Internal recycling of respired CO2 may be important for plant functioning under changing climate regimes

Recent studies have provided evidence of a large flux of root-respired CO₂ in the transpiration stream of trees. In our study, we investigated the potential impact of this internal CO₂ transport on aboveground carbon assimilation and CO₂ efflux. To trace the transport of root-respired CO₂, we infused a ¹³C label at the stem base of field-grown Populus deltoides Bartr. ex. Marsh trees. The ¹³C label was transported to the top of the stem and throughout the crown via the transpiration stream. Up to 17% of the ¹³C label was assimilated by chlorophyll-containing tissues. Our results provide evidence of a mechanism for recycling respired CO₂ within trees. Such a mechanism may have important implications for how plants cope with predicted increases in intensity and frequency of droughts. Here, we speculate on the potential significance of this recycling mechanism within the context of plant responses to climate change and plants currently inhabiting arid environments.     

Effects of Atmospheric CO(2) Enrichment on Photosynthesis, Respiration, and Growth of Sour Orange Trees

Numerous net photosynthetic and dark respiratory measurements were made over a period of 4 years on leaves of 24 sour orange (Citrus aurantium) trees; 8 of them growing in ambient air at a mean CO₂ concentration of 400 microliters per liter, and 16 growing in air enriched with CO₂ to concentrations approaching 1000 microliters per liter. Over this CO₂ concentration range, net photosynthesis increased linearly with CO₂ by more than 200%, whereas dark respiration decreased linearly to only 20% of its initial value. These results, together with those of a comprehensive fine-root biomass determination and two independent aboveground trunk and branch volume inventories, suggest that a doubling of the air's current mean CO₂ concentration of 360 microliters per liter would enhance the growth of the trees by a factor of 3.8.     

Responses of Gmelina arborea, a tropical deciduous tree species, to elevated atmospheric CO2: Growth, biomass productivity and carbon sequestration efficacy

The photosynthetic response of trees to rising CO₂ concentrations largely depends on source-sink relations, in addition to differences in responsiveness by species, genotype, and functional group. Previous studies on elevated CO₂ responses in trees have either doubled the gas concentration (>700 μmol mol⁻¹) or used single large addition of CO₂ (500-600 μmol mol⁻¹). In this study, Gmelina arborea, a fast growing tropical deciduous tree species, was selected to determine the photosynthetic efficiency, growth response and overall source-sink relations under near elevated atmospheric CO₂ concentration (460 μmol mol⁻¹). Net photosynthetic rate of Gmelina was ∼30% higher in plants grown in elevated CO₂ compared with ambient CO₂-grown plants. The elevated CO₂ concentration also had significant effect on photochemical and biochemical capacities evidenced by changes in F_V/F_M, ABS/CSm, ET₀/CSm and RuBPcase activity. The study also revealed that elevated CO₂ conditions significantly increased absolute growth rate, above ground biomass and carbon sequestration potential in Gmelina which sequestered ∼2100 g tree⁻¹ carbon after 120 days of treatment when compared to ambient CO₂-grown plants. Our data indicate that young Gmelina could accumulate significant biomass and escape acclimatory down-regulation of photosynthesis due to high source-sink capacity even with an increase of 100 μmol mol⁻¹ CO₂.     

Water savings in mature deciduous forest trees under elevated CO2

Stomatal conductance of plants exposed to elevated CO₂ is often reduced. Whether this leads to water savings in tall forest‐trees under future CO₂ concentrations is largely unknown but could have significant implications for climate and hydrology. We used three different sets of measurements (sap flow, soil moisture and canopy temperature) to quantify potential water savings under elevated CO₂ in a ca. 35 m tall, ca. 100 years old mixed deciduous forest. Part of the forest canopy was exposed to 540 ppm CO₂ during daylight hours using free air CO₂ enrichment (FACE) and the Swiss Canopy Crane (SCC). Across species and a wide range of weather conditions, sap flow was reduced by 14% in trees subjected to elevated CO₂, yielding ca. 10% reduction in evapotranspiration. This signal is likely to diminish as atmospheric feedback through reduced moistening of the air comes into play at landscape scale. Vapour pressure deficit (VPD)‐sap flow response curves show that the CO₂ effect is greatest at low VPD, and that sap flow saturation tends to occur at lower VPD in CO₂‐treated trees. Matching stomatal response data, the CO₂ effect was largely produced by Carpinus and Fagus, with Quercus contributing little. In line with these findings, soil moisture at 10 cm depth decreased at a slower rate under high‐CO₂ trees than under control trees during rainless periods, with a reversal of this trend during prolonged drought when CO₂‐treated trees take advantage from initial water savings. High‐resolution thermal images taken at different heights above the forest canopy did detect reduced water loss through altered energy balance only at <5 m distance (0.44 K leaf warming of CO₂‐treated Fagus trees). Short discontinuations of CO₂ supply during morning hours had no measurable canopy temperature effects, most likely because the stomatal effects were small compared with the aerodynamic constraints in these dense, broad‐leaved canopies. Hence, on a seasonal basis, these data suggest a <10% reduction in water consumption in this type of forest when the atmosphere reaches 540% ppm CO₂.     

Effects of CO(2) Enrichment and Carbohydrate Content on the Dark Respiration of Soybeans

During the period of most active leaf expansion, the foliar dark respiration rate of soybeans (Glycine max cv Williams), grown for 2 weeks in 1000 microliters CO₂ per liter air, was 1.45 milligrams CO₂ evolved per hour leaf density thickness, and this was twice the rate displayed by leaves of control plants (350 microliters CO₂ per liter air). There was a higher foliar nonstructural carbohydrate level (e.g. sucrose and starch) in the CO₂ enriched compared with CO₂ normal plants. For example, leaves of enriched plants displayed levels of nonstructural carbohydrate equivalent to 174 milligrams glucose per gram dry weight compared to the 84 milligrams glucose per gram dry weight found in control plant leaves. As the leaves of CO₂ enriched plants approached full expansion, both the foliar respiration rate and carbohydrate content of the CO₂ enriched leaves decreased until they were equivalent with those same parameters in the leaves of control plants. A strong positive correlation between respiration rate and carbohydrate content was seen in high CO₂ adapted plants, but not in the control plants.

Mitochondria, isolated simultaneously from the leaves of CO₂ enriched and control plants, showed no difference in NADH or malate-glutamate dependent O₂ uptake, and there were no observed differences in the specific activities of NAD⁺ linked isocitrate dehydrogenase and cytochrome c oxidase. Since the mitochondrial O₂ uptake and total enzyme activities were not greater in young enriched leaves, the increase in leaf respiration rate was not caused by metabolic adaptations in the leaf mitochondria as a response to long term CO₂ enrichment. It was concluded, that the higher respiration rate in the enriched plant's foliage was attributable, in part, to a higher carbohydrate status.

     

Stomatal conductance in mature deciduous forest trees exposed to elevated CO<Subscript>2</Subscript>

Stomatal conductance (g _s) of mature trees exposed to elevated CO₂ concentrations was examined in a diverse deciduous forest stand in NW Switzerland. Measurements of g _s were carried out on upper canopy foliage before noon, over four growing seasons, including an exceptionally dry summer (2003). Across all species reductions in stomatal conductance were smaller than 25% most likely around 10%, with much variation among species and trees. Given the large heterogeneity in light conditions within a tree crown, this signal was not statistically significant, but the responses within species were surprisingly consistent throughout the study period. Except during a severe drought, stomatal conductance was always lower in trees of Carpinus betulus exposed to elevated CO₂ compared to Carpinus trees in ambient air, but the difference was only statistically significant on 2 out of 15 days. In contrast, stomatal responses in Fagus sylvatica and Quercus petraea varied around zero with no consistent trend in relation to CO₂ treatment. During the 2003 drought in the third treatment year, the CO₂ effect became reversed in Carpinus, resulting in higher g _s in trees exposed to elevated CO₂ compared to control trees, most likely due to better water supply because of the previous soil water savings. This was supported by less negative predawn leaf water potential in CO₂ enriched Carpinus trees, indicating an improved water status. These findings illustrate (1) smaller than expected CO₂-effects on stomata of mature deciduous forest trees, and (2) the possibility of soil moisture feedback on canopy water relations under elevated CO₂.     

Effects of High Atmospheric CO(2) and Sink Size on Rates of Photosynthesis of a Soybean Cultivar

The effect of sink strength on photosynthetic rates under conditions of long-term exposure to high CO₂ has been investigated in soybean. Soybean plants (Merr. cv. Fiskeby V) were grown in growth chambers containing 350 microliters CO₂ per liter air until pod set. At that time, plants were trimmed to three trifoliolate leaves and either 21 pods (high sink treatment) or 6 pods (low sink treatment). Trimmed plants were either left in 350 microliters CO₂ per liter of air or placed in 1000 microliters CO₂ per liter of air (high CO₂ treatment) until pod maturity. Whole plant net photosynthetic rates of all plants were measured twice weekly, both at 350 microliters CO₂ per liter of air and 1000 microliters CO₂ per liter of air. Plants were also harvested at this time for dry weight measurements. Photosynthetic rates of high sink plants at both measurement CO₂ concentrations were consistently higher than those of low sink plants, and those of plants given the 350 microliter CO₂ per liter of air treatment were higher at both measurement CO₂ concentrations than those of plants given the 1000 microliters CO₂ per liter of air treatment. When plants were measured under treatment CO₂ levels, however, rates were higher in 1,000 microliter plants than 350 microliter CO₂ plants. Dry weights of all plant parts were higher in the 1,000 microliters CO₂ per liter air treatment than in the 350 microliters CO₂ per liter air treatment, and were higher in the low sink than in the high sink treatments.     

Effects of CO2 and sugars on photosynthesis and composition of avocado leaves grown in vitro

The effects of micropropagation conditions on avocado (Persea americana Mill.) have been measured in leaves and plants cultured in vitro. The consequences of the type and concentration of sugar in the medium and of carbon dioxide concentration in the atmosphere on the rates of photosynthesis and amounts of ribulose 1,5-biphosphate carboxylase-oxygenase (EC 4.1.1.39; Rubisco) and total soluble protein (TSP) were measured. At the highest sucrose supply (87.6 mM), Rubisco content was substantially decreased in leaves, and even more when elevated CO₂ (1 000 μmol·mol⁻¹) was supplied. Maximum photosynthetic rate (P_max) was significantly decreased when plants developed in high sucrose and elevated CO₂. However, Rubisco concentration was significantly greater when glucose was supplied at the same molar concentration or when the concentration of sucrose was small (14.6 mM), and no differences were observed due to the CO₂ concentration in the air in these treatments. The ratio of Rubisco to total soluble protein (Rubisco/TSP) was dramatically decreased in plants grown in the highest concentration of sucrose and with elevated CO₂. Leaf area and ratio of leaf fresh weight/(stem + root) fresh weight, were greater in plants grown with CO₂ enriched air. However, upon transplanting, survival was poorer in plants grown on low sucrose/high CO₂ compared to those grown on high sucrose/high CO₂.     

Induction of reduced photorespiratory activity in submersed and amphibious aquatic macrophytes

Incubation under water in a 30 C/14-hour or 12 C/10-hour photoperiod caused the CO₂ compensation points of 10 aquatic macrophytes to decrease below 25 or increase above 50 microliters CO₂ per liter, respectively. Submerged and aerial leaves of two amphibious angiosperms (Myriophyllum brasiliense and Proserpinaca palustris) maintained high compensation points when incubated in air but, when the submerged or aerial leaves of Proserpinaca were incubated under water, the compensation points dropped as low as 10. This suggests that, in addition to temperature and photoperiod, some factor associated with submergence regulates the compensation point of aquatic plants. In the high-compensation point plants, photorespiration, as a percentage of net photosynthesis, was equivalent to that in terrestrial C₃ plants. For Hydrilla verticillata, the decreasing CO₂ compensation points (110, 40, and 10) were associated with reduced photorespiration, as indicated by decreased O₂ inhibition, decreased rates of CO₂ evolution into CO₂-free air, and increased net photosynthetic rates.     

Elevated Atmospheric CO2 Impacts Abundance and Diversity of Nitrogen Cycling Functional Genes in Soil

The concentration of CO₂ in the Earth's atmosphere has increased over the last century. Although this increase is unlikely to have direct effects on soil microbial communities, increased atmospheric CO₂ may impact soil ecosystems indirectly through plant responses. This study tested the hypothesis that exposure of plants to elevated CO₂ would impact soil microorganisms responsible for key nitrogen cycling processes, specifically denitrification and nitrification. We grew trembling aspen (Populus tremuloides) trees in outdoor chambers under ambient (360 ppm) or elevated (720 ppm) levels of CO₂ for 5 years and analyzed the microbial communities in the soils below the trees using quantitative polymerase chain reaction and clone library sequencing targeting the nitrite reductase (nirK) and ammonia monooxygenase (amoA) genes. We observed a more than twofold increase in copy numbers of nirK and a decrease in nirK diversity with CO₂ enrichment, with an increased predominance of Bradyrhizobia-like nirK sequences. We suggest that this dramatic increase in nirK-containing bacteria may have contributed to the significant loss of soil N in the CO₂-treated chambers. Elevated CO₂ also resulted in a significant decrease in copy numbers of bacterial amoA, but no change in archaeal amoA copy numbers. The decrease in abundance of bacterial amoA was likely a result of the loss of soil N in the CO₂-treated chambers, while the lack of response for archaeal amoA supports the hypothesis that physiological differences in these two groups of ammonia oxidizers may enable them to occupy distinct ecological niches and respond differently to environmental change.     

Overexpression of sucrose-phosphate synthase in tomato plants grown with CO2 enrichment leads to decreased foliar carbohydrate accumulation relative to untransformed controls

Tomato plants expressing the maize sucrose-phosphate synthase (SPS) cDNA under the control of the promoterof the small subunit of ribulose-1,5-bisphosphate carboxylase oxygenase (rbcS) promoter were grown 5 weeks in air (450 μmol.m^–2.s^–1 irradiance, 350 ppm CO₂) and then either maintained in air or exposed to CO₂ enrichment (1 000 ppm CO₂) for 8 d. A linear relationship between the foliar sucrose to starch ratio and maximal extractable SPS activity was found both in air and high CO₂. Starch accumulation was dramatically increased in all plants subjected to CO₂ enrichment but the CO₂-dependent increase in foliar starch accumulation was much lower in the leaves of the SPS transformants than in those of the untransformed controls in the same conditions. Maximal extractable ribulose-1,5-bisphosphate carboxylase/oxygenase activity was reduced by growth at high CO₂ to a similar extent in both plant types. The carbon/nitrogen ratios were similar in both plant lines in both growth conditions after 20 d exposure to high CO₂. A small (5 %) increase in carbon export capacity was observed at high CO₂ in the leaves of transformed plants compared to leaves from untransformed controls. Increased foliar SPS activity did not, however, prevent acclimation of photosynthesis in plants grown with long-term CO₂ enrichment.     

Seasonal fine-root biomass development of sour orange trees grown in atmospheres of ambient and elevated CO2 concentration

Sour orange trees have been grown from the seedling stage out-of-doors at Phoenix, Arizona, USA, in open-top enclosures with clear plastic walls for 3.5 years. For the last 3 years of this period, half of the trees have been continuously exposed to air enriched with CO₂ to 300 μmol mol^?1 above the ambient concentration. At 2-month intervals over the last 12 months, we have determined the fine-root biomass in the top 0.4 m of the soil profile beneath the trees. Results from both treatments define a single relationship between fine-root biomass and trunk cross-sectional area. The data also show the CO₂-enriched trees to have approximately 2.3 times more fine-root biomass in this soil layer than the trees grown in ambient air.     

Limitations on photosynthesis under environment-simulating culturein vitro

Limitations on photosynthesis, characterized by leaf CO₂ exchange, chlorophyll fluorescence, and thylakoid structure, were studied under environmental conditions simulating culturein vitro. These were simulated by growingPhaseolus vulgaris plants in nutrient solution under high relative humidity of air (>90%), and CO₂ concentrations (c_a) that decreased with the development of photosynthetic activities during plant ontogeny (1200 to 300 mg m^?3). The ontogeny of such model plants was more rapid, primary leaves reached photosynthetic maturity 2 to 3 d earlier and their life span was 7 to 14 d shorter than in control plants. Their photosynthetic activityin situ was limited, after reaching “photosynthetic maturity”, similarly to plants grownin vitro. When measured under optimal conditions, however, 50 to 70% higher net photosynthetic rates (P_N) were found in leaves of different ages as compared with plants grown under c_a of 700 mg m^?3 and a lower air humidity (30–35%). This increase in P_N was associated with a high conductance for CO₂ transfer by adaxial and abaxial epidermes. In model plants, the dark respiration rate (R_D) was almost twice that in the control, while the photorespiration rates were similar to controls; CO₂ compensation concentration was about 50% of that in controls. The ratios P_N/R_D were similar in control and in model plants. Chlorophylla+b content in leaves of the model plants was lower than that in the control plants. Grana extent increased with plant age in the model plants while it decreased in the control ones. In both the stromal and granal membranes of the chloroplasts in model plants, a marked accumulation of carotenoids occurred independent of age. The ratio of variable to maximal fluorescence, F_v/F_m, did not differ in the model and the control plants. In the control plants, photochemical quenching (q_P) slightly increased with plant age and was not affected by CO₂ concentration present during measurement. In the model plants, q_P increased with elevated CO₂ concentration in young plants and decreased in saturating CO₂ concentrations in older plants. Nonphotochemical quenching (q_NP) was lower in the model plants and increased under CO₂ saturating conditions. Vitality index, Rfd, was markedly lower in the model plants than in the control ones and a decline was found in saturating CO₂ concentration.     

A simple method for testing leaf responses of tall tropical forest trees to elevated CO2

The effects of atmospheric CO₂ enrichment on mature trees in their natural environment are largely unknown. Here we present a new, and inexpensive technique which can be used in situ to address some key physiological questions related to the CO₂ problem. Small, light-weight cups mounted on the lower side of rigid leaves at the top of tall tropical forest trees were supplied with CO₂-enriched air derived from a low-technology air mixing device utilizing forest floor CO₂ evolution. We present the scientific rationale for such field experiments, technical details, an assessment of potential cup artifacts and first results illustrating effects of elevated CO₂ on stomata and carbohydrate accumulation in the canopies of mature trees.     

Prechilling of Xanthium strumarium L. Reduces Net Photosynthesis and, Independently, Stomatal Conductance, While Sensitizing the Stomata to CO(2)

Greenhouse-grown plants of Xanthium strumarium L. were exposed in a growth cabinet to 10 C during days and 5 C during nights for periods of up to 120 hours. Subsequently, CO₂ exchange, transpiration, and leaf temperature were measured on attached leaves and in leaf sections at 25 or 30 C, 19 C dew point of the air, 61 milliwatts per square centimeter irradiance, and CO₂ concentrations between 0 and 1000 microliters per liter ambient air. Net photosynthesis and stomatal conductance decreased and dark respiration increased with increasing duration of prechilling. The reduction in net photosynthesis was not a consequence of decreased stomatal conductance because the intercellular CO₂ concentration in prechilled leaves was equal to or greater than that in greenhouse-grown controls. The intercellular CO₂ concentration at which one-half maximum net photosynthesis occurred remained the same in prechilled leaves and controls (175 to 190 microliters per liter). Stomata of the control plants responded to changes in the CO₂ concentration of the air only slightly. Prechilling for 24 hours or more sensitized stomata to CO₂; they responded to changes in CO₂ concentration in the range from 100 to 1000 microliters per liter.     

Responses of cotton and wheat photosynthesis and growth to cyclic variation in carbon dioxide concentration

The carbon dioxide concentration in free air carbon dioxide enrichment (FACE) systems typically has rapid fluctuations. In our FACE system, power spectral analysis of CO₂ concentration measured every second with an open path analyzer indicated peaks in variation with a period of about one minute. I used open-top chambers to expose cotton and wheat plants to either a constant elevated CO₂ concentration of 180 ??mol mol^?1 above that of outside ambient air, or to the same mean CO₂ concentration, but with the CO₂ enrichment cycling between about 30 and 330 ??mol mol^?1 above the concentration of outside ambient air, with a period of one minute. Three short-term replicate plantings of cotton were grown in Beltsville, Maryland with these CO₂ concentration treatments imposed for 27-day periods over two summers, and one winter wheat crop was grown from sowing to maturity. In cotton, leaf gas-exchange measurements of the continuously elevated treatment and the fluctuating treatment indicated that the fluctuating CO₂ concentration treatment consistently resulted in substantial down-regulation of net photosynthetic rate (P _N) and stomatal conductance (g _s). Total shoot biomass of the vegetative cotton plants in the fluctuating CO₂ concentration treatment averaged 30% less than in the constantly elevated CO₂ concentration treatment at 27 days after planting. In winter wheat, leaf gas-exchange measurements also indicated that down-regulation of P _N and g _s occurred in flag leaves in the fluctuating CO₂ concentration treatment, but the effect was not as consistent in other leaves, nor as severe as found in cotton. However, wheat grain yields were 12% less in the fluctuating CO₂ concentration treatment compared with the constant elevated CO₂ concentration treatment. Comparison with wheat yields in chambers without CO₂ addition indicated a nonsignificant increase of 5% for the fluctuating elevated CO₂ concentration treatment, and a significant increase of 19% for the constant elevated treatment. The results suggest that treatments with fluctuating elevated CO₂ concentrations could underestimate plant growth at projected future atmospheric CO₂ concentrations.     

Use of sediment CO2 by submersed rooted plants

Background and Aims

Submersed plants have different strategies to overcome inorganic carbon limitation. It is generally assumed that only small rosette species (isoetids) are able to utilize the high sediment CO₂ availability. The present study examined to what extent five species of submersed freshwater plants with different morphology and growth characteristics (Lobelia dortmanna, Lilaeopsis macloviana, Ludwigia repens, Vallisneria americana and Hydrocotyle verticillata) are able to support photosynthesis supplied by uptake of CO₂ from the sediment.

Methods

Gross photosynthesis was measured in two-compartment split chambers with low inorganic carbon availability in leaf compartments and variable CO₂ availability (0 to >8 mmol L⁻¹) in root compartments. Photosynthetic rates based on root-supplied CO₂ were compared with maximum rates obtained at saturating leaf CO₂ availability, and ¹⁴C experiments were conducted for two species to localize bottlenecks for utilization of sediment CO₂.

Key Results

All species except Hydrocotyle were able to use sediment CO₂, however, with variable efficiency, and with the isoetid, Lobelia, as clearly the most effective and the elodeid, Ludwigia, as the least efficient. At a water column CO₂ concentration in equilibrium with air, Lobelia, Lilaeopsis and Vallisneria covered >75% of their CO₂ requirements by sediment uptake, and sediment CO₂ contributed substantially to photosynthesis at water CO₂ concentrations up to 1000 µmol L⁻¹. For all species except Ludwigia, the shoot to root ratio on an areal basis was the single factor best explaining variability in the importance of sediment CO₂. For Ludwigia, diffusion barriers limited uptake or transport from roots to stems and transport from stems to leaves.

Conclusions

Submersed plants other than isoetids can utilize sediment CO₂, and small and medium sized elodeids with high root to shoot area in particular may benefit substantially from uptake of sediment CO₂ in low alkaline lakes.Key words: Submersed rooted plants, CO₂ uptake, sediment CO₂, Lobelia dortmanna, Lilaeopsis macloviana, Ludwigia repens, Vallisneria americana, Hydrocotyle verticillata     

Leaf gas exchange and nitrogen dynamics of N2-fixing, field-grown Alnus glutinosa under elevated atmospheric CO2

Few studies have investigated the effects of elevated CO₂ on the physiology of symbiotic N₂-fixing trees. Tree species grown in low N soils at elevated CO₂ generally show a decline in photosynthetic capacity over time relative to ambient CO₂ controls. This negative adjustment may be due to a reallocation of leaf N away from the photosynthetic apparatus, allowing for more efficient use of limiting N. We investigated the effect of twice ambient CO₂ on net CO₂ assimilation (A), photosynthetic capacity, leaf dark respiration, and leaf N content of N2-fixing Alnus glutinosa (black alder) grown in field open top chambers in a low N soil for 160 d. At growth CO₂, A was always greater in elevated compared to ambient CO₂ plants. Late season A vs. internal leaf p(CO₂) response curves indicated no negative adjustment of photosynthesis in elevated CO₂ plants. Rather, elevated CO₂ plants had 16% greater maximum rate of CO₂ fixation by Rubisco. Leaf dark respiration was greater at elevated CO₂ on an area basis, but unaffected by CO₂ on a mass or N basis. In elevated CO₂ plants, leaf N content (μg N cm^?2) increased 50% between Julian Date 208 and 264. Leaf N content showed little seasonal change in ambient CO₂ plants. A single point acetylene reduction assay of detached, nodulated root segments indicated a 46% increase in specific nitrogenase activity in elevated compared to ambient CO₂ plants. Our results suggest that N₂-fixing trees will be able to maintain high A with minimal negative adjustment of photosynthetic capacity following prolonged exposure to elevated CO₂ on N-poor soils.     

Postillumination burst of carbon dioxide in crassalacean Acid metabolism plants

Immediately following exposure to light, a postillumination burst of CO₂ has been detected in Crassulacean acid metabolism plants. A detailed study with pineapple (Ananas comosus) leaves indicates that the postillumination burst changes its amplitude and kinetics during the course of a day. In air, the postillumination burst in pineapple leaves generally is exhibited as two peaks. The postillumination burst is sensitive to atmospheric CO₂ and O₂ concentrations as well as to the light intensity under which plants are grown. We propose that the CO₂ released in the first postillumination burst peak is indicative of photorespiration since it is sensitive to either O₂ or CO₂ concentration while the second CO₂ evolution peak is likely due to decarboxylation of organic acids involved in Crassulacean acid metabolism.     

CO2-enriched atmosphere and supporting material impact the growth,morphophysiology and ultrastructure of in vitro Brazilian-ginseng [Pfaffia glomerata (Spreng.) Pedersen] plantlets

This study aimed to evaluate under photoautotrophic conditions the effect of CO₂-enriched atmosphere (360 or 1,000 μmol CO₂ mol^?1 air) combined with two substrate types (agar or Florialite^®) in vitro on plants of Pfaffia glomerata, an endangered medicinal species with promising applications in phytotherapy and phytomedicine. The effects of the treatments on the growth, stomatal density, Rubisco activity, carbon isotopic discrimination, metabolite accumulation, photosynthetic pigments and ultrastructural characteristics were investigated. After a 35-day cultivation period, the in vitro-growth of P. glomerata nodal segments under the different treatments resulted in plants with substantial differences in relation to their growth, photosynthetic pigments, stomatal density and leaf ultrastructural characteristics. The enrichment with CO₂ coupled with a porous substrate increased the growth of P. glomerata. The stomatal density in the abaxial epidermis more than doubled in response to the high CO₂ supply in both supporting types, whereas the Rubisco activity and activation state were both unresponsive to the treatments. Regardless of the CO₂ supply, the plants grown in agar displayed higher carbon isotope discrimination than their counterparts grown in Florialite^®. We propose that the long-term photosynthetic performance was improved using Florialite^® as a growth support in combination with a high CO₂ supply. No apparent signs of photosynthetic down-regulation could be found under elevated CO₂ conditions. The enrichment of in vitro atmospheres with CO₂ coupled with a porous substrate offers new possibilities for improving the growth and production on a commercial scale of high morphological and physiological quality Pfaffia plants.