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ABSTRACT Phenolic glycosides are effective reactive oxygen scavengers and peroxidase substrates, suggesting that compounds in addition to ascorbate may have functional importance in defence responses against ozone (O3), especially in the leaf apoplast. The apoplastic concentrations of ascorbic acid (AA) and phenolic glycosides in Arabidopsis thaliana L. Col-0 wild-type plants were determined following exposure to a range of O3 concentrations (5, 125 or 175 nL L−1) in controlled environment chambers. AA in leaf apoplast extracts was almost entirely oxidized in all treatments, suggesting that O3 scavenging by direct reactions with reduced AA was very limited. In regard to phenolics, O3 stimulated transcription of numerous phenylpropanoid pathway genes and increased the apoplastic concentration of sinapoyl malate. However, modelling of O3 scavenging in the apoplast indicated that sinapoyl malate concentrations were too low to be effective protectants. Furthermore, null mutants for sinapoyl esters ( fah1-7), kaempferol glycosides ( tt4-1) and the double mutant ( tt4-1/fah1-7) were equally sensitive to chronic O3 as Ler-0 wild-type plants. These results indicate that current understanding of O3 defence schemes deserves reassessment as mechanisms other than direct scavenging of O3 by extracellular AA and antioxidant activity of some phenolics may predominate in some plant species. [ABSTRACT FROM AUTHOR]

The heterotrimeric G-protein complex provides signal amplification and target specificity. The Arabidopsis (Arabidopsis thaliana) Gβ-subunit of this complex (AGB1) interacts with and modulates the activity of target cytoplasmic proteins. This specificity resides in the structure of the interface between AGB1 and its targets. Important surface residues of AGB1, which were deduced from a comparative evolutionary approach, were mutated to dissect AGBl-dependent physiological functions. Analysis of the capacity of these mutants to complement well-established phenotypes of Gβ-null mutants revealed AGB1 residues critical for specific AGB1-mediated biological processes, including growth architecture, pathogen resistance, stomata-mediated leaf-air gas exchange, and possibly photosynthesis. These findings provide promising new avenues to direct the finely tuned engineering of crop yield and traits. [ABSTRACT FROM AUTHOR]

Climate change factors such as elevated atmospheric carbon dioxide (CO2) and ozone (O3) can exert significant impacts on soil microbes and the ecosystem level processes they mediate. However, the underlying mechanisms by which soil microbes respond to these environmental changes remain poorly understood. The prevailing hypothesis, which states that CO2- or O3-induced changes in carbon (C) availability dominate microbial responses, is primarily based on results from nitrogen (N)-limiting forests and grasslands. It remains largely unexplored how soil microbes respond to elevated CO2 and O3 in N-rich or N-aggrading systems, which severely hinders our ability to predict the long-term soil C dynamics in agroecosystems. Using a long-term field study conducted in a no-till wheat-soybean rotation system with open-top chambers, we showed that elevated CO2 but not O3 had a potent influence on soil microbes. Elevated CO2 (1.56ambient) significantly increased, while O3 (1.46ambient) reduced, aboveground (and presumably belowground) plant residue C and N inputs to soil. However, only elevated CO2 significantly affected soil microbial biomass, activities (namely heterotrophic respiration) and community composition. The enhancement of microbial biomass and activities by elevated CO2 largely occurred in the third and fourth years of the experiment and coincided with increased soil N availability, likely due to CO2- stimulation of symbiotic N2 fixation in soybean. Fungal biomass and the fungi:bacteria ratio decreased under both ambient and elevated CO2 by the third year and also coincided with increased soil N availability; but they were significantly higher under elevated than ambient CO2. These results suggest that more attention should be directed towards assessing the impact of N availability on microbial activities and decomposition in projections of soil organic C balance in N-rich systems under future CO2 scenarios. [ABSTRACT FROM AUTHOR]

Elevated atmospheric CO2 and ozone (O3) may affect productivity of legumes in part by altering symbiotic N2 fixation. To investigate this possibility, measurements of plant biomass, N levels and natural 15N abundance (δ15N) were used to examine the effects of elevated 002 and O3 on N acquisition in field-grown peanut (Arachis hypogaea L.) using open-top chambers. Seasonal 12-h daily average CO2 treatment concentrations were 376, 550, and 730 μmol mol1. Carbon dioxide treatments were applied in reciprocal combinations with seasonal 12-h daily average O3 concentrations of 21, 49, and 79 nmol mol-1. At mid-vegetative growth, elevated 002 significantly reduced leaf N concentrations by up to 44%, but not δ15N values. Elevated O3 did not significantly affect N concentrations or δ15N values. At harvest, plant N concentrations were similar among treatments except for a 14% reduction in the highest-level 002-O3 treatment. Plant N accumulation varied in proportion with treatment effects on biomass production, which was increased with elevated 002 when averaged over the O3 treatments and suppressed by high-level O3 at ambient CO2. Elevated 002 reduced plant δ15N values in low- and mid-level O3 treatments while mid- and high-level O3 increased them at ambient CO2. The changes in ö15N values suggested that N2 fixation activity was stimulated with elevated CO2 and inhibited by elevated O3. Elevated CO2 ameliorated detrimental O3 effects to varying extents depending on the concentrations of the two gases. These results indicated that interactions between CO2 and O3 on plant physiology can alter N acquisition processes, with impacts on peanut productivity likely dependent in part on these changes. [ABSTRACT FROM AUTHOR]

We examined the effects of elevated CO2 and O3 and their interaction on leaf litter chemistry and decomposition in pure stands of aspen ( Populus tremuloides) and mixed stands of birch ( Betula papyrifera) and aspen at the Aspen Free Air CO2 Enrichment (FACE) experiment. A 935-day in situ incubation study was performed using litterbags filled with naturally senesced leaf litter. We found that elevated CO2 had no overall effects on litter decomposition rates, whereas elevated O3 reduced litter mass loss (−13%) in the first year. The effect of O3 on mass loss disappeared in the second year. For aspen litter but not mixed birch-aspen litter, decomposition rates were negatively correlated with initial concentrations of condensed tannins and phenolics. Most soluble components (94% of soluble sugars, 99% of condensed tannins, and 91% of soluble phenolics) and any treatment effects on their initial concentrations disappeared rapidly. However, the mean residence time (MRT) of birch-aspen litter (3.1 years) was significantly lower than that of aspen litter (4.8 years). Further, because of variation in total litterfall, total litter mass, C, lignin and N remaining in the ecosystem was highest under elevated CO2 and lowest under elevated O3 during the incubation period. Our results indicate that elevated CO2 and O3 can alter short-term litter decomposition dynamics, but longer-term effects will depend more on indirect effects mediated through changes in forest community composition. Treatment effects on soluble components are likely to influence cyclical microbial processes and carbon pulses in the ecosystem only when coupled with increased (CO2) or decreased (O3) litter inputs. [ABSTRACT FROM AUTHOR]

Elevated CO2 has been shown to stimulate plant productivity and change litter chemistry. These changes in substrate availability may then alter soil microbial processes and possibly lead to feedback effects on N availability. However, the strength of this feedback, and even its direction, remains unknown. Further, uncertainty remains whether sustained increases in net primary productivity will lead to increased long-term C storage in soil. To examine how changes in litter chemistry and productivity under elevated CO2 influence microbial activity and soil C formation, we conducted a 230-day microcosm incubation with five levels of litter addition rate that represented 0, 0.5, 1.0, 1.4 and 1.8 × litterfall rates observed in the field for aspen stand growing under control treatments at the Aspen FACE experiment in Rhinelander, WI, USA. Litter and soil samples were collected from the corresponding field control and elevated CO2 treatment after trees were exposed to elevated CO2 (560 ppm) for 7 years. We found that small decreases in litter [N] under elevated CO2 had minor effects on microbial biomass carbon, microbial biomass nitrogen and dissolved inorganic nitrogen. Increasing litter addition rates resulted in linear increase in total C and new C (C from added litter) that accumulated in whole soil as well as in the high density soil fraction (HDF), despite higher cumulative C loss by respiration. Total N retained in whole soil and in HDF also increased with litter addition rate as did accumulation of new C per unit of accumulated N. Based on our microcosm comparisons and regression models, we expected that enhanced C inputs rather than changes in litter chemistry would be the dominant factor controlling soil C levels and turnover at the current level of litter production rate (230 g C m−2 yr−1 under ambient CO2). However, our analysis also suggests that the... [ABSTRACT FROM AUTHOR]

Soybean [Glycine max (L.) Merr.] cultivars Essex and Forrest that exhibit differences in ozone (O3) sensitivity were used in greenhouse experiments to investigate the role of leaf extracellular antioxidants in O3 injury responses. Charcoal-filtered air and elevated O3 conditions were used to assess genetic, leaf age, and O3 effects. In both cultivars, the extracellular ascorbate pool consisted of 80–98% dehydroascorbic acid, the oxidized form of ascorbic acid (AA) that is not an antioxidant. For all combinations of genotype and O3 treatments, extracellular AA levels were low (1–30nmolg−1 FW) and represented 3–30% of the total antioxidant capacity. Total extracellular antioxidant capacity was twofold greater in Essex compared with Forrest, consistent with greater O3 tolerance of Essex. The results suggest that extracellular antioxidant metabolites in addition to ascorbate contribute to detoxification of O3 in soybean leaves and possibly affect plant sensitivity to O3 injury. [Copyright &y& Elsevier]

Prior work showed that CO2 enhancement ratios (ER) were similar for plants grown in open-top chambers (OTCs) whether grown in the ground or in insulated containers aboveground. Per plant comparisons were suspect since the ground-grown plants were cultivated in rows at normal densities making it difficult to separate the effects of plant competition from the variables of interest. Soybean [Glycine max (L.) Merr. cv. Essex] was grown in the ground and in aboveground containers in OTCs in ambient and elevated CO2 at equal planting densities. The hypothesis was that at equal densities, container- and ground-grown plants would exhibit both equivalent ERs and equivalent per plant yields. Although the only differences in net photosynthetic rate (An:μmol m-2 s-1) and conductance to water vapor (gs:mol m-2 s-1) were due to CO2 and container- and ground-grown plants had similar ERs (mean = 20%), per plant yields were still less in the container-grown plants at both levels of CO2 (mean = -17%). Reproductive measures, except mass per seed, as well as total stem biomass were significantly reduced in the containers. High CO2 increased seed oil concentration and the level of fatty acid saturation. The only observed environmental difference was higher daytime root zone temperatures in containers (2-6°C). The robust ERs suggest that neither above- nor below-ground resource limitations was the cause of the yield discrepancies. [ABSTRACT FROM AUTHOR]

• The model of Farquhar, von Caemmerer and Berry is the standard in relating photosynthetic carbon assimilation and concentration of intercellular CO2. The techniques used in collecting the data from which its parameters are estimated have been the object of extensive optimization, but the statistical aspects of estimation have not received the same attention. • The model segments assimilation into three regions, each modeled by a distinct function. Three parameters of the model, namely the maximum rate of Rubisco carboxylation ( Vc max), the rate of electron transport ( J), and nonphotorespiratory CO2 evolution ( Rd), are customarily estimated from gas exchange data through separate fitting of the component functions corresponding to the first two segments. This disjunct approach is problematic in requiring preliminary arbitrary subsetting of data into sets believed to correspond to each region. • It is shown how multiple segments can be estimated simultaneously, using the entire data set, without predetermination of transitions by the investigator. • Investigation of the number of parameters that can be estimated in the two-segment model suggests that, under some conditions, it is possible to estimate four or even five parameters, but that only Vc max, J, and Rd, have good statistical properties. Practical difficulties and their solutions are reviewed, and software programs are provided. [ABSTRACT FROM AUTHOR]

The effects of elevated CO2 and ozone (O3) on net photosynthetic rate (A) and growth are generally antagonistic although plant responses are highly dependent on crop sensitivity to the individual gases and their concentrations. In this experiment, we evaluated the effects of various CO2 and O3 mixtures on leaf gas-exchange, harvest biomass, and leaf chemistry in peanut (Arachis hypogaea L.), an O3–sensitive species, using open-top field chambers. Treatments included ambient CO2 (about 375 μmol mol-1) and CO2 enrichment of approximately 173 and 355 μmol mol-1 in combination with charcoal-filtered air (22 nmol O3 mol-1), nonfiltered air (46 nmol O3 mol-1), and nonfiltered air plus O3 (75 nmol O3 mol-1). Twice-ambient CO2 in charcoal-filtered air increased A by 23% while decreasing seasonal stomatal conductance (gs) by 42%. Harvest biomass was increased 12 to 15% by elevated CO2. In ambient CO2, non-filtered air and added O3 lowered A by 21% and 48%, respectively, while added O3 reduced gs by 18%. Biomass was not significantly affected by nonfiltered air, but was 40% lower in the added O3 treatment. Elevated CO2 generally suppressed inhibitory effects of O3 on A and harvest biomass. Leaf starch concentration was increased by elevated CO2 and decreased by O3. Treatment effects on foliar N and total phenolic concentrations were minor. Increasing atmospheric CO2 concentrations should attenuate detrimental effects of ambient O3 and promote growth in peanut but its effectiveness declines with increasing O3 concentrations. [ABSTRACT FROM AUTHOR]

Many adverse effects of tropospheric O3 on C3 crop plants are ameliorated by elevated concentrations of atmospheric CO2, but the extent of the interaction can vary, depending on the species, gas concentrations, and other experimental conditions. A 2-yr open-top field chamber experiment was conducted to examine this interaction in peanut (Arachis hypogaea L.) by testing the effects of O3 and CO2 mixtures on yield and seed quality. Treatments were ambient CO2 (375 μmol mol-1) and CO2 additions of approximately 173 and 355 μmol mol-1 in combination with charcoal-filtered (CF) air (22 nmol O3 mol-1), nonfiltered (NE) air (46 nmol O3 mol-1), and NF air plus 03 (75 nmol O3 mol-1). At ambient CO2, pod number was suppressed 16% in NF air and 44% in elevated O3. Pod and seed mass were not significantly affected in NF air but were lowered 33 to 37% in elevated O3. Elevated CO2 increased yield parameters 7 to 17% for plants grown in CF air and restored yield in NF air and elevated O3 treatments to control or higher levels. Gas treatment effects on peanut market grade characteristics were small. No treatment effects were observed on the protein and oil contents of seeds, but there were changes in fatty acid composition. Overall results indicate that increasing concentrations of tropospheric O3 will suppress yield of O3–sensitive peanut cultivars, while elevated CO2 will moderate this response. Elevated O3 and CO2 are not expected to have major effects on peanut seed composition and quality. [ABSTRACT FROM AUTHOR]

Mycorrhizas are ubiquitous plant–fungus mutualists in terrestrial ecosystems and play important roles in plant resource capture and nutrient cycling. Sporadic evidence suggests that anthropogenic nitrogen (N) input may impact the development and the functioning of arbuscular mycorrhizal (AM) fungi, potentially altering host plant growth and soil carbon (C) dynamics. In this study, we examined how mineral N inputs affected mycorrhizal mediation of plant N acquisition and residue decomposition in a microcosm system. Each microcosm unit was separated into HOST and TEST compartments by a replaceable mesh screen that either prevented or allowed AM fungal hyphae but not plant roots to grow into the TEST compartments. Wild oat ( Avena fatua L.) was planted in the HOST compartments that had been inoculated with either a single species of AM fungus, Glomus etunicatum, or a mixture of AM fungi including G. etunicatum. Mycorrhizal contributions to plant N acquisition and residue decomposition were directly assessed by introducing a mineral 15N tracer and 13C-rich residues of a C4 plant to the TEST compartments. Results from 15N tracer measurements showed that AM fungal hyphae directly transported N from the TEST soil to the host plant. Compared with the control with no penetration of AM fungal hyphae, AM hyphal penetration led to a 125% increase in biomass 15N of host plants and a 20% reduction in extractable inorganic N in the TEST soil. Mineral N inputs to the HOST compartments (equivalent to 5.0 g N m−2 yr−1) increased oat biomass and total root length colonized by mycorrhizal fungi by 189% and 285%, respectively, as compared with the no-N control. Mineral N inputs to the HOST plants also reduced extractable inorganic N and particulate residue C proportion by 58% and 12%, respectively, in the corresponding TEST soils as compared to the no-N control, by stimulating AM fungal growth and activities. The species mixture of mycorrhizal fungi was more effective in facilitating N transport and residue decomposition than the single AM species. These findings indicate that low-level mineral N inputs may significantly enhance nutrient cycling and plant resource capture in terrestrial ecosystems via stimulation of root growth, mycorrhizal functioning, and residue decomposition. The long-term effects of these observed alterations on soil C dynamics remain to be investigated. [ABSTRACT FROM AUTHOR]

The inhibitory effects of tropospheric O3 on crop photosynthesis, growth, and yield have been documented in numerous studies over the past 35 years. In large part, the results of this research supported governmental regulations designed to limit tropospheric O3 levels to concentrations that affected crop production at economically acceptable levels. Recent studies have brought into question the efficacy of these concentration-based O3 standards compared with flux-based approaches that incorporate O3 uptake along with environmental and biotic factors that influence plant responses. In addition, recent studies provide insight into the biochemical mechanisms of O3 injury to plants. Current interpretations suggest that upon entry into the leaf intercellular space O3 rapidly reacts with components of the leaf apoplast to initiate a complex set of responses involving the formation of toxic metabolites and generation of plant defence responses that constitute variably effective countermeasures. Plant species and cultivars exhibit a range of sensitivity to O3, evident as heritable characteristics, that must reflect identifiable biochemical and molecular processes that affect sensitivity to O3 injury, although their exact makeup remains unclear. Ozone clearly impairs photosynthetic processes, which might include the effects on electron transport and guard cell homeostasis as well as the better-documented effects on carbon fixation via decreased Rubisco activity. Translocation of photosynthate could be inhibited by O3 exposure as well. Further, the influence of tropospheric O3 needs to be considered when assessing potential effects of rising concentrations of atmospheric CO2 on crop production. Advances in O3 flux modelling and improved understanding of biochemical and molecular effects of O3 on photosynthetic gas exchange and plant defence processes are leading to more complete, integrated assessments of O3 impacts on crop physiology that continue to support the rationale for maintaining or improving current O3 air quality standards as well as providing a basis for development of more O3-tolerant crop lines. [ABSTRACT FROM AUTHOR]

The projected rise in atmospheric CO2 concentration is expected to increase growth and yield of many agricultural crops. The magnitude of this stimulus will partly depend on interactions with other components of the atmosphere such as tropospheric O3. Elevated CO2 concentrations often lessen the deleterious effects of O3, but the mechanisms responsible for this response have received little direct examination. Previous studies have indicated that protection against O3 injury by elevated CO2 can be attributed to reduced O3 uptake, while other studies suggest that CO2 effects on anti-oxidant metabolism might also be involved. The aim of this experiment was to test further the roles of O3 flux and antioxidant metabolism in the suppression of O3 injury by elevated CO2. In a two-year experiment, soybean [Glycine max (L.) Merr.] was exposed from emergence to maturity to charcoal-filtered air or charcoal-filtered air plus a range of O3 concentrations in combination with ambient or approximately twice-ambient CO2 concentrations in open-top field chambers. Experimental manipulation of O3 concentrations and estimates of plant O3 uptake indicated that equivalent O3 fluxes that suppressed net photosynthesis, growth, and yield at ambient concentrations of CO2 were generally much less detrimental to plants treated concurrently with elevated CO2. These responses appeared unrelated to treatment effects on superoxide dismutase, glutathione reductase, and peroxidase activities and glutathione concentration. Total ascorbic acid concentration increased by 28–72% in lower canopy leaves in response to elevated CO2 and O3 but not in upper canopy leaves. Increasing concentrations of atmospheric CO2 will likely ameliorate O3 damage to many crops due to reduced O3 uptake, increased carbon assimilation, and possibly as yet undetermined additional factors. The results of this study further suggest that elevated CO2 may increase the threshold O3 flux for biomass and yield loss in soybean. [ABSTRACT FROM PUBLISHER]

In studies of CO2-enrichment effects on plants, the applicability of results derived from experiments using container-grown plants for predictions of future crop performance in a CO2-enriched atmosphere has been questioned. Concerns also have been expressed about plant growth studies with the air pollutant O3 in pot-grown plants. Further, since elevated CO2 and O3 co-occur, studies are required with the combination of gases. In this 2-yr experiment, soybean [Glycine max (L.) Merr.] plants grown in large pots (15 and 21 L) and in the ground were exposed to mixtures of CO2 and O3 in open-top chambers. The CO2 treatments were ambient and CO2 enrichment of approximately 337 μmol mol-1 added 24 h d-1. Ozone treatments were charcoalfiltered (CF) air (23 nmol mol-1) and approximately 1.5 times ambient O3 levels (71 nmol mol-1) given 12 h d-1. Relative effects of elevated CO2 and O3 on aboveground biomass and seed yield were quite similar for plants grown in pots compared with plants grown in the ground. Elevated COz increased total seed mass and O3 suppressed it to similar magnitudes in both rooting environments. Elevated CO2 also reduced the toxic effects of O3. Net photosynthesis (A) was similar while stomatal conductance (gs) was higher in pot-grown compared with ground-grown plants, possibly due to better soil moisture status. The results indicated that planting density and rooting environment affected plant morphology, but relative responses of seed yield to elevated CO2 and O3 were not fundamentally different between soybean plants grown in large pots and in the ground in open-top chambers. [ABSTRACT FROM AUTHOR]

A critical global climate change issue is how increasing concentrations of atmospheric CO2 and ground-level O3 will affect agricultural productivity. This includes effects on decomposition of residues left in the field and availability of mineral nutrients to subsequent crops. To address questions about decomposition processes, a 2-year experiment was conducted to determine the chemistry and decomposition rate of aboveground residues of soybean (Glycine max(L.) Merr.) grown under reciprocal combinations of low and high concentrations of CO2 and O3 in open-top field chambers. The CO2 treatments were ambient (370 μmol mol−1) and elevated (714 μmol mol−1) levels (daytime 12 h averages). Ozone treatments were charcoal-filtered air (21 nmol mol−1) and nonfiltered air plus 1.5 times ambient O3 (74 nmol mol−1) 12 h day−1. Elevated CO2 increased aboveground postharvest residue production by 28–56% while elevated O3 suppressed it by 15–46%. In combination, inhibitory effects of added O3 on biomass production were largely negated by elevated CO2. Plant residue chemistry was generally unaffected by elevated CO2, except for an increase in leaf residue lignin concentration. Leaf residues from the elevated O3 treatments had lower concentrations of nonstructural carbohydrates, but higher N, fiber, and lignin levels. Chemical composition of petiole, stem, and pod husk residues was only marginally affected by the elevated gas treatments. Treatment effects on plant biomass production, however, influenced the content of chemical constituents on an areal basis. Elevated CO2 increased the mass per square meter of nonstructural carbohydrates, phenolics, N, cellulose, and lignin by 24–46%. Elevated O3 decreased the mass per square meter of these constituents by 30–48%, while elevated CO2 largely ameliorated the added O3 effect. Carbon mineralization rates of component residues from the elevated gas treatments were not significantly different from the control. However, N immobilization increased in soils containing petiole and stem residues from the elevated CO2, O3, and combined gas treatments. Mass loss of decomposing leaf residue from the added O3 and combined gas treatments was 48% less than the control treatment after 20 weeks, while differences in decomposition of petiole, stem, and husk residues among treatments were minor. Decreased decomposition of leaf residues was correlated with lower starch and higher lignin levels. However, leaf residues only comprised about 20% of the total residue biomass assayed so treatment effects on mass loss of total aboveground residues were relatively small. The primary influence of elevated atmospheric CO2 and O3 concentrations on decomposition processes is apt to arise from effects on residue mass input, which is increased by elevated CO2 and suppressed by O3. [ABSTRACT FROM AUTHOR]

A critical global climate change issue is how increasing concentrations of atmospheric CO2 and ground-level O3 will affect agricultural productivity. This includes effects on decomposition of residues left in the field and availability of mineral nutrients to subsequent crops. To address questions about decomposition processes, a 2-year experiment was conducted to determine the chemistry and decomposition rate of aboveground residues of soybean (Glycine max(L.) Merr.) grown under reciprocal combinations of low and high concentrations of CO2 and O3 in open-top field chambers. The CO2 treatments were ambient (370 μmol mol−1) and elevated (714 μmol mol−1) levels (daytime 12 h averages). Ozone treatments were charcoal-filtered air (21 nmol mol−1) and nonfiltered air plus 1.5 times ambient O3 (74 nmol mol−1) 12 h day−1. Elevated CO2 increased aboveground postharvest residue production by 28–56% while elevated O3 suppressed it by 15–46%. In combination, inhibitory effects of added O3 on biomass production were largely negated by elevated CO2. Plant residue chemistry was generally unaffected by elevated CO2, except for an increase in leaf residue lignin concentration. Leaf residues from the elevated O3 treatments had lower concentrations of nonstructural carbohydrates, but higher N, fiber, and lignin levels. Chemical composition of petiole, stem, and pod husk residues was only marginally affected by the elevated gas treatments. Treatment effects on plant biomass production, however, influenced the content of chemical constituents on an areal basis. Elevated CO2 increased the mass per square meter of nonstructural carbohydrates, phenolics, N, cellulose, and lignin by 24–46%. Elevated O3 decreased the mass per square meter of these constituents by 30–48%, while elevated CO2 largely ameliorated the added O3 effect. Carbon mineralization rates of component residues from the elevated gas treatments were not significantly different from the control. However, N immobilization increased in soils containing petiole and stem residues from the elevated CO2, O3, and combined gas treatments. Mass loss of decomposing leaf residue from the added O3 and combined gas treatments was 48% less than the control treatment after 20 weeks, while differences in decomposition of petiole, stem, and husk residues among treatments were minor. Decreased decomposition of leaf residues was correlated with lower starch and higher lignin levels. However, leaf residues only comprised about 20% of the total residue biomass assayed so treatment effects on mass loss of total aboveground residues were relatively small. The primary influence of elevated atmospheric CO2 and O3 concentrations on decomposition processes is apt to arise from effects on residue mass input, which is increased by elevated CO2 and suppressed by O3. [ABSTRACT FROM AUTHOR]

Ribonucleases (RNases) degrade RNA and exert a major influence on gene expression during development and in response to biotic and abiotic stresses. RNase activity typically increases in response to pathogen attack, wounding and phosphate (Pi) deficiency. Activity also increases during senescence and other programmed cell death processes. The air pollutant ozone (O3) often induces injury and accelerated senescence in many plants, but the biochemical mechanisms involved in these responses remain unclear. The objective of this study was to determine whether RNase activity and isozyme expression was stimulated in wheat ( Triticum aestivum L.) flag leaves following treatment with O3. Plants were treated in open-top chambers with charcoal-filtered air (27 nmol O3 mol−1) (control) or non-filtered air plus O3 (90 nmol O3 mol−1) (O3) from seedling to reproductive stage. After exposure for 56 days, RNase activity was 2.1 times higher in flag leaf tissues from an O3-sensitive cultivar in the O3 treatment compared with the control, which generally coincided with foliar injury and lower soluble protein concentration, but not soluble leaf [Pi]. Soluble [Pi] in leaf tissue extracts from the O3 and control treatments was not significantly different. RNase activity gels indicated the presence of three major RNases and two nucleases, and their expression was enhanced by the O3 treatment. Isozymes stimulated in the O3 treatment were also stimulated in naturally senescent flag leaf tissues from plants in the control. However, soluble [Pi] in extracts from naturally senescent flag leaves was 50% lower than that found in green flag leaves in the control treatment. Thus, senescence-like pathological responses induced by O3 were accompanied by increased RNase and nuclease activities that also were observed in naturally senescent leaves. However, [Pi] in the leaf tissue samples suggested that O3-induced injury and accelerated senescence was atypical of normal senescence processes in that Pi export was not observed in O3-treated plants. [ABSTRACT FROM AUTHOR]

Concentrations of total soluble phenolics, catechin, proanthocyanidins (PA), lignin and nitrogen (N) were measured in loblolly pine (Pinus taeda L.) needles exposed to either ambient CO2 concentration ([CO2]), ambient plus 175 or ambient plus 350 μmol CO2 mol−1 in branch chambers for 2 years. The CO2 treatments were superimposed on a 2 × 2 factorial combination of irrigation and fertilization treatments. In addition, we compared the effects of branch chambers and open-top chambers on needle chemistry. Proanthocyanidin and N concentrations were measured in needles from branch chambers and from trees in open-top chambers exposed concurrently for 2 years to either ambient [CO2] or ambient plus 200 μmol CO2 mol−1 in combination with a fertilization treatment. In the branch chambers, concentrations of total soluble phenolics in needles generally increased with needle age. Concentrations of total soluble phenolics, catechin and PA in needle extracts increased about 11% in response to the elevated [CO2] treatments. There were no significant treatment effects on foliar lignin concentrations. Nitrogen concentrations were about 10% lower in needles from the elevated [CO2] treatments than in needles from the ambient [CO2] treatments. Soluble phenolic and PA concentrations were higher in the control and irrigated soil treatments in about half of the comparisons; otherwise, differences were not statistically significant. Needle N concentrations increased 23% in response to fertilization. Treatment effects on PA and N concentrations were similar between branch and open-top chambers, although in this part of the study N concentrations were not significantly affected by the CO2 treatments in either the branch or open-top chambers. We conclude that elevated [CO2] and low N availability affected foliar chemical composition, which could in turn affect plant–pathogen interactions, decomposition rates and mineral nutrient cycling. [ABSTRACT FROM PUBLISHER]

Southern pine beetles and associated pathogenic fungi represent the largest biotic threat to pine forests in the southeastern USA. The two primary defensive mechanisms of the tree to the beetle–fungal complex are the primary oleoresin flow and the concentrations of preformed and induced secondary compounds. We compared oleoresin flow and concentrations of phloem nutrients, soluble sugars, starch, total phenolics and proanthocyanidins in Pinus taeda L. trees in fertilized and control plots in the Sandhills region of North Carolina. Four blocks of 10 trees per treatment were sampled on five dates from May to November 1995. Phloem nitrogen and potassium concentrations were elevated in trees on fertilized plots, whereas phloem calcium concentrations were decreased. Fertilization significantly enhanced (10–20%) concentrations of phloem phenolics and proanthocyanidins. In contrast, phloem soluble sugars and starch concentrations were up to 30% lower in fertilized trees than in control trees. Increased phenolic concentrations and lower nonstructural carbohydrates should correlate with reduced tissue palatability and decreased pathogen susceptibility in fertilized trees; however, resin flows were significantly lower (30–100%) in fertilized trees compared with control trees, which may facilitate pine bark beetle establishment. Furthermore, fertilization-induced increases in phloem nitrogen concentration may be more important than tissue carbohydrate or phenolic content in determining tissue palatability. [ABSTRACT FROM AUTHOR]