Elevated atmospheric CO2 increases microbial growth rates in soil: results of three CO2 enrichment experiments

Increasing the belowground translocation of assimilated carbon by plants grown under elevated CO₂ can cause a shift in the structure and activity of the microbial community responsible for the turnover of organic matter in soil. We investigated the long‐term effect of elevated CO₂ in the atmosphere on microbial biomass and specific growth rates in root‐free and rhizosphere soil. The experiments were conducted under two free air carbon dioxide enrichment (FACE) systems: in Hohenheim and Braunschweig, as well as in the intensively managed forest mesocosm of the Biosphere 2 Laboratory (B2L) in Oracle, AZ. Specific microbial growth rates (μ) were determined using the substrate‐induced respiration response after glucose and/or yeast extract addition to the soil. For B2L and both FACE systems, up to 58% higher μ were observed under elevated vs. ambient CO₂, depending on site, plant species and N fertilization. The μ‐values increased linearly with atmospheric CO₂ concentration at all three sites. The effect of elevated CO₂ on rhizosphere microorganisms was plant dependent and increased for: Brassica napus=Triticum aestivum<Beta vulgaris<Populus deltoides. N deficiency affected microbial growth rates directly (N limitation) and indirectly (changing the quantity of fine roots). So, 50% decrease in N fertilization caused the overall increase or decrease of microbial growth rates depending on plant species. The μ‐value increase was lower for microorganisms growing on yeast extract then for those growing on glucose, i.e. the effect of elevated CO₂ was smoothed on rich vs. simple substrate. So, the r/K strategies ratio can be better revealed by studying growth on simple (glucose) than on rich substrate mixtures (yeast extract). Our results clearly showed that the functional characteristics of the soil microbial community (i.e. specific growth rates) rather than total microbial biomass amount are sensitive to increased atmospheric CO₂. We conclude that the more abundant available organics released by roots at elevated CO₂ altered the ecological strategy of the soil microbial community specifically a shift to a higher contribution of fast‐growing r‐selected species was observed. These changes in functional structure of the soil microbial community may counterbalance higher C input into the soil under elevated atmospheric CO₂ concentration.     

Evaluation of the preservative properties of Thymus vulgaris essential oil in topically applied formulations under a challenge test

The preservative properties of thyme essential oil (3%) with a known composition were evaluated in two types of final formulations, suitable for use as pharmaceutical or cosmetic vehicles, by means of a standard challenge test proposed by the latest European Pharmacopoeia. The required preservation efficacy criteria were satisfied against the bacterial strains, against the yeast in one of the formulations, but not against the mould strain involved in this study. Interactions between the essential oil compounds and other factors present in the final formulation might have influenced the activity of this essential oil, leading to an incomplete satisfaction of the criteria.     

Mitochondrial dysfunction increases oxidative stress and decreases chronological life span in fission yeast

Background

Oxidative stress is a probable cause of aging and associated diseases. Reactive oxygen species (ROS) originate mainly from endogenous sources, namely the mitochondria.

Methodology/Principal Findings

We analyzed the effect of aerobic metabolism on oxidative damage in Schizosaccharomyces pombe by global mapping of those genes that are required for growth on both respiratory-proficient media and hydrogen-peroxide-containing fermentable media. Out of a collection of approximately 2700 haploid yeast deletion mutants, 51 were sensitive to both conditions and 19 of these were related to mitochondrial function. Twelve deletion mutants lacked components of the electron transport chain. The growth defects of these mutants can be alleviated by the addition of antioxidants, which points to intrinsic oxidative stress as the origin of the phenotypes observed. These respiration-deficient mutants display elevated steady-state levels of ROS, probably due to enhanced electron leakage from their defective transport chains, which compromises the viability of chronologically-aged cells.

Conclusion/Significance

Individual mitochondrial dysfunctions have often been described as the cause of diseases or aging, and our global characterization emphasizes the primacy of oxidative stress in the etiology of such processes.     

Elevated atmospheric partial pressure of CO2 and plant growth

Cotton plants were grown in late spring under full sunlight in glasshouses containing normal ambient partial pressure of CO₂ (32±2Pa) and enriched partial pressure of CO₂ (64±1.5Pa) and at four levels of nitrogen nutrition. Thirty-five days after planting, the total dry weights of high CO₂-grown plants were 2- to 3.5-fold greater than plants grown in normal ambient CO₂ partial pressure. Depending on nitrogen nutrition level, non-structural carbohydrate content (mainly starch) in the leaves of plants grown in normal CO₂ was between 4 and 37% of the total leaf dry weight compared to 39 to 52% in the leaves of high CO₂-grown plants. Specific leaf weight calculated using total dry weight was 1.6- to 2-fold greater than that based on structural dry weight. In high CO₂-grown plants the amount of non-structural carbohydrate translocated from the leaves at night was between 10 and 20% of the level at the end of the photoperiod. This suggests that the plant was unable to utilize all the carbohydrate it assimilated in elevated CO₂ atmosphere. While there was a 1.5-fold enhancement in the rate of CO₂ assimilation in plants grown in 64 Pa CO₂, there was, however, some evidence to suggest that the activities of other metabolic pathways in the plants were not stimulated to the same extent by the enriched CO₂ atmosphere. This resulted in massive accumulation of non-structural carbohydrate, particularly at low level of nitrogen nutrition.Abbreviations A rate of CO₂ assimilation - PPFD photosynthetic photo flux density - NAR net assimilation rate - pCO₂ partial pressure of CO₂ - RGR relative growth rate     

Elevated atmospheric partial pressure of CO2 and plant growth

Summary Cotton and maize plants were grown under full sunlight in glass houses containing normal ambient partial pressure of CO₂ (330±20 bar) and enriched partial pressure of CO₂ (640 ±15 bar) with four levels of nitrogen nutrient. In 40 day old cotton plants grown in high CO₂, there was a 2-fold increase in day weight and a 1.6-fold increase in leaf area compared with plants grown in ambient CO₂. In 30 day old maize plants there was only 20% increase in dry weight in plants grown in 640 bar CO₂ compared with plants grown in 330 bar and no significant increase in leaf area. In both species, at both CO₂ treatments, dry weight and leaf area decreased in similar proportion with decreased nitrogen nutrient.The increase of leaf area in cotton plants at high CO₂ caused a reduction of total nitrogen on a dry weight basis. In cotton assimilation rate increased 1.5 fold when plants were grown with high nitrogen and high CO₂. The increase was less at lower levels of nitrate nutrient. There was a 1.2 fold increase in assimilation rate in maize grown at high CO₂ with high nitrate nutrient.Cotton and maize grown in high CO₂ had a lower assimilation rate in ambient CO₂ compared to plants grown in normal ambient air. This difference was due to the reduction in RuBP carboxylase activity. Water use efficiency was doubled in both cotton and maize plants grown at high CO₂ in all nutrient treatments. However, this increase in water use efficiency was due primarily to reduced transpiration in some treatments and to increased assimilation in others. These data show that plant responses to elevated atmospheric partial pressure of CO₂ depend on complex of partially compensatory processes which are not readily predictable.     

Mating increases starvation resistance and decreases oxidative stress resistance in Drosophila melanogaster females

Mating stimulates complex physiological changes in females of Drosophila melanogaster. Long-term effects of mating are manifested in increased fecundity and shortened lifespan. It is not clear how mating affects stress resistance in fly females. We addressed this question here and found that mated and highly fecund wild-type D. melanogaster females have significantly higher resistance to starvation throughout their lifetime than age-matched virgin females. Mean survival time under starvation was age dependent with maximum survival time observed in 15-day-old mated females. Mating-induced increase in starvation resistance was associated with significantly higher fat reserves stored as triacylglycerols. While mated females had higher resistance to starvation, their resistance to oxidative stress was significantly lower than in age-matched virgins. Our study revealed that mating leads to an opposing relationship between resistance to starvation and resistance to oxidative stress in Drosophila females. Thus, shortened lifespan of mated females is associated with their high-fat content and greater susceptibility to oxidative stress.     

Rubrerythrin and rubredoxin oxidoreductase in Desulfovibrio vulgaris: a novel oxidative stress protection system

Evidence is presented for an alternative to the superoxide dismutase (SOD)-catalase oxidative stress defense system in Desulfovibrio vulgaris (strain Hildenborough). This alternative system consists of the nonheme iron proteins, rubrerythrin (Rbr) and rubredoxin oxidoreductase (Rbo), the product of the rbo gene (also called desulfoferrodoxin). A Deltarbo strain of D. vulgaris was found to be more sensitive to internal superoxide exposure than was the wild type. Unlike Rbo, expression of plasmid-borne Rbr failed to restore the aerobic growth of a SOD-deficient strain of Escherichia coli. Conversely, plasmid-borne expression of two different Rbrs from D. vulgaris increased the viability of a catalase-deficient strain of E. coli that had been exposed to hydrogen peroxide whereas Rbo actually decreased the viability. A previously undescribed D. vulgaris gene was found to encode a protein having 50% sequence identity to that of E. coli Fe-SOD. This gene also encoded an extended N-terminal sequence with high homologies to export signal peptides of periplasmic redox proteins. The SOD activity of D. vulgaris is not affected by the absence of Rbo and is concentrated in the periplasmic fraction of cell extracts. These results are consistent with a superoxide reductase rather than SOD activity of Rbo and with a peroxidase activity of Rbr. A joint role for Rbo and Rbr as a novel cytoplasmic oxidative stress protection system in D. vulgaris and other anaerobic microorganisms is proposed.     

Ethylene perception inhibitor 1-MCP decreases oxidative damage of leaves through enhanced antioxidant defense mechanisms in soybean plants grown under high temperature stress

Ethylene is a stress hormone involved in early senescence and abscission of vegetative and reproductive organs under stress conditions. Ethylene perception inhibitors can minimize the impact of ethylene-mediated stress. The effects of high temperature (HT) stress during flowering on ethylene production rate in leaf, flower and pod and the effects of ethylene inhibitor on ethylene production rate, oxidative damage and physiology of soybean are not understood. We hypothesize that HT stress induces ethylene production, which causes premature leaf senescence and flower and pod abscission, and that application of the ethylene perception inhibitor 1-Methyl cyclopropene (1-MCP) can minimize HT stress induced ethylene response in soybean. The objectives of this study were to (1) determine whether ethylene is produced in HT stress; (2) quantify the effects of HT stress and 1-MCP application on oxidative injury; and (3) evaluate the efficacy of 1-MCP at minimizing HT-stress-induced leaf senescence and flower abscission. Soybean plants were exposed to HT (38/28 °C) or optimum temperature (OT; 28/18 °C) for 14 d at flowering stage (R2). Plants at each temperature were treated with 1-MCP (1 μg L⁻¹) gas for 5 h or left untreated (control). High temperature stress increased rate of ethylene production in leaves, flowers and pods, production of reactive oxygen species (ROS), membrane damage, and total soluble carbohydrate content in leaves and decreased photosynthetic rate, sucrose content, F_v/F_m ratio and antioxidant enzyme activities compared with OT. Foliar spray of 1-MCP decreased rate of ethylene production and ROS and leaf senescence traits but enhanced antioxidant enzyme activities (e.g. superoxide dismutase and catalase). In conclusion, HT stress increased ethylene production rates, caused oxidative damage, decreased antioxidant enzyme activity, caused premature leaf senescence, increased flower abscission and decreased pod set percentage. Application of 1-MCP lowered ethylene and ROS production, enhanced antioxidant enzyme activity, increased membrane stability, delayed leaf senescence, decreased flower abscission and increased pod set percentage. The beneficial effects of 1-MCP were greater under HT stress compared to OT in terms of decreased ethylene production, decreased ROS production, increased antioxidant protection, decreased flower abscission and increased pod set percentage.     

Antifungal activity of thyme (Thymus vulgaris L.) essential oil and thymol against moulds from damp dwellings

AIMS: To characterize antifungal activities of essential oil of thyme (Thymus vulgaris L.) and pure thymol, as comparative substance, on different mould species isolated from damp dwellings. METHODS AND RESULTS: Fifty samples of wall scrapes were collected from damp dwellings in Zagreb, the capital of Croatia. The members of the following mould genera were recovered from the samples: Aspergillus (44%), Penicillium (18%) Alternaria, Ulocladium, Absidia and Mucor (8%) Cladosporium, Trichoderma and Rhizopus (6%), and Chaetomium (2%). Two strains of Stachybotrys chartarum were isolated from damp dwellings in Slovakia. Antifungal activities of the thyme essential oil, which contains p-cymene (36.5%), thymol (33.0%) and 1,8-cineole (11.3%) as main components, and pure thymol were determined by the dilution method and exposure to vaporous phase of the oil. Minimum inhibitory concentrations (MIC) of both thymol and essential oil were bellow 20 microg ml(-1), except for Mucor spp. (50.20 microg ml(-1)). Thymol exhibited approximately three-times stronger inhibition than essential oil of thyme. The vaporous phase of the thyme essential oil (82 microg l(-1)) in glass chambers strongly suppressed the sporulation of moulds during 60 days of exposure. CONCLUSION: The thyme essential oil possesses a wide range spectrum of fungicidal activity. The vaporous phase of the oil exhibited long-lasting suppressive activity on moulds from damp dwellings. SIGNIFICANCE AND IMPACT OF THE STUDY: Essential oil of thyme and thymol could be used for disinfection of mouldy walls in the dwellings in low concentration.     

Effect of orange peel essential oil on oxidative stress in AOM animals

The processing parameters of pump speed, inlet air temperature, outlet air temperature and homogenization pressure were evaluated. Encapsulation efficiency is high with a satisfied releasing rate. Then, acute otitis media (AOM) animal model was built and diet containing orange peel essential oil microcapsules were administrated to AOM animals. Pharmacological test showed that orange peel essential oil treatment could decrease serum and cochlea malondialdehyde (MDA), immunoglobulins A (IgA), immunoglobulins G (IgG), immunoglobulins M (IgM) levels and increase antioxidant enzymes activities. It can be concluded that orange peel essential oil treatment could decrease oxidative injury in acute otitis media rats.     

Elevated CO2 increases fungal-based micro-foodwebs in soils of contrasting plant species

Plant and Soil - As different plant species support different soil communities, assessment of climate change impacts on soil communities must consider how these impacts depend on the vegetation. We...     

Rising atmospheric CO2 may affect oil quality and seed yield of sunflower (Helianthus annus L.)

The impact of rising atmospheric CO₂ on crop productivity and quality is very important for global food and nutritional security under the changing climatic scenario. A study was conducted to investigate the effect of elevated CO₂ on seed oil quality and yield in a sunflower hybrid DRSH 1 and variety DRSF 113, raised inside open top chambers and exposed to elevated CO₂ (550 ± 50 µl l^?1). Elevated CO₂ exposure significantly influenced the rate of photosynthesis, seed yield and the quality traits in both hybrid and variety. Plants grown under elevated CO₂ concentration showed 61–68 % gain in biomass and 35–46 % increase in seed yield of both the genotypes, but mineral nutrient and protein concentration decreased in the seeds. The reduction in seed protein was up to 13 %, while macro and micronutrients decreased drastically (up to 43 % Na in hybrid seeds) under elevated CO₂ treatment. However, oil content increased significantly in DRSF 113 (15 %). Carbohydrate seed reserves increased with similar magnitudes in both the genotypes under elevated CO₂ treatment (13 %). Fatty acid composition in seed oil contained higher proportion of unsaturated fatty acids (oleic and linoleic acid) under elevated CO₂ treatment, which is a desirable change in oil quality for human consumption. These findings conclude that rising atmospheric CO₂ in changing future climate can enhance biomass production and seed yield in sunflower and alter their seed oil quality in terms of increased concentration of unsaturated fatty acids compared with saturated fatty acids and lower seed proteins and mineral nutrients.     

Elevated CO2 increases nitrogen rhizodeposition and microbial immobilization of root-derived nitrogen

With this study, we aimed to determine how elevated CO(2) affects rhizodeposition and the cycling of rhizodeposited nitrogen (N) in the soil under C(3) and C(4) plants. In addition, we examined how cultivated genotypes of wheat (Triticum turgidum) and maize (Zea mays) responded to elevated CO(2) in comparison with their wild relatives. By constructing an N-transfer experiment we could directly assess cycling of the rhizodeposited N and trace the fate of rhizodeposited N in the soil and in receiver plants. Biomass production, rhizodeposition and cycling of root-borne N in maize genotypes were not affected by elevated CO(2). Elevated CO(2) stimulated above- and below-ground biomass production of the wheat genotypes on average by 38%, and increased rhizodeposition and immobilization of root-derived N on average by 30%. Concurrently, elevated CO(2) reduced mineral (15)N and re-uptake of the root-derived N by 50% in wheat. This study shows that elevated CO(2) may enhance N limitation by increasing N rhizodeposition and subsequent immobilization of the root-derived N.     

Acute knockdown of uncoupling protein-2 increases uncoupling via the adenine nucleotide transporter and decreases oxidative stress in diabetic kidneys

Increased O(2) metabolism resulting in chronic hypoxia is common in models of endstage renal disease. Mitochondrial uncoupling increases O(2) consumption but the ensuing reduction in mitochondrial membrane potential may limit excessive oxidative stress. The present study addressed the hypothesis that mitochondrial uncoupling regulates mitochondria function and oxidative stress in the diabetic kidney. Isolated mitochondria from kidney cortex of control and streptozotocin-induced diabetic rats were studied before and after siRNA knockdown of uncoupling protein-2 (UCP-2). Diabetes resulted in increased UCP-2 protein expression and UCP-2-mediated uncoupling, but normal mitochondria membrane potential. This uncoupling was inhibited by GDP, which also increased the membrane potential. siRNA reduced UCP-2 protein expression in controls and diabetics (-30-50%), but paradoxically further increased uncoupling and markedly reduced the membrane potential. This siRNA mediated uncoupling was unaffected by GDP but was blocked by ADP and carboxyatractylate (CAT). Mitochondria membrane potential after UCP-2 siRNA was unaffected by GDP but increased by CAT. This demonstrated that further increased mitochondria uncoupling after siRNA towards UCP-2 is mediated through the adenine nucleotide transporter (ANT). The increased oxidative stress in the diabetic kidney, manifested as increased thiobarbituric acids, was reduced by knocking down UCP-2 whereas whole-body oxidative stress, manifested as increased circulating malondialdehyde, remained unaffected. All parameters investigated were unaffected by scrambled siRNA. In conclusion, mitochondrial uncoupling via UCP-2 regulates mitochondria membrane potential in diabetes. However, blockade of the diabetes-induced upregulation of UCP- 2 results in excessive uncoupling and reduced oxidative stress in the kidney via activation of ANT.     

大气CO2浓度升高对毛竹叶片膜脂过氧化和抗氧化系统的影响

为了解竹子对大气CO2浓度升高的生理响应,为气候变化背景下的竹林适应性管理提供理论依据,运用开顶式气室(OTCs)设置了3个CO2处理浓度(环境大气、500和700μmol·mol-1),探讨了大气CO2浓度升高对毛竹(Phyllostachys edulis)叶片光合色素、膜脂过氧化和抗氧化系统的影响。结果表明:与环境大气比较,500μmol·mol-1浓度处理30d时对毛竹叶片光合色素、膜脂过氧化和抗氧化系统影响并不明显,仅叶片CAT活性显著降低;随着处理时间的延长,对毛竹叶片膜脂过氧化和抗氧化系统的影响逐渐显现,至处理90d时,除叶片可溶性糖含量无明显变化,其他测定指标均有显著变化;700μmol·mol-1浓度处理在不同时间上对毛竹叶片膜脂过氧化和抗氧化系统的影响均较500μmol·mol-1CO2浓度处理明显,处理30d时毛竹叶片可溶性糖含量和抗氧化酶活性有明显变化,处理90d时,各项测定指标均有显著变化;研究表明大气CO2浓度升高一定程度上能增强毛竹的抗氧化能力,但光合产物的过量积累也会造成碳水化合物源-库失衡和Rubisco的再生受到反馈作用抑制。     

High glucose increases expression of cyclooxygenase‐2, increases oxidative stress and decreases the generation of nitric oxide in mouse microvessel endothelial cells

Hyperglycaemia is a key factor that contributes to the development of diabetes‐related microvascular disease. Both cyclooxygenase I and cyclooxygenase II are expressed in endothelial cells and play key roles in the regulation of cardiovascular function. In the current study we tested the hypothesis that hyperglycaemia‐induced increased expression of cyclooxygenase II is a contributing factor both to the increased oxidative stress and to the reduction in the generation of nitric oxide in microvessel endothelial cells following their exposure to high glucose. We demonstrated that the exposure of mouse microvascular endothelial cells to high glucose for 3 days decreased the generation of nitric oxide and enhanced production of superoxide. Western blots illustrated that exposure to high glucose also increased endothelial nitric oxide synthase and cyclooxygenase II protein expression levels and decreased the dimer/monomer ratio of endothelial nitric oxide synthase protein. All the changes induced by the high glucose culture media could be reversed by either the cyclooxygenase II inhibitor CAY10404, the non‐selective cyclooxygenase inhibitor indomethacin or the protein kinase C inhibitor chelerythrine, but not solely by preincubation with the antioxidant and putative NADPH oxidase inhibitor, apocynin. Our data indicate that high glucose induced oxidative stress is linked to an increase in the expression of cyclooxygenase II and a reduced generation of nitric oxide that is associated with an uncoupled endothelial nitric oxide synthase, possibly due to decreased dimer/monomer ratio. J. Cell. Physiol. 222: 669–675, 2010. © 2009 Wiley‐Liss, Inc.     

Effect of temperature on net CO2 assimilation and photosystem II quantum yield of electron transfer of French bean (Phaseolus vulgaris L.) leaves during drought stress

The effect of leaf temperature on stomatal conductance and net CO₂ uptake was studied on French bean (Phaseolus vulgaris L.) using either dehydrated attached leaves (25–40% water deficit) or cut leaves supplied with 10^–4 M abscisic acid (ABA) solution to the transpiration stream. Decreasing leaf temperature caused stomatal opening and increased net CO₂ uptake (which was close to zero at around 25° C) to a level identical to that of control leaves (without water deficit) at around 15° C. (i) The ABA effect on stomatal closure was modulated by temperature and, presumably, ABA is at least partly responsible for stomatal closure of french bean submitted to a drought stress. (ii) For leaf temperatures lower than 15° C, net CO₂ uptake was no longer limited by water deficit even on very dehydrated leaves. This shows that dehydrated leaves retain a substantial part of their photosynthetic capacity which can be revealed at normal CO₂ concentrations when stomata open at low temperature. In contrast to leaves fed with ABA, decreasing the O₂ concentration from 21% to 1% O₂ did not increase either the rate of net CO₂ uptake or the thermal optimum for photosynthesis of dehydrated leaves. The quantum yield of PSII electron flow (measured by F/F_m) was lower in 1% O₂ than in 21% O₂ for each leaf pretreatment given (non-dehydrated leaves, dehydrated leaves, and leaves fed with ABA) even within a temperature range in which leaf photosynthesis at normal CO₂ concentration was the same in these two O₂ concentrations. It is concluded that this probably indicates an heterogeneity of photosynthesis, since this difference in quantum yield disappears when using high CO₂ concentrations during measurements.Abbreviations and Symbols ABA abscisic acid - F_m maximum chlorophyll fluorescence - F difference between steady-state chlorophyll fluorescence and F_m - PPFD photosynthetic photon flux density We would like to thank Dr. J.-M. Briantais (Laboratoire d'écologie végétale, Orsay, France) for help during fluorescence measurements and Ms. J. Liebert for technical assistance.     

Ozone increases root respiration but decreases leaf CO2 assimilation in cotton and melon

It is well established that exposure of plant foliage to tropospheric ozone (O3) inhibits photosynthetic gas exchange in leaves and the translocation of current photosynthate to sink tissues. It is less clear what impact O3-reduced source strength has on the physiological responses of sink tissue such as fine roots. The responses were investigated of carbon acquisition in leaves and carbon utilization in the respiration of fine roots, following chronic (weeks) and acute (hours) exposures to O3 in open top chambers. Previous reports indicate increased, decreased, and unchanged rates of root respiration following exposure to O3. A decline in source activity is confirmed, but an increase in sink respiration is reported in fine roots of Pima cotton (cv. S-6) and muskmelon (cv. Ambrosia hybrid). Leaf source strength and root sink activity changed in opposing directions, thus there was no positive correlation that might indicate direct substrate control of root function. Additional linkages between shoot and root following exposure to O3 may be involved.     

Long-term photosynthetic response in single leaves of A C3 and C4 salt marsh species grown at elevated atmospheric CO2 in situ

Summary Mono-specific communities of the C₃ sedge, Scirpus olneyi and the C₄ grass, Spartina patens, were exposed to normal ambient or elevated CO₂, (ca. 680 l l^–1) throughout the 1987 and 1988 growing seasons in open-top field chambers located on a tidal marsh. Single stems of C₃ plants grown in ambient or elevated CO₂ showed an increased photosynthetic rate when tested at elevated CO₂ for both seasons. This increase in photosynthetic response in the C₃ species was maintained throughout the 1987 and 1988 growing season. The stimulation of photosynthesis with elevated CO₂ appeared to increase as temperature increased and decreased as photosynthetic photon flux (PPF) increased. Analysis of the photosynthetic response of the C₃ species during the 1988 season indicated that significant differences in light-saturated photosynthetic rate between ambient and elevated CO₂ conditions continued until October. In contrast to the C₃ sedge, the C₄ grass showed no significant photosynthetic increase to elevated CO₂ except at the beginning of the 1988 season.