The influence of elevated CO2 on non-structural carbohydrate distribution and fructan accumulation in wheat canopies

We grew 2.4 m² wheat canopies in a large growth chamber under high photosynthetic photon flux (1000 μmol m⁻² s⁻¹) and using two CO₂ concentrations, 360 and 1200 μmol mol⁻¹. Photosynthetically active radiation (400–700 nm) was attenuated slightly faster through canopies grown in 360μmol mol⁻¹ than through canopies grown in 1200μmol mol⁻¹, even though high-CO₂ canopies attained larger leaf area indices. Tissue fractions were sampled from each 5-cm layer of the canopies. Leaf tissue sampled from the tops of canopies grown in 1200μmol mol⁻¹ accumulated significantly more total non-structural carbohydrate, starch, fructan, sucrose, and glucose (p≤ 0.05) than for canopies grown in 360μmol mol⁻¹. Non-structural carbohydrate did not significantly increase in the lower canopy layers of the elevated CO₂ treatment. Elevated CO₂ induced fructan synthesis in all leaf tissue fractions, but fructan formation was greatest in the uppermost leaf area. A moderate temperature reduction of 10 °C over 5d increased starch, fructan and glucose levels in canopies grown in 1200μmol mol⁻¹, but concentrations of sucrose and fructose decreased slightly or remained unchanged. Those results may correspond with the use of fructosyl-residues and release of glucose when sucrose is consumed in fructan synthesis.     

Consequences of CO2 and light interactions for leaf phenology, growth, and senescence in Quercus rubra

We investigated how light and CO₂ levels interact to influence growth, phenology, and the physiological processes involved in leaf senescence in red oak (Quercus rubra) seedlings. We grew plants in high and low light and in elevated and ambient CO₂. At the end of three years of growth, shade plants showed greater biomass enhancement under elevated CO₂ than sun plants. We attribute this difference to an increase in leaf area ratio (LAR) in shade plants relative to sun plants, as well as to an ontogenetic effect: as plants increased in size, the LAR declined concomitant with a decline in biomass enhancement under elevated CO₂ Elevated CO₂ prolonged the carbon gain capacity of shade‐grown plants during autumnal senescence, thus increasing their functional leaf lifespan. The prolongation of carbon assimilation, however, did not account for the increased growth enhancement in shade plants under elevated CO₂. Elevated CO₂ did not significantly alter leaf phenology. Nitrogen concentrations in both green and senesced leaves were lower under elevated CO₂ and declined more rapidly in sun leaves than in shade leaves. Similar to nitrogen concentration, the initial slope of A/C_i curves indicated that Rubisco activity declined more rapidly in sun plants than in shade plants, particularly under elevated CO₂. Absolute levels of chlorophyll were affected by the interaction of CO₂ and light, and chlorophyll content declined to a minimal level in sun plants sooner than in shade plants. These declines in N concentration, in the initial slope of A/C_i curves, and in chlorophyll content were consistent with declining photosynthesis, such that elevated CO₂ accelerated senescence in sun plants and prolonged leaf function in shade plants. These results have implications for the carbon economy of seedlings and the regeneration of red oak under global change conditions.     

Growth, light interception and yield responses of spring wheat (Triticum aestivum L.) grown under elevated CO2 and O3 in open-top chambers

Spring wheat cv. Minaret was grown to maturity under three carbon dioxide (CO₂) and two ozone (O₃) concentrations in open-top chambers (OTC). Green leaf area index (LAI) was increased by elevated CO₂ under ambient O₃ conditions as a direct result of increases in tillering, rather than individual leaf areas. Yellow LAI was also greater in the 550 and 680 μmol mol^–1 CO₂ treatments than in the chambered ambient control; individual leaves on the main shoot senesced more rapidly under 550 μmol mol^–1 CO₂, but senescence was delayed at 680 μmol mol^–1 CO₂. Fractional light interception (f) during the vegetative period was up to 26% greater under 680 μmol mol^–1 CO₂ than in the control treatment, but seasonal accumulated intercepted radiation was only increased by 8%. As a result of greater carbon assimilation during canopy development, plants grown under elevated CO₂ were taller at anthesis and stem and ear biomass were 27 and 16% greater than in control plants. At maturity, yield was 30% greater in the 680 μmol mol^–1 CO₂ treatment, due to a combination of increases in the number of ears per m^–2, grain number per ear and individual grain weight (IGW). Exposure to a seasonal mean (7 h d^–1) of 84 nmol mol^–1 O₃ under ambient CO₂ decreased green LAI and increased yellow LAI, thereby reducing both f and accumulated intercepted radiation by ≈ 16%. Individual leaves senesced completely 7–28 days earlier than in control plants. At anthesis, the plants were shorter than controls and exhibited reductions in stem and ear biomass of 15 and 23%. Grain yield at maturity was decreased by 30% due to a combination of reductions in ear number m^–2, the numbers of grains per spikelet and per ear and IGW. The presence of elevated CO₂ reduced the rate of O₃-induced leaf senescence and resulted in the maintenance of a higher green LAI during vegetative growth under ambient CO₂ conditions. Grain yields at maturity were nevertheless lower than those obtained in the corresponding elevated CO₂ treatments in the absence of elevated O₃. Thus, although the presence of elevated CO₂ reduced the damaging impact of ozone on radiation interception and vegetative growth, substantial yield losses were nevertheless induced. These data suggest that spring wheat may be susceptible to O₃-induced injury during anthesis irrespective of the atmospheric CO₂ concentration. Possible deleterious mechanisms operating through effects on pollen viability, seed set and the duration of grain filling are discussed.     

Effects of free-air CO2 enrichment on the development of the photosynthetic apparatus in wheat, as indicated by changes in leaf proteins

A spring wheat crop was grown at ambient and elevated (550 μmol mol^?1) CO₂ concentrations under free-air CO₂ enrichment (FACE) in the field. Four experimental blocks, each comprising 21-m-diameter FACE and control experimental areas, were used. CO₂ elevation was maintained day and night from crop emergence to final grain harvest. This experiment provided a unique opportunity to examine the hypothesis that CO₂ elevation in the field would lead to acclimatory changes within the photosynthetic apparatus under open field conditions and lo assess whether acclimation was affected by crop developmental stage, leaf ontogeny and leaf age. Change in the photosynthetic apparatus was assessed by measuring changes in the composition of total leaf and thylakoid polypeptides separated by SDS-PAGE. For leaves at completion of emergence of the blade, growth at the elevated CO₂ concentration had no apparent effect on the amount of any of the major proteins of the photosynthetic apparatus regardless of the leaf examined. Leaf 5 on the main stem was in full sunlight at emergence, but then became shaded progressively as 3–4 further leaves formed above with continued development of the crop. By 35 d following completion of blade emergence, leaf 5 was in shade. At this point, the chlorophyll alb ratio had declined by 26% both in plants grown at the control CO₂ concentration and in those grown at the elevated CO₂ concentration, which is indicative of shade acclimation. The ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content declined by 45% in the control leaves, but by 60% in the leaves grown at the elevated CO₂ concentration. The light- harvesting complex of photosystcm II (LHCII) and the chlorophyll content showed no decrease and no difference between treatments, indicating that the decrease in Rubisco was not an effect of earlier senescence in the leaves at the elevated CO₂ concentration. Following completion of the emergence of the flag-leaf blade, the elevated-CO₂ treatment inhibited the further accumulation of Rubisco which was apparent in control leaves over the subsequent 14 d. From this point onwards, the flag leaves from both treatments showed a loss of Rubisco, which was far more pronounced in the elevated-CO₂ treatment, so that by 36 d the Rubisco content of these leaves was just 70% of that of the controls and by 52 d it was only 20%. At 36 d, there was no decline in chlorophyll, LHCII or the chloroplast ATPase coupling factor (CFI) in the elevated CO₂ concentration treatment relative to the control. By 52 d, all of these proteins showed a significant decline relative to the control. This indicates that the decreased concentration of Rubisco at this final stage probably reflected earlier senescence in the elevated-CO₂ treatment, but that this was preceded by a CO₂-concentration-dependent decline in Rubisco.     

Effect of increased CO2 concentration and temperature on the phyllosphere mycoflora of winter wheat flag leaves during ripening

The impact of elevated carbon dioxide (CO₂, 600/700 μmol mol^-1) and temperature (+ 4°C) on phyllosphere fungi colonising flag leaves of mini crops of winter wheat cv. Mercia between anthesis and harvest was determined in a computer-controlled environment facility in 1993 and 1994. In both years the total fungal populations (cm² leaf) were found to have increased due to exposure to either elevated CO₂ and elevated CO₂+ temperature treatments. This was mainly due to significant increases in populations of Cladosporium spp. (C. cladosporioides and C. herbarum) on the flag leaves during ripening. Other phyllosphere component species such as white and pink yeasts were not markedly affected by treatments. The range of fungal species found in such controlled environment chambers was narrower than that commonly found on flag leaves of field grown crops. Common and important colonisers of leaves and ripening ears such as Aureobasidium pullulans, Epicoccum nigrum and Fusarium spp. were seldom isolated.     

Effects of elevated CO2 concentration and increased temperature on winter wheat: test of ARCWHEAT1 simulation model

Winter wheat (Triticum aestivum L., ev. Mercia) was grown in a controlled-environment facility at two CO₂ concentrations (targets 350 and 700 μmol mol^?1), and two temperature regimes (tracking ambient and ambient + 4°C). Observations of phenology, canopy growth, dry matter production and grain yield were used to test the ARCWHEAT1 simulation model. Dry-matter production and grain yield were increased at elevated CO₂ concentration (27 and 39%, respectively) and reduced at increased temperature (?16 and ?35%, respectively). ARCWHEAT1 substantially underestimated canopy growth for all treatments. However, differences in the facility environment from field conditions over the winter, indicated by the unusually rapid canopy growth observed in this period, meant that empirical model relationships were being used outside the conditions for which they were developed. The ARCWHEAT productivity submodel, given observed green area indices as inputs, overestimated the effect of CO₂ on productivity. An alternative, more mechanistic submodel of productivity, based on the SUCROS87 and Farquhar & von Caemmerer models, simulated observed crop biomass very closely. When these productivity simulations were inputed into the ARCWHEAT1 partitioning and grain-fill submodels, grain yield was predicted poorly, mainly as a result of the assumption that the number of grains is proportional to total growth during a short pre-anthesis phase. While yield was not correlated with growth in this phase, it was correlated with growth in longer pre-anthesis phases, indicating that ARCWHEAT1 could be improved by taking into account the contribution of earlier growth in determining yield.     

Future atmospheric CO2 leads to delayed autumnal senescence

Growing seasons are getting longer, a phenomenon partially explained by increasing global temperatures. Recent reports suggest that a strong correlation exists between warming and advances in spring phenology but that a weaker correlation is evident between warming and autumnal events implying that other factors may be influencing the timing of autumnal phenology. Using freely rooted, field‐grown Populus in two Free Air CO₂ Enrichment Experiments (AspenFACE and PopFACE), we present evidence from two continents and over 2 years that increasing atmospheric CO₂ acts directly to delay autumnal leaf coloration and leaf fall. In an atmosphere enriched in CO₂ (by ～45% of the current atmospheric concentration to 550 ppm) the end of season decline in canopy normalized difference vegetation index (NDVI) – a commonly used global index for vegetation greenness – was significantly delayed, indicating a greener autumnal canopy, relative to that in ambient CO₂. This was supported by a significant delay in the decline of autumnal canopy leaf area index in elevated as compared with ambient CO₂, and a significantly smaller decline in end of season leaf chlorophyll content. Leaf level photosynthetic activity and carbon uptake in elevated CO₂ during the senescence period was also enhanced compared with ambient CO₂. The findings reveal a direct effect of rising atmospheric CO₂, independent of temperature in delaying autumnal senescence for Populus, an important deciduous forest tree with implications for forest productivity and adaptation to a future high CO₂ world.     

Seasonal changes in the response of winter wheat to elevated atmospheric CO2 concentration grown in Open-Top Chambers and field tracking enclosures

Winter wheat was grown at ambient and elevated (ambient plus 350 μL L^–1) CO₂ concentrations in open top chambers and in field-tracking sun-lit climatized enclosures (elevated is 718 μL L^–1). There was no significant effect of CO₂ concentration on sheath, leaf and root biomass and leaf area in the early spring (January to April). 24-h canopy CO₂ exchange rate (CCER) was not significantly affected either. However, elevated CO₂ concentration increased CCER at midday, decreased evapotranspiration rate and increased instantaneous water-use-efficiency during early spring. Leaf, sheath and root nitrogen concentration per unit dry weight decreased and nonstructural carbohydrate concentration increased under elevated CO₂, and N-uptake per unit ground area decreased significantly (– 22%) towards the end of this period. These results contrast with results from the final harvest, when grain yield and biomass were increased by 19% under elevated CO₂. N concentration per dry weight was reduced by 5%, but N-uptake per unit ground area was significantly higher (+ 11%) for the elevated CO₂ treatment. 24-h and midday-CCER increased significantly more in late spring (period of 21 April to 30 May) (respectively by + 40% and 53%) than in the early spring (respectively 5% and 19%) in response to elevated CO₂. Midday evapotranspiration rate was reduced less by elevated CO₂ in the late spring (– 13%) than in early spring (– 21%). The CO₂ response of midday and 24-h CCER decreased again (+ 27% and + 23% resp.) towards the end of the growing season. We conclude that the low response to CO₂ concentration during the early spring was associated with a growth-restriction, caused by low temperature and irradiance levels. The reduction of nitrogen concentration, the increase of nonstructural carbohydrate, and the lower evapotranspiration indicated that CO₂ did have an effect towards the end of early spring, but not on biomass accumulation. Regression analysis showed that both irradiance and temperature affected the response to CO₂.     

Seasonal CO2 exchange patterns of developing peach (Prunus persica) fruits in response to temperature, light and CO2 concentration

CO₂ exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m^?2S^?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO₂ evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO₂ (g dry weight)^?1 S^?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m^?2 S^?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O₁₀ value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO₂ exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO₂ concentrations via CO₂ refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m^?2 S^?1. Young peach fruits responded to increasing ambient CO₂ concentrations with decreasing net CO₂ exchange rates in light, but more mature fruits did not respond to increases in ambient CO₂. Fruit CO₂ exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO₂ concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO₂ response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO₂1^?2. In CO₂ free air, fruit photosynthesis was dependent on CO₂ refixation since CO₂ uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO₂-free air and 370 μl CO₂ 1^?1 indicated that young peach fruits were apparently able to take up CO₂ from the external atmosphere. CO₂ uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO₂ refixation.     

Effects of increased CO2 concentration and temperature on growth and yield of winter wheat at two levels of nitrogen application

Winter wheat (Triticum aestivum L., cv. Mercia) was grown in chambers under light and temperature conditions similar to the UK field environment for the 1990/1991 growing season at two levels each of atmospheric CO₂ concentration (seasonal means: 361 and 692 μmol mol^?1), temperature (tracking ambient and ambient +4°C) and nitrogen application (equivalent to 87 and 489 kg ha^?1 total N applied). Total dry matter productivity through the season, the maximum number of shoots and final ear number were stimulated by CO₂ enrichment at both levels of the temperature and N treatments. At high N, there was a CO₂-induced stimulation of grain yield (+15%) similar to that for total crop dry mass (+12%), and there was no significant interaction with temperature. This contrasts with other studies, where positive interactions between the effects of increases in temperature and CO₂ have been found. Temperature had a direct, negative effect on yield at both levels of the N and CO₂ treatments. This could be explained by the temperature-dependent shortening of the phenological stages, and therefore, the time available for accumulating resources for grain formation. At high N, there was also a reduction in grain set at ambient +4°C temperature, but the overall negative effect of warmer temperature was greater on the number of grains (-37%) than on yield (-18%), due to a compensating increase in average grain mass. At low N, despite increasing total crop dry mass and the number of ears, elevated CO₂ did not increase grain yield and caused a significant decrease under ambient temperature conditions. This can be explained in terms of a stimulation of early vegetative growth by CO₂ enrichment leading to a reduction in the amount of N available later for the formation and filling of grain.     

Effects of elevated CO2 and O3 on the rate and duration of grain growth and harvest index in spring wheat (Triticum aestivum L.)

Wheat (Triticum aestivum L.) cv. Minaret was grown in open-top chambers (OTCs) in 1995 and 1996 under three carbon dioxide (CO₂) and two ozone (O₃) levels. Plants were harvested regularly between anthesis and maturity to examine the rate of grain growth (dG/dt; mg d^–1) and the rate of increase in harvest index (dHI/dt;% d^–1). The duration of grain filling was not affected by elevated CO₂ or O₃, but was 12 days shorter in 1995, when the daily mean temperature was over 3 °C higher than in 1996. Season-long exposure to elevated CO₂ (680 μmol mol^–1) significantly increased the rate of grain growth in both years and mean grain weight at maturity (MGW) was up to 11% higher than in the chambered ambient air control (chAA; 383 μmol mol^–1). However, the increase in final yield obtained under elevated CO₂ relative to the chAA control in 1996 resulted primarily from a 27% increase in grain number per unit ground area. dG/dt was significantly reduced by elevated O₃ under ambient CO₂ conditions in 1995, but final grain yield was not affected because of a concurrent increase in grain number. Neither dG/dt nor dHI/dt were affected by the higher mean O₃ concentrations applied in 1996 (77 vs. 66 nmol mol^–1); the differing effects of O₃ on grain growth in 1995 and 1996 observed in both the ambient and elevated CO₂ treatments may reflect the contrasting temperature environments experienced. Grain yield was nevetheless reduced under elevated O₃ in 1996, primarily because of a substantial decrease in grain number. The data obtained show that, although exposure to elevated CO₂ and O₃ individually or in combination may affect both dG/dt and dHI/dt, the presence of elevated CO₂ does not protect against substantial O₃-induced yield losses resulting from its direct deleterious impact on reproductive processes. The implications of these results for food production under future climatic conditions are considered.     

Sap flow in wheat under free-air CO2 enrichment

The effects of elevated carbon dioxide (CO₂) concentration on plant water use are best evaluated on plants grown under field conditions and with measurement techniques that do not disturb the natural function of the plant. Heat balance sap flow gauges were used on individual main stems of wheat (Triticum aestivum L. cv Yecora rojo) grown under normal ambient conditions (control) and in a free-air CO₂ enrichment (FACE) system in Arizona with either high (control + high H₂O = CW; FACE + high H₂O = FW) or low (control + low H₂O = CD; FACE + low H₂O = FD) irrigation regimens. Over a 30d period (stem elongation to anthesis), combinations of treatments were monitored with,10–40 gauges per treatment. The effects of increased CO₂ on tiller water use were inconsistent in both the diurnal patterns of sap flow and the statistical analyses of daily sap flow (F_tot). Initial results suggested that the reductions in F_tot, from CO₂ enrichment were small (,0–10%) in relation to the H₂O treatment effect (,20–30%). For a 3d period, F_tot of FW was,19–26% less than that of CW (P = 0.10). Examination of the different sources of variation in the study revealed that the location of gauges within the experimental plots influenced the variance of the sap flow measurements. This variation was probably related to positional variation in subsurface drip lines used to irrigate plots. A sampling design was proposed for use of sap flow gauges in FACE systems with subsurface irrigation that takes into account the main treatment effects of CO₂ enrichment and the other sources of variation identified in this study. Despite the small and often statistically non-significant differences in F_tot between the CW and FW treatments, cumulative water use of the FW treatment at the end of the first three test periods ranged from 7 to 23% lower than that of the CW treatment. Differences in sap flow between FW and CW compared well with treatment differences in evapotranspiration. The results of the study, based on the first reported sap flow measurements of wheat, suggest that irrigation requirements for wheat production, in the present climatic regimen of the south-western US, may be predicted to decrease slightly because of increasing atmospheric CO₂.     

A burst of CO2 from photosynthesizing leaves after a temperature decrease under constant light conditions

A transient CO₂ burst from seedlings of some plant species was observed after a rapid temperature decrease. The magnitude of the CO₂ release depended on initial temperature, oxygen concentration and light intensity. To obtain a maximal value of CO₂ release, the temperature had to decrease by more than 8°C. The phenomenon was detected only in the light, and was confined to C₃ species. It was inhibited by low oxygen concentration, indicating its possible connection with photorespiration.