase (POD) and the concentrations of chlorophylls, free polyamines and soluble proteins were determined from the leaves of six genotypes of silver birch (Betula pendula Roth) seedlings exposed to short-term elevated carbon dioxide (CO₂), temperature (T), ultraviolet-B irradiation (UV-B, 280-315 nm) and their combinations. Results showed that the activity of PPO in the leaves was low but increased by elevated CO₂ and elevated T. The POD activity varied between the genotypes due to an interactive effect of CO₂ × UV-B. The soluble proteins were clearly decreased by elevated CO₂, but the level of response varied among the genotypes. The concentrations of chl a and total chlorophylls were lower in the leaves treated with elevated CO₂ than in leaves grown at ambient CO₂. An interactive effect of CO₂ × UV-B on the chl a/b ratio was found. Elevated T increased chl b concentration and decreased chl a/b ratio. Temperature treatments also caused variation in the concentrations of chl a, chl b and total chlorophylls among the genotypes. Polyamine analyses showed that the concentrations of putrescine were increased and spermine decreased in leaves treated with elevated T. However, the change in putrescine by elevated T was clearer at ambient CO₂ than in eCO₂ environment (significant effect of T × CO₂). In conclusion, the defensive enzymes, photosynthetic pigments, soluble proteins and growth-regulating polyamines in silver birch leaves were not susceptible to enhanced UV-B radiation. In contrast, all the variables responded to elevated T and/or elevated CO₂, reflecting the enhancive effects of climate change conditions not only on leaf productivity, but also on leaf turn-over rate. Most of these climate-driven changes were not regulated by UV-B radiation.     

Response of nodulated alfalfa to water supply,temperature and elevated CO2: productivity and water relations

Exposing plants to long-term CO₂ enrichment generally leads to increases in plant biomass, total leaf area and alterations on leaf net photosynthetic rates, stomatal conductance and water use efficiency. However, the magnitude of such effects is dependent on the availability of other potentially limiting resources. The aim of our study was to elucidate the effects of elevated CO₂, applied at different temperature and water availability regimes, on nodulated alfalfa plants. Regardless of water supply, elevated CO₂ enhanced plant growth, especially when combined with increased temperature although no differences were detected until 30 days of treatment. Absence of differences in leaf relative growth rate, and gas exchange measurements, suggested that plants grown in a low water regime adjusted their growth to the amount of available water. Elevated CO₂ enhanced water use efficiency because of reduced water consumption and a greater dry mass production. Increased dry matter production of plants grown under elevated CO₂ and temperature was the result of stimulated photosynthetic rates, greater leaf area and water use efficiency. Lack of CO₂ effect on photosynthesis of plants grown at ambient temperature might be consequence of down-regulation phenomena. Plants grown at 700 μmol mol⁻¹ CO₂ maintained control nitrogen levels, discarding enhanced nitrogen availability as the main factor explaining enhanced dry matter.     

Impact of in vitro CO2 enrichment and sugar deprivation on acclimatory responses of Phalaenopsis plantlets to ex vitro conditions

We assessed the effect of growth at either 400 μmol mol^?1 (ambient) or 1000 μmol mol^?1 (elevated) CO₂ and 0 g L^?1 (deprivation) or 30 g L^?1 (supplementation) sugar on morphological traits, photosynthetic attributes and intrinsic elements of the CAM pathway using the CAM orchid Phalaenopsis ‘Amaglade’. The growth of shoot (retarded) and root (induced) was differently affected by CO₂ enrichment and mixotrophic regime (+sugar). The Fv/Fm ratio was 14% more in CO₂-enriched treatment than at ambient level during in vitro growth. At elevated level of CO₂ and sugar treatment, the content of Chl(a + b), Chl a/b and Chl/Car was enhanced while carotenoid content remained unaltered. During in vitro growth, gas-exchange analysis indicated that increased uptake of CO₂ accorded with the increased rate of transpiration and unchanged stomatal conductance at elevated level of CO₂ under both photo- and mixotrophic growth condition. At elevated level of CO₂ and sugar deprivation, activities of Rubisco (26.4%) and PEPC (74.5%) was up-regulated. Among metabolites, the content of sucrose and starch was always higher under CO₂ enrichment during both in vitro and ex vitro growth. Our results indicate that plantlets grown under CO₂ enrichment developed completely viable photosynthetic apparatus ready to be efficiently transferred to ex vitro condition that has far-reaching implications in micropropagation of Phalaenopsis.     

Elevated CO2 increases water use efficiency by sustaining photosynthesis of water-limited maize and sorghum

Maize and grain sorghum seeds were sown in pots and grown for 39 days in sunlit controlled-environment chambers at 360 (ambient) and 720 (double-ambient, elevated) μmol mol⁻¹ carbon dioxide concentrations [CO₂]. Canopy net photosynthesis (PS) and evapotranspiration (TR) was measured throughout and summarized daily from 08:00 to 17:00 h Eastern Standard Time. Irrigation was withheld from matched pairs of treatments starting on 26 days after sowing (DAS). By 35 DAS, cumulative PS of drought-stress maize, compared to well-watered plants, was 41% lower under ambient [CO₂] but only 13% lower under elevated [CO₂]. In contrast, by 35 DAS, cumulative PS of drought-stress grain sorghum, compared to well-watered plants, was only 9% lower under ambient [CO₂] and 7% lower under elevated [CO₂]. During the 27-35 DAS drought period, water use efficiency (WUE, mol CO₂ Kmol⁻¹ H₂O), was 3.99, 3.88, 5.50, and 8.65 for maize and 3.75, 4.43, 5.26, and 9.94 for grain sorghum, for ambient-[CO₂] well-watered, ambient-[CO₂] stressed, elevated-[CO₂] well-watered and elevated-[CO₂] stressed plants, respectively. Young plants of maize and sorghum used water more efficiently at elevated [CO₂] than at ambient [CO₂], especially under drought. Reductions in biomass by drought for young maize and grain sorghum plants were 42 and 36% at ambient [CO₂], compared to 18 and 14% at elevated [CO₂], respectively. Results of our water stress experiment demonstrated that maintenance of relatively high canopy photosynthetic rates in the face of decreased transpiration rates enhanced WUE in plants grown at elevated [CO₂]. This confirms experimental evidence and conceptual models that suggest that an increase of intercellular [CO₂] (or a sustained intercellular [CO₂]) in the face of decreased stomatal conductance results in relative increases of growth of C₄ plants. In short, drought stress in C₄ crop plants can be ameliorated at elevated [CO₂] as a result of lower stomatal conductance and sustaining intercellular [CO₂]. Furthermore, less water might be required for C₄ crops in future higher CO₂ atmospheres, assuming weather and climate similar to present conditions.     

Leaf photosynthesis of Haberlea rhodopensis before and during drought

Haberlea rhodopensis is a homoiochlorophyllous resurrection plant that shows a low rate of leaf net CO₂ uptake (4–6 μmol m^?2 s^?1) under saturating photosynthetic photon flux densities in air (21% O₂ and about 390 ppm CO₂). However, leaf net CO₂ uptake reaches values of 17–18 μmol m^?2 s^?1 under saturating CO₂ and light. H. rhodopensis leaves have a very low mesophyll CO₂ conductance that can partly explain the low rate of leaf net CO₂ uptake in normal air. Experimental evidences suggest that mesophyll conductance is not sensitive to temperature in the 20–35 °C range. In addition, it is shown that the (1) transpiration rate of H. rhodopensis is nearly linearly related to the vapour pressure difference between the leaf and the ambient air within the interval from 0.5 kPa to 2.5 kPa at a leaf temperature of 25 °C and (2) leaf net CO₂ uptake in normal air under saturating light does not change much with leaf temperature (between 20 °C and 30 °C). At a leaf relative water content of between 90% and 30%, the decrease of leaf net CO₂ assimilation during drought can be explained by a decrease of leaf CO₂ diffusional conductance. Accordingly the non-photochemical chlorophyll fluorescence quenching decreases only at relative water contents lower than 20%, indicating that photosynthetic activity maintains a trans-thylakoidal proton gradient over a wide range of leaf water contents. Moreover, PSII photochemistry (as estimated by the Fv/Fm ratio and the thermoluminescence B band intensity) is only affected at leaf relative water contents lower than about 20%, thus confirming that primary photosynthetic reactions are resistant to drought. Interestingly, the effect of leaf desiccation on photosynthetic capacity, measured at very high ambient CO₂ molar ratios under saturating PPFD, is identical to that observed for three non-resurrection C₃ mesophytes. This demonstrates that the photosynthetic apparatus of H. rhodopensis is not more resistant to desiccation when compared to other C₃ plants. Since the leaf area decreases by more than 50% when the leaf relative water content is reduced to about 40% during drought it is supposed, following Farrant et al. [Farrant, J.M., Vander, W.C., Lofell, D.A., Bartsch, S., Whittaker, A., 2003. An investigation into the role of light during desiccation of three angiosperms resurrection plants. Plant Cell Environ. 26, 1275–1286], that H. rhodopensis leaf cells avoid mechanical stress.     

Elevated CO2 and water-availability effect on gas exchange and nodule development in N2-fixing alfalfa plants

N₂-fixing alfalfa plants were grown in controlled conditions at different CO₂ levels (350 μmol mol^?1 versus 700 μmol mol^?1) and water-availability conditions (WW, watered at maximum pot water capacity versus WD, watered at 50% of control treatments) in order to determine the CO₂ effect (and applied at two water regimes) on plant growth and nodule activity in alfalfa plants. The CO₂ stimulatory effect (26% enhancement) on plant growth was limited to WW plants, whereas no CO₂ effect was observed in WD plants. Exposure to elevated CO₂ decreased Rubisco carboxylation capacity of plants, caused by a specific reduction in Rubisco (EC 4.1.1.39) concentration (11% in WW and 43% in WD) probably explained by an increase in the leaf carbohydrate levels. Plants grown at 700 μmol mol^?1 CO₂ maintained control photosynthetic rates (at growth conditions) by diminishing Rubisco content and by increasing nitrogen use efficiency. Interestingly, our data also suggest that reduction in shoot N demand (reflected by the TSP and especially Rubisco depletion) affected negatively nodule activity (malate dehydrogenase, EC 1.1.1.37, and glutamate-oxaloacetate transaminase, EC 2.6.1.1, activities) particularly in water-limited conditions. Furthermore, nodule DM and TSS data revealed that those nodules were not capable to overcome C sink strength limitations.     

The mechanism(s) involved in the photoprotection of PSII at elevated CO2 in nodulated alfalfa plants

In a previous study, we found that enhanced CO₂ subjected to nodulated alfalfa plants grown at different temperatures (ambient and ambient + 4 °C) and water availability regimes could protect PSII from photodamage. The main objective of this study was to determine the mechanism(s) involved in the photoprotection of PSII at elevated CO₂ levels in this plant. Elevated CO₂ reduced carboxylation capacity-induced photosynthetic acclimation and reduced enzymatic and/or nonenzymatic antioxidant activities, suggesting that changes in electron flow did not cause any photooxidative damage (which was also confirmed by H₂O₂ and lipid peroxidation analyses). Enhanced nonphotochemical quenching and xanthophyll cycle pigments revealed that plants grown at 700 μmol mol⁻¹ CO₂ compensated for the reduction in energy sink with a larger capacity for nonphotochemical dissipation of excitation energy as heat, i.e., modulating the status of the VAZ components. Elevated CO₂ induced the de-epoxidation of violaxanthin to zeaxanthin, facilitating thermal dissipation and protecting the photosynthetic apparatus against the deleterious effect of excess excitation energy.     

The mechanism(s) involved in the photoprotection of PSII at elevated CO2 in nodulated alfalfa plants

In a previous study, we found that enhanced CO₂ subjected to nodulated alfalfa plants grown at different temperatures (ambient and ambient + 4 °C) and water availability regimes could protect PSII from photodamage. The main objective of this study was to determine the mechanism(s) involved in the photoprotection of PSII at elevated CO₂ levels in this plant. Elevated CO₂ reduced carboxylation capacity-induced photosynthetic acclimation and reduced enzymatic and/or nonenzymatic antioxidant activities, suggesting that changes in electron flow did not cause any photooxidative damage (which was also confirmed by H₂O₂ and lipid peroxidation analyses). Enhanced nonphotochemical quenching and xanthophyll cycle pigments revealed that plants grown at 700 μmol mol^?1 CO₂ compensated for the reduction in energy sink with a larger capacity for nonphotochemical dissipation of excitation energy as heat, i.e., modulating the status of the VAZ components. Elevated CO₂ induced the de-epoxidation of violaxanthin to zeaxanthin, facilitating thermal dissipation and protecting the photosynthetic apparatus against the deleterious effect of excess excitation energy.     

Elevated CO2 alleviates the impact of drought on barley improving water status by lowering stomatal conductance and delaying its effects on photosynthesis

We analysed the impact of elevated CO₂ on water relations, water use efficiency and photosynthetic gas exchange in barley (Hordeum vulgare L.) under wet and drying soil conditions. Soil moisture was less depleted under elevated compared to ambient [CO₂]. Elevated CO₂ had no significant effect on the water relations of irrigated plants, except on whole plant hydraulic conductance, which was markedly decreased at elevated compared to ambient CO₂ concentrations. The values of relative water content, water potential and osmotic potential were higher under elevated CO₂ during the entire drought period. The better water status of water-limited plants grown at elevated CO₂ was the result of stomatal control rather than of osmotic adjustment. Despite the low stomatal conductance produced by elevated CO₂, net photosynthesis was higher under elevated than ambient CO₂ concentrations. With water shortage, photosynthesis was maintained for longer at higher rates under elevated CO₂. The reduction of stomatal conductance and therefore transpiration, and the enhancement of carbon assimilation by elevated CO₂, increased instantaneous and whole plant water use efficiency in both irrigated and droughted plants. Thus, the metabolism of barley plants grown under elevated CO₂ and moderate or mild water deficit conditions is benefited by increased photosynthesis and lower transpiration. The reduction in plant water use results in a marked increase in soil water content which delays the onset and severity of water deficit.     

Elevated atmospheric CO2 decreases oxidative stress and increases essential oil yield in leaves of Thymus vulgaris grown in a mini-FACE system

Potted one-year-old plants of Thymus vulgaris L. (thyme, Lamiaceae, C3 metabolism), were grown for three months (10 June–10 September, 2004) in a “mini-free-air-CO₂-enrichment” (“mini-FACE”) system, under 500 μmol mol^?1 and ambient concentrations of atmospheric carbon dioxide (CO₂). Compared to ambient CO₂, elevated CO₂ stimulated leaf superoxide dismutase (SOD, EC 1.15.1.1) activity only at the first sampling-time (July), followed by no variation or even a trend of decreased activity on the other two sampling-times (August and September). Under high CO₂, guaiacol peroxidase (GPX, EC 1.11.1.7) and catalase (CAT, EC 1.11.1.6) leaf activities showed no variation or drop throughout the duration of the experiment. By contrast, under elevated CO₂, leaf glutathione reductase (GR, EC 1.6.4.2) activity increased on all the sampling-times, and also a duration-dependent upward trend of glutathione (GSH) content was recorded, with this increase becoming significant – compared with ambient CO₂ – at the third sampling-time (September). Simultaneously, leaves from plants grown under high CO₂ showed a marked increase in essential oil yield, with slight increments in phenolic component and decrements in mono- and sesquiterpene components. Also, a drop in thiobarbituric acid reactive substances (TBARS) content under elevated CO₂ was displayed. Thus, in general, the results reported here point to a downregulation of leaf antioxidant enzymes under elevated CO₂, supporting the notion of reduced reactive oxygen species (ROS) formation under these circumstances. Relying instead on antioxidant-regenerating enzymes, namely GR, fairly high GSH content and essential oils, might be a ‘low cost’ life strategy for growth under elevated CO₂, not requiring synthesis/activation of energy-intensive and expensive metabolic processes.     

How is the relationship between the C4 cereal Sorghum bicolor and the C3 root hemi-parasites Striga hermonthica and Striga asiatica affected by elevated CO2?

The C₄ cereal Sorghum bicolor was grown under either ambient (350 μmol mol^?1) or elevated (700 μmol mol^?1) [CO₂] in either the presence or absence of the C₃ obligate root hemi-parasites Striga hermonthica or S. asiatica. Both uninfected and infected sorghum plants were taller and had greater biomass, photosynthetic rates, water-use efficiencies and leaf areas under elevated compared with ambient [CO₂]. There was no evidence of any downregula-tion of photosynthesis in sorghum grown at elevated [CO₂]. Biomass of infected sorghum was lower under both ambient and elevated [CO₂], and although infected plants were larger under elevated [CO₂] the relative impact of infection on host biomass was either the same (S. asiatica) or only slightly less (S. hermonthica) than under ambient [CO₂]. In contrast, biomass of S. hermonthica and S. asiatica per host was lower under elevated than ambient [CO₂], although rates of photosynthesis were higher at elevated [CO₂] and parasite stomatal conductance was not responsive to [CO₂]. Parasites emerged above-ground and flowered earlier under ambient compared with elevated [CO₂]. It appears that the mechanism(s) by which the parasites affect host growth is (are) relatively insensitive to increased atmospheric [CO₂], although the parasites themselves were adversely affected by growth at elevated [CO₂].     

Differential Effects of Pseudomonas mendocina and Glomus intraradices on Lettuce Plants Physiological Response and Aquaporin PIP2 Gene Expression Under Elevated Atmospheric CO2 and Drought

Arbuscular mycorrhizal (AM) symbiosis and plant-growth-promoting rhizobacterium (PGPR) can alleviate the effects of water stress in plants, but it is unknown whether these benefits can be maintained at elevated CO₂. Therefore, we carried out a study where seedlings of Lactuca sativa were inoculated with the AM fungus (AMF) Glomus intraradices N.C. Schenk & G.S. Sm. or the PGPR Pseudomonas mendocina Palleroni and subjected to two levels of watering and two levels of atmospheric CO₂ to ascertain their effects on plant physiological parameters and gene expression of one PIP aquaporin in roots. The inoculation with PGPR produced the greatest growth in lettuce plants under all assayed treatments as well as the highest foliar potassium concentration and leaf relative water content under elevated [CO₂] and drought. However, under such conditions, the PIP2 gene expression remained almost unchanged. G. intraradices increased significantly the AMF colonization, foliar phosphorus concentration and leaf relative water content in plants grown under drought and elevated [CO₂]. Under drought and elevated [CO₂], the plants inoculated with G. intraradices showed enhanced expression of the PIP2 gene as compared to P. mendocina or control plants. Our results suggest that both microbial inoculation treatments could help to alleviate drought at elevated [CO₂]. However, the PIP2 gene expression was increased only by the AMF but not by the PGPR under these conditions.     

Elevated CO2 reduces the drought effect on nitrogen metabolism in barley plants during drought and subsequent recovery

The objective of this study was to determine the response of nitrogen metabolism to drought and recovery upon rewatering in barley (Hordeum vulgare L.) plants under ambient (350 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂ conditions. Barley plants of the cv. Iranis were subjected to drought stress for 9, 13, or 16 days. The effects of drought under each CO₂ condition were analysed at the end of each drought period, and recovery was analysed 3 days after rewatering 13-day droughted plants. Soil and plant water status, protein content, maximum (NR_max) and actual (NR_act) nitrate reductase, glutamine synthetase (GS), and aminant (NADH-GDH) and deaminant (NAD-GDH) glutamate dehydrogenase activities were analysed. Elevated CO₂ concentration led to reduced water consumption, delayed onset of drought stress, and improved plant water status. Moreover, in irrigated plants, elevated CO₂ produced marked changes in plant nitrogen metabolism. Nitrate reduction and ammonia assimilation were higher at elevated than at ambient CO₂, which in turn yielded higher protein content. Droughted plants showed changes in water status and in foliar nitrogen metabolism. Leaf water potential (Ψ_w) and nitrogen assimilation rates decreased after the onset of water deprivation. NR_act and NR_max activity declined rapidly in response to drought. Similarly, drought decreased GS whereas NAD-GDH rose. Moreover, protein content fell dramatically in parallel with decreased leaf Ψ_w. In contrast, elevated CO₂ reduced the water stress effect on both nitrate reduction and ammonia assimilation coincident with a less-steep decrease in Ψ_w. On the other hand, Ψ_w practically reached control levels after 3 days of rewatering. In parallel with the recovery of plant water status, nitrogen metabolism was also restored. Thus, both NR_act and NR_max activities were restored to about 75-90% of control levels when water supply was restored; the GS activity reached 80-90% of control values; and GDH activities and protein content were similar to those of control plants. The recovery was always faster and slightly higher in plants grown under elevated CO₂ conditions compared to those grown in ambient CO₂, but midday Ψ_w dropped to similar values under both CO₂ conditions. The results suggest that elevated CO₂ improves nitrogen metabolism in droughted plants by maintaining better water status and enhanced photosynthesis performance, allowing superior nitrate reduction and ammonia assimilation. Ultimately, elevated CO₂ mitigates many of the effects of drought on nitrogen metabolism and allows more rapid recovery following water stress.