Elevated CO2 and water deficit effects on photosynthesis, ribulose bisphosphate carboxylase‐oxygenase, and carbohydrate metabolism in rice

Rice (Oryza sativa[L.] cv. IR-72) was grown for a season in sunlit, controlled-environment chambers at 350 or 700 µmol CO₂ mol^?1 under continuously flooded (unstressed) or drought-imposed periods at panicle initiation (stressed). The midday canopy photosynthetic rates (P_n), measured at the CO₂ concentration ([CO₂]) used for growth, were enhanced by high [CO₂] but reduced by drought. High [CO₂] increased P_n by 18 to 34% for the unstressed plants, and 6 to 12% for the stressed plants. In the unstressed plants, CO₂ enrichment increased water-use efficiency (WUE) by 26%, and reduced evapotranspiration (ET) by 8 to 14%. Both high [CO₂] and severe drought decreased the activity and content of ribulose bisphosphate carboxylase-oxygenase (Rubisco). High-CO₂-unstressed plants had 6 to 22% smaller content and 5 to 25%, lower activity of Rubisco than ambient-CO₂-unstressed plants. Under severe drought, reductions of Rubisco were 53 and 27% in activity and 40 and 12% in content, respectively, for ambient- and high-CO₂ treatments. The apparent catalytic turnover rate (K_cat) of midday fully activated Rubisco was not altered by high [CO₂], but severe drought reduced K_cat by 17 to 23%. Chloroplasts of the high-CO₂ leaves contained more, and larger starch grains than those of the ambient CO₂ leaves. High [CO₂] did not affect the leaf sucrose content of unstressed plants. In contrast, severe drought reduced the leaf starch and increased the sucrose content in both CO₂ treatments. The activity of leaf sucrose phosphate synthase of unstressed plants was not affected by high [CO₂], whereas that of ambient-CO₂-grown plants was reduced 45% by severe drought. Reduction in ET and enhancements in both P_n and WUE for rice grown under high [CO₂] helped to delay the adverse effects of severe drought and allowed the stressed plants to assimilate CO₂ for an extra day. Thus, rice grown in the next century may utilize less water, use water more efficiently, and be able to tolerate drought better under some situations.     

Regulation of hormonal responses of sweet pepper as affected by salinity and elevated CO2 concentration

This study examines the extent to which the predicted CO₂‐protective effects on the inhibition of growth, impairment of photosynthesis and nutrient imbalance caused by saline stress are mediated by an effective adaptation of the endogenous plant hormonal balance. Therefore, sweet pepper plants (Capsicum annuum, cv. Ciclón) were grown at ambient or elevated [CO₂] (400 or 800 µmol mol^–1) with a nutrient solution containing 0 or 80 mM NaCl. The results show that, under saline conditions, elevated [CO₂] increased plant dry weight, leaf area, leaf relative water content and net photosynthesis compared with ambient [CO₂], whilst the maximum potential quantum efficiency of photosystem II was not modified. In salt‐stressed plants, elevated [CO₂] increased leaf NO₃^– concentration and reduced Cl^– concentration. Salinity stress induced ABA accumulation in the leaves but it was reduced in the roots at high [CO₂], being correlated with the stomatal response. Under non‐stressed conditions, IAA was dramatically reduced in the roots when high [CO₂] was applied, which resulted in greater root DW and root respiration. Additionally, the observed high CK concentration in the roots (especially tZR) could prevent downregulation of photosynthesis at high [CO₂], as the N level in the leaves was increased compared with the ambient [CO₂], under salt‐stress conditions. These results demonstrate that the hormonal balance was altered by the [CO₂], which resulted in significant changes at the growth, gas exchange and nutritional levels.     

Enhanced isoprene emission capacity and altered light responsiveness in aspen grown under elevated atmospheric CO2 concentration

Controversial evidence of CO₂‐responsiveness of isoprene emission has been reported in the literature with the response ranging from inhibition to enhancement, but the reasons for such differences are not understood. We studied isoprene emission characteristics of hybrid aspen (Populus tremula x P. tremuloides) grown under ambient (380 μmol mol^?1) and elevated (780 μmol mol^?1) [CO₂] to test the hypothesis that growth [CO₂] effects on isoprene emission are driven by modifications in substrate pool size, reflecting altered light use efficiency for isoprene synthesis. A novel in vivo method for estimation of the pool size of the immediate isoprene precursor, dimethylallyldiphosphate (DMADP) and the activity of isoprene synthase was used. Growth at elevated [CO₂] resulted in greater leaf thickness, more advanced development of mesophyll and moderately increased photosynthetic capacity due to morphological “upregulation”, but isoprene emission rate under growth light and temperature was not significantly different among ambient‐ and elevated‐[CO₂]‐grown plants independent of whether measured at 380 μmol mol^?1 or 780 μmol mol^?1 CO₂. However, DMADP pool size was significantly less in elevated‐[CO₂]‐grown plants, but this was compensated by increased isoprene synthase activity. Analysis of CO₂ and light response curves of isoprene emission demonstrated that the [CO₂] for maximum isoprene emission was shifted to lower [CO₂] in elevated‐[CO₂]‐grown plants. The light‐saturated isoprene emission rate (I_max,Q) was greater, but the quantum efficiency at given I_max,Q was less in elevated‐[CO₂]‐grown plants, especially at higher CO₂ measurement concentration, reflecting stronger DMADP limitation at lower light and higher [CO₂]. These results collectively demonstrate important shifts in light and CO₂‐responsiveness of isoprene emission in elevated‐[CO₂]‐acclimated plants that need consideration in modeling isoprene emissions in future climates.     

Atmospheric carbon dioxide and fertilizer nitrogen effects on radiation interception by rice 1

In order to predict the potential impacts of global change, it is important to understand the impact of increasing global atmospheric [CO₂] on the growth and yield of crop plants. The objectives of this study were to determine the interaction of N fertilization rates and atmospheric [CO₂] on radiation interception and radiation-use efficiency of rice (Oryza sativa L. cv. IR72) grown under tropical field conditions. Rice plants were grown inside open top chambers in a lowland rice field at the International Rice Research Institute in the Philippines at ambient (about 350 μmol mol^-1) or elevated (about 600 μmol mol^-1 during the 1993 wet season and 700 μmol mol^-1 during the 1994 dry season) in combination with three levels of applied N (0, 50 or 100 kg N ha^-1 in the wet season; 0, 90 or 200 kg N ha-1 in the dry season). Light interception was not directly affected by [CO₂], but elevated [CO₂] indirectly increased light interception through increasing total absorbed N. Plant N requirement for radiation interception was similar for rice grown under ambient [CO₂] or elevated [CO₂] treatments. The conversion efficiency of intercepted radiation to dry matter, radiation-use efficiency (RUE), was about 35% greater at elevated [CO₂] than at ambient [CO₂]. The relationship between leaf N and RUE was curvilinear. At ambient [CO₂], RUE was fairly stable across levels of leaf N, but leaf N less than about 2.5% resulted in lower RUE for plants grown with elevated [CO₂] than for plant grown at ambient [CO₂]. Decreased leaf N with increased [CO₂], therefore decreased RUE of rice plants grown at elevated [CO₂]. When predicting responses of rice to elevated [CO₂], RUE should be adjusted with a decrease in leaf N. This revised version was published online in June 2006 with corrections to the Cover Date.     

The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C₄ species can be stimulated by increasing CO₂ concentration, [CO₂], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO₂ leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO₂]. Three C₄ species that differed in the reported degree of bundle sheath leakiness to CO₂, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO₂] of 350 μl l^?1 (ambient) or 700 μl l^?1 (elevated). Assimilation as a function of increasing [CO₂] at high photosynthetic photon flux density (PPFD, 1 600 μmol m^?2 s^?1) indicated that leaf photosynthesis was not saturated under current ambient [CO₂] for any of the three C₄ species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO₂] was light dependent for all three C₄ species. The stimulation of leaf photosynthesis at elevated [CO₂] was not associated with previously published values of CO₂ leak rates from the bundle sheath, changes in the ratio of activities of PEP-carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO₂] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C₄ species can respond directly to increased [CO₂] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves.     

Influence of carbon dioxide enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition

The objective of this study was to test whether elevated [CO₂], [O₃] and nitrogen (N) fertility altered leaf mass per area (LMPA), non‐structural carbohydrate (TNC), N, lignin (LTGA) and proanthocyanidin (PA) concentrations in cotton (Gossypium hirsutum L.) leaves and roots. Cotton was grown in 14 dm³ pots with either sufficient (0·8 g N dm ^{? 3}) or deficient (0·4 and 0·2 g N dm ^{? 3}) N fertilization, and treated in open‐top chambers with either ambient or elevated ( + 175 and + 350 μ mol mol ^{? 1}) [CO₂] in combination with either charcoal‐filtered air (CF) or non‐filtered air plus 1·5 times ambient [O₃]. At about 50 d after planting, LMPA, starch and PA concentrations in canopy leaves were as much as 51–72% higher in plants treated with elevated [CO₂] compared with plants treated with ambient [CO₂], whereas leaf N concentration was 29% lower in elevated [CO₂]‐treated plants compared with controls. None of the treatments had a major effect on LTGA concentrations on a TNC‐free mass basis. LMPA and starch levels were up to 48% lower in plants treated with elevated [O₃] and ambient [CO₂] compared with CF controls, although the elevated [O₃] effect was diminished when plants were treated concurrently with elevated [CO₂]. On a total mass basis, leaf N and PA concentrations were higher in samples treated with elevated [O₃] in ambient [CO₂], but the difference was much reduced by elevated [CO₂]. On a TNC‐free basis, however, elevated [O₃] had little effect on tissue N and PA concentrations. Fertilization treatments resulted in higher PA and lower N concentrations in tissues from the deficient N fertility treatments. The experiment showed that suppression by elevated [O₃] of LMPA and starch was largely prevented by elevated [CO₂], and that interpretation of [CO₂] and [O₃] effects should include comparisons on a TNC‐free basis. Overall, the experiment indicated that allocation to starch and PA may be related to how environmental factors affect source–sink relationships in plants, although the effects of elevated [O₃] on secondary metabolites differed in this respect.     

Seasonal patterns of photosynthetic response and acclimation to elevated carbon dioxide in field-grown strawberry

Strawberry (Fragaria × ananassa) plants were grown in field plots at the current ambient [CO₂], and at ambient + 300 and ambient + 600 μmol mol⁻¹ [CO₂]. Approximately weekly measurements were made of single leaf gas exchange of upper canopy leaves from early spring through fall of two years, in order to determine the temperature dependence of the stimulation of photosynthesis by elevated [CO₂], whether growth at elevated [CO₂] resulted in acclimation of photosynthesis, and whether any photosynthetic acclimation was reduced when fruiting created additional demand for the products of photosynthesis. Stimulation of photosynthetic CO₂ assimilation by short-term increases in [CO₂] increased strongly with measurement temperature. The stimulation exceeded that predicted from the kinetic characteristics of ribulose-1,5-bisphosphate carboxylase at all temperatures. Acclimation of photosynthesis to growth at elevated [CO₂] was evident from early spring through summer, including the fruiting period in early summer, with lower rates under standard measurement conditions in plants grown at elevated [CO₂]. The degree of acclimation increased with growth [CO₂]. However, there were no significant differences between [CO₂] treatments in total nitrogen per leaf area, and photosynthetic acclimation was reversed one day after switching the [CO₂] treatments. Tests showed that acclimation did not result from a limitation of photosynthesis by triose phosphate utilization rate at elevated [CO₂]. Photosynthetic acclimation was not evident during dry periods in midsummer, when the elevated [CO₂] treatments conserved soil water and photosynthesis declined more at ambient than at elevated [CO₂]. Acclimation was also not evident during the fall, when plants were vegetative, despite wet conditions and continued higher leaf starch content at elevated [CO₂]. Stomatal conductance responded little to short-term changes in [CO₂] except during drought, and changed in parallel with photosynthetic acclimation through the seasons in response to the long-term [CO₂] treatments. The data do not support the hypothesis that source-sink balance controls the seasonal occurrence of photosynthetic acclimation to elevated [CO₂] in this species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Increased growth in elevated [CO2]: an early,short-term response?

Saplings of four clones of Sitka spruce and cherry were grown for three and two growing seasons, respectively, in open top chambers at two CO₂ concentrations (≈ 350 and ≈ 700 μmol mol^–1) to determine whether the increase in total biomass brought about by enhanced [CO₂] is a result of a transient or persistent effect in nonlimiting conditions. Classical growth analysis was applied to both species and mean current relative growth rate of total dry mass (R_T) and leaf dry mass (R_L), and period relative growth rate of total dry mass ( ) and leaf dry mass ( ) were calculated. Sitka spruce saplings and cherry seedlings showed a positive growth response to elevated [CO₂], and at the end of the experiments both species were ≈ 40% larger in elevated [CO₂] than in ambient [CO₂]. As a result, the period mean and were significantly higher in elevated [CO₂]. The differences in plant dry mass at the end of the experiments were a consequence of the more rapid growth in the early phase of exposure to elevated [CO₂]. After this initial phase mean R_T and R_L were similar or even lower in elevated [CO₂] than in ambient [CO₂]. NAR of both species was much higher in elevated [CO₂], whereas both LAR, SLA, and LMR showed the opposite trend. The higher LAR and SLA of plants in ambient [CO₂] contributed to a compensation by which they maintained R_T similar to that of elevated [CO₂] saplings despite lower NAR and photosynthetic rate. However, when the same size the trees were similar amongst the [CO₂] treatments, indicating that one of the main effect of elevated [CO₂] on tree growth is to speed-up early development in all aspects.     

Leaf photosynthesis and carbohydrates of CO₂-enriched maize and grain sorghum exposed to a short period of soil water deficit during vegetative development

Among C₄ species, sorghum is known to be more drought tolerant than maize. The objective was to evaluate differences in leaf gas exchanges, carbohydrates, and two enzyme activities of these nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME) C₄ subtype monocots in response to water deficit and CO₂ concentration ([CO₂]). Maize and sorghum were grown in pots in sunlit environmental-controlled chambers. Treatments included well watered (WW) and water stressed (WS) (water withheld at 26 days) and daytime [CO₂] of 360 (ambient) and 720 (elevated) μmol mol⁻¹. Midday gas exchange rates, concentrations of nonstructural carbohydrates, and activities of sucrose-phosphate synthase (SPS) and adenosine 5′-diphosphoglucose pyrophosphorylase (ADGP) were determined for fully expanded leaf sections. There was no difference in leaf CO₂ exchange rates (CER) between ambient and elevated [CO₂] control plants for both maize and sorghum. After withholding water, leaf CER declined to zero after 8 days in maize and 10 days for sorghum. Sorghum had lower stomatal conductance and transpiration rates than maize, which resulted in a longer period of CER under drought. Nonstructural carbohydrates of both control maize and sorghum were hardly affected by elevated [CO₂]. Under drought, however, increases in soluble sugars and decreases in starch were generally observed for maize and sorghum at both [CO₂] levels. For stressed maize and sorghum, decreases in starch occurred earlier and were greater at ambient [CO₂] than at elevated [CO₂]. For maize, drought did not meaningfully affect SPS activity. However, a decline in SPS activity was observed for drought-stressed sorghum under both [CO₂] treatments. There was an increase in ADGP activity in maize under drought for both [CO₂] treatments. Such a response in ADGP to drought, however, did not occur for sorghum. The generally more rapid response of maize than sorghum to drought might be related to the more rapid growth of leaf area of maize.     

Elevated CO2 and limited nitrogen nutrition can restrict excitation energy dissipation in photosystem II of Japanese white birch (Betula platyphylla var. japonica) leaves

Elevated atmospheric CO₂ concentration [CO₂] and different levels of nitrogen (N) nutrition can influence the amount of excess excitation energy in photosystem (PS) II and related photosynthetic properties. The interactive effect of two [CO₂] levels (ambient: 360 µM M⁻¹ and elevated: 720 µM M⁻¹) and two N levels (high: 700 mg N plant⁻¹ and low: 100 mg N plant⁻¹) on these properties was examined in seedlings of Japanese white birch (Betula platyphylla var. japonica) using simultaneous measurements of gas exchange and chlorophyll fluorescence. Photosynthetic acclimation to elevated [CO₂], as indicated by a decline in carboxylation efficiency (CE), was observed in plants grown at elevated [CO₂] especially under low N. Elevated [CO₂] resulted in a decrease in area-based leaf N content (N_area) irrespective of N treatment. The adverse effect of elevated [CO₂] and low N on CE may have been exacerbated by a greater accumulation of leaf sugar and starch contents in these plants leading to a lower electron transport rate (ETR). While these plants also showed higher non-photochemical quenching (Nq_P) that could offset the reduction in energy dissipation through ETR to some extent, they still have a higher risk of photoinhibition from excessive excitation energy in PSII as indicated by a decrease in photochemical quenching (q_P). However, chronic photoinhibition was not observed in plant grown at elevated [CO₂] and low N because they showed no difference in F_v/F_m (the maximum photochemical efficiency of PSII) from those grown at ambient [CO₂] and low N after an overnight dark adaptation. High levels of Nq_P in plants grown at elevated [CO₂] and low N reflect a near saturation of thermal energy dissipation. This impaired capacity of photoprotection would render these plants more vulnerable to photoinhibition in the event of additional environmental stresses such as drought, low or high temperature.     

Effect of panicle removal on photosynthetic acclimation under elevated CO<Subscript>2</Subscript> in rice

To examine the role of sink size on photosynthetic acclimation under elevated atmospheric CO₂ concentrations ([CO₂]), we tested the effects of panicle-removal (PR) treatment on photosynthesis in rice (Oryza sativa L.). Rice was grown at two [CO₂] levels (ambient and ambient + 200 μmol mol⁻¹) throughout the growing season, and at full-heading stage, at half the plants, a sink-limitation treatment was imposed by the removal of the panicles. The PR treatment alleviated the reduction of green leaf area, the contents of chlorophyll (Chl) and Rubisco after the full-heading stage, suggesting delay of senescence. Nonetheless, elevated [CO₂] decreased photosynthesis (measured at current [CO₂]) of plants exposed to the PR treatment. No significant [CO₂] × PR interaction on photosynthesis was observed. The decrease of photosynthesis by elevated [CO₂] of plants was associated with decreased leaf Rubisco content and N content. Leaf glucose content was increased by the PR treatment and also by elevated [CO₂]. In conclusion, a sink-limitation in rice improved N status in the leaves, but this did not prevent the photosynthetic down-regulation under elevated [CO₂].     

Root-zone CO2 and root-zone temperature effects on photosynthesis and nitrogen metabolism of aeroponically grown lettuce (Lactuca sativa L.) in the tropics

Effects of elevated root-zone (RZ) CO₂ concentration (RZ [CO₂]) and RZ temperature (RZT) on photosynthesis, productivity, nitrate (NO₃ ^?), total reduced nitrogen (TRN), total leaf soluble and Rubisco proteins were studied in aeroponically grown lettuce plants in a tropical greenhouse. Three weeks after transplanting, four different RZ [CO₂] concentrations (ambient, 360 ppm, and elevated concentrations of 2,000; 10,000; and 50,000 ppm) were imposed on plants at 20°C-RZT or ambient(A)-RZT (24–38°C). Elevated RZ [CO₂] resulted in significantly higher light-saturated net photosynthetic rate, but lower light-saturated stomatal conductance. Higher elevated RZ [CO₂] also protected plants from both chronic and dynamic photoinhibition (measured by chlorophyll fluorescence Fv/Fm ratio) and reduced leaf water loss. Under each RZ [CO₂], all these variables were significantly higher in 20°C-RZT plants than in A-RZT plants. All plants accumulated more biomass at elevated RZ [CO₂] than at ambient RZ [CO₂]. Greater increases of biomass in roots than in shoots were manifested by lower shoot/root ratios at elevated RZ [CO₂]. Although the total biomass was higher at 20°C-RZT, the increase in biomass under elevated RZ [CO₂] was greater at A-RZT. Shoot NO₃ ^? and TRN concentrations, total leaf soluble and Rubisco protein concentrations were higher in all elevated RZ [CO₂] plants than in plants under ambient RZ [CO₂] at both RZTs. Under each RZ [CO₂], total leaf soluble and Rubisco protein concentrations were significantly higher at 20°C-RZT than at A-RZT. Our results demonstrated that increased P _Nmax and productivity under elevated [CO₂] was partially due to the alleviation of midday water loss, both dynamic and chronic photoinhibition as well as higher turnover of Calvin cycle with higher Rubisco proteins.     

Acclimation of photosynthesis to elevated CO2 and temperature in five British native species of contrasting functional type

Acclimation of photosynthesis to growth at elevated CO₂ concentration varies markedly between species. Species functionally classified as stress-tolerators (S) and ruderals (R), are thought to be incapable, or the least capable, of responding positively in terms of growth to elevated [CO₂]. Is this pattern of response also apparent in leaf photosynthesis of wild S- and R-strategists? Acclimatory loss of a photosynthetic and growth response to elevated [CO₂] is assumed to reflect limitation on capacity to utilize additional photosynthate. The doubling of pre-industrial global [CO₂] is expected to coincide with a 3 °C increase in mean temperature which could stimulate growth; will photosynthetic capacity at elevated [CO₂] be greater when the concurrent temperature increase is simulated? Five species from natural grassland of NW Europe and of contrasting ecological strategy were grown in hemispherical greenhouses, environmentally controlled to track the external microclimate. Within a replicated design, plants were grown at (i) current ambient [CO₂] and temperature, (ii) elevated [CO₂] (ambient + 340 μmol mol^–1) and ambient temperature, (iii) ambient [CO₂] and elevated temperature (ambient + 3 °C), or (iv) elevated [CO₂] and elevated temperature. After 75–104 days, the CO₂ response of light-saturated rates of photosynthesis (A_sat) was analysed in controlled-environment cuvettes in a field laboratory. There was no acclimatory loss of photosynthetic capacity with growth in elevated [CO₂] or elevated temperature over this period in Poa alpina (S), Bellis perennis (R) or Plantago lanceolata (mixed C-S-R strategist), and a significant (P ? ? bl 0.05) increase in capacity in Helianthemum nummularium (S) and Poa annua (R). Photosynthetic rates of leaves grown and measured in elevated [CO₂] were therefore significantly higher than rates for leaves grown and measured in ambient [CO₂], for all species. With the exception of Poa alpina, stomatal conductance and stomatal limitation on A_sat showed no acclimatory response to growth in elevated [CO₂]. Carboxylation efficiency, determined from the initial slope of the response of A_sat to intercellular CO₂ concentration was significantly increased by elevated [CO₂] and elevated temperature in H.nummularium, implying a possible increase in in vivo RubisCO activity. Increased carboxylation efficiency of this species was also reflected by an increase in the CO₂- and light-saturated rates of photosynthesis, indicating an increased capacity for regeneration of the primary CO₂ acceptor in photosynthesis. The results show that R-strategists and slow-growing S-strategists, are inherently capable of large increases in leaf photosynthetic capacity with growth in elevated [CO₂] in contrast to expectations from growth studies. With the exception of P.annua, where there was a significant negative interaction between CO₂ and temperature, concurrent increase in growth temperature had little effect on this pattern of response.     

Inoculation with an enhanced N2‐fixing Bradyrhizobium japonicum strain (USDA110) does not alter soybean (Glycine max Merr.) response to elevated [CO2]

This study tested the hypothesis that inoculation of soybean (Glycine max Merr.) with a Bradyrhizobium japonicum strain (USDA110) with greater N₂ fixation rates would enhance soybean response to elevated [CO₂]. In field experiments at the Soybean Free Air CO₂ Enrichment facility, inoculation of soybean with USDA110 increased nodule occupancy from 5% in native soil to 54% in elevated [CO₂] and 34% at ambient [CO₂]. Despite this success, inoculation with USDA110 did not result in greater photosynthesis, growth or seed yield at ambient or elevated [CO₂] in the field, presumably due to competition from native rhizobia. In a growth chamber experiment designed to study the effects of inoculation in the absence of competition, inoculation with USDA110 in sterilized soil resulted in nodule occupation of >90%, significantly greater ¹⁵N₂ fixation, photosynthetic capacity, leaf N and total plant biomass compared with plants grown with native soil bacteria. However, there was no interaction of rhizobium fertilization with elevated [CO₂]; inoculation with USDA110 was equally beneficial at ambient and elevated [CO₂]. These results suggest that selected rhizobia could potentially stimulate soybean yield in soils with little or no history of prior soybean production, but that better quality rhizobia do not enhance soybean responses to elevated [CO₂].     

Nitrogen assimilation and transpiration: key processes conditioning responsiveness of wheat to elevated [CO2] and temperature

Although climate scenarios have predicted an increase in [CO₂] and temperature conditions, to date few experiments have focused on the interaction of [CO₂] and temperature effects in wheat development. Recent evidence suggests that photosynthetic acclimation is linked to the photorespiration and N assimilation inhibition of plants exposed to elevated CO₂. The main goal of this study was to analyze the effect of interacting [CO₂] and temperature on leaf photorespiration, C/N metabolism and N transport in wheat plants exposed to elevated [CO₂] and temperature conditions. For this purpose, wheat plants were exposed to elevated [CO₂] (400 vs 700 µmol mol^?1) and temperature (ambient vs ambient + 4°C) in CO₂ gradient greenhouses during the entire life cycle. Although at the agronomic level, elevated temperature had no effect on plant biomass, physiological analyses revealed that combined elevated [CO₂] and temperature negatively affected photosynthetic performance. The limited energy levels resulting from the reduced respiratory and photorespiration rates of such plants were apparently inadequate to sustain nitrate reductase activity. Inhibited N assimilation was associated with a strong reduction in amino acid content, conditioned leaf soluble protein content and constrained leaf N status. Therefore, the plant response to elevated [CO₂] and elevated temperature resulted in photosynthetic acclimation. The reduction in transpiration rates induced limitations in nutrient transport in leaves of plants exposed to elevated [CO₂] and temperature, led to mineral depletion and therefore contributed to the inhibition of photosynthetic activity.     

Soybean nodulation and N2 fixation response to drought under carbon dioxide enrichment

The combined effects of carbon dioxide (CO₂) enrichment and water deficits on nodulation and N₂ fixation were analysed in soybean [Glycine max (L.) Merr.]. Two short-term experiments were conducted in greenhouses with plants subjected to soil drying, while exposed to CO₂ atmospheres of either 360 or 700 μmol CO₂ mol^–1. Under drought-stressed conditions, elevated [CO₂] resulted in a delay in the decrease in N₂ fixation rates associated with drying of the soil used in these experiments. The elevated [CO₂] also allowed the plants under drought to sustain significant increases in nodule number and mass relative to those under ambient [CO₂]. The total non-structural carbohydrate (TNC) concentration was lower in the shoots of the plants exposed to drought; however, plants under elevated CO₂ had much higher TNC levels than those under ambient CO₂. For both [CO₂] treatments, drought stress induced a substantial accumulation of TNC in the nodules that paralleled N₂ fixation decline, which indicates that nodule activity under drought may not be carbon limited. Under drought stress, ureide concentration increased in all plant tissues. However, exposure to elevated [CO₂] resulted in substantially less drought-induced ureide accumulation in leaf and petiole tissues. A strong negative correlation was found between ureide accumulation and TNC levels in the leaves. This relationship, together with the large effect of elevated [CO₂] on the decrease of ureide accumulation in the leaves, indicated the importance of ureide breakdown in the response of N₂ fixation to drought and of feedback inhibition by ureides on nodule activity. It is concluded that an important effect of CO₂ enrichment on soybean under drought conditions is an enhancement of photoassimilation, an increased partitioning of carbon to nodules and a decrease of leaf ureide levels, which is associated with sustained nodule growth and N₂ rates under soil water deficits. We suggest that future [CO₂] increases are likely to benefit soybean production by increasing the drought tolerance of N₂ fixation.     

Effects of fully open-air [CO2] elevation on leaf ultrastructure, photosynthesis, and yield of two soybean cultivars

The objective of this study was to investigate the effect of elevated (550 ± 17 ??mol mol^?1) CO₂ concentration ([CO₂]) on leaf ultrastructure, leaf photosynthesis and seed yield of two soybean cultivars [Glycine max (L.) Merr. cv. Zhonghuang 13 and cv. Zhonghuang 35] at the Free-Air Carbon dioxide Enrichment (FACE) experimental facility in North China. Photosynthetic acclimation occurred in soybean plants exposed to long-term elevated [CO₂] and varied with cultivars and developmental stages. Photosynthetic acclimation occurred at the beginning bloom (R1) stage for both cultivars, but at the beginning seed (R5) stage only for Zhonghuang 13. No photosynthetic acclimation occurred at the beginning pod (R3) stage for either cultivar. Elevated [CO₂] increased the number and size of starch grains in chloroplasts of the two cultivars. Soybean leaf senescence was accelerated under elevated [CO₂], determined by unclear chloroplast membrane and blurred grana layer at the beginning bloom (R1) stage. The different photosynthesis response to elevated [CO₂] between cultivars at the beginning seed (R5) contributed to the yield difference under elevated [CO₂]. Elevated [CO₂] significantly increased the yield of Zhonghuang 35 by 26% with the increased pod number of 31%, but not for Zhonghuang 13 without changes of pod number. We conclude that the occurrence of photosynthetic acclimation at the beginning seed (R5) stage for Zhonghuang 13 restricted the development of extra C sink under elevated [CO₂], thereby limiting the response to elevated [CO₂] for the seed yield of this cultivar.     

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.     

Acquisition and within-plant allocation of 13C and 15N in CO2-enriched Quercus robur plants

We assessed the effects of doubling atmospheric CO₂ concentration, [CO₂], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual ¹³CO₂ and ¹⁵NO₃^? labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l^?1 [CO₂] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l^?1 [CO₂] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO₂ assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO₂ respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO₂]. ¹³C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO₂] both after the 12-h ¹³CO₂ pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO₂ (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO₂] on fine root-specific ¹⁵N uptake (amount of recently assimilated ¹⁵N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO₂] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO₂]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h ¹³C chase phase, leaves of the first and second growth flushes were almost completely depleted in recent ¹³C under ambient [CO₂], whereas these leaves retained important amounts of recently assimilated ¹³C (carbohydrate reserves?) under elevated [CO₂].