Does oxygen limit nitrogenase activity in soybean exposed to elevated CO2?

Soybean (Glycine max L. Merr) plants grown under control (360 µmol mol^?1) or elevated CO₂ concentration (800 µmol mol^?1) from 33 to 42 d after sowing were assayed for various components of in vivo nitrogenase activity to test the hypothesis that increasing carbohydrate supply to nodules would increase the potential (i.e. O₂ saturated) nitrogenase activity and impose a more severe O₂ limitation on both nodule metabolism and total nitrogenase activity. Within 51 h of elevated CO₂ treatment, significant increases relative to control plants were seen in total nitrogenase activity expressed per plant. After 6 d of elevated CO₂, the total nitrogenase activity per plant was 18% higher than that in control. This was attributed to an initial increase in nodule size, and a subsequent increase in nodule number following plant exposure to elevated CO₂. However, after 9 d of elevated CO₂, the potential and total nitrogenase activities per gram nodule dry weight were lower, not higher than corresponding values in plants in the control treatment. These results did not support the hypothesis. It was concluded that the metabolic capacity of the control nodules were not limited by carbohydrate supply, at least at the assay temperatures employed here.     

Does long-term cultivation of saplings under elevated CO2 concentration influence their photosynthetic response to temperature?

Background and Aims Plants growing under elevated atmospheric CO₂ concentrations often have reduced stomatal conductance and subsequently increased leaf temperature. This study therefore tested the hypothesis that under long-term elevated CO₂ the temperature optima of photosynthetic processes will shift towards higher temperatures and the thermostability of the photosynthetic apparatus will increase.Methods The hypothesis was tested for saplings of broadleaved Fagus sylvatica and coniferous Picea abies exposed for 4–5 years to either ambient (AC; 385 µmol mol⁻¹) or elevated (EC; 700 µmol mol⁻¹) CO₂ concentrations. Temperature response curves of photosynthetic processes were determined by gas-exchange and chlorophyll fluorescence techniques.Key Results Initial assumptions of reduced light-saturated stomatal conductance and increased leaf temperatures for EC plants were confirmed. Temperature response curves revealed stimulation of light-saturated rates of CO₂ assimilation (A_max) and a decline in photorespiration (R_L) as a result of EC within a wide temperature range. However, these effects were negligible or reduced at low and high temperatures. Higher temperature optima (T_opt) of A_max, Rubisco carboxylation rates (V_Cmax) and R_L were found for EC saplings compared with AC saplings. However, the shifts in T_opt of A_max were instantaneous, and disappeared when measured at identical CO₂ concentrations. Higher values of T_opt at elevated CO₂ were attributed particularly to reduced photorespiration and prevailing limitation of photosynthesis by ribulose-1,5-bisphosphate (RuBP) regeneration. Temperature response curves of fluorescence parameters suggested a negligible effect of EC on enhancement of thermostability of photosystem II photochemistry.Conclusions Elevated CO₂ instantaneously increases temperature optima of A_max due to reduced photorespiration and limitation of photosynthesis by RuBP regeneration. However, this increase disappears when plants are exposed to identical CO₂ concentrations. In addition, increased heat-stress tolerance of primary photochemistry in plants grown at elevated CO₂ is unlikely. The hypothesis that long-term cultivation at elevated CO₂ leads to acclimation of photosynthesis to higher temperatures is therefore rejected. Nevertheless, incorporating acclimation mechanisms into models simulating carbon flux between the atmosphere and vegetation is necessary.     

Does elevated atmospheric CO2 concentration inhibit mitochondrial respiration in green plants?

ATP, adenosine triphosphate
K_m, Michaelis-Menton coefficient
C_a, concentration of CO₂ in the air (μmol mol^–1)
NAD, oxidized nicotin adenine dinucleotide
NADH, reduced nicotin adenine dinucleotide
NADP, oxidized nicotin adenine phosphate dinucleotide
NADPH, reduced nicotine adenine phosphate dinucleotide
R, rate of respiration per unit DW [μmol g
DW^–1], Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase
V_c,_max, maximum in vivo rate of carboxylation at Rubisco (μmol m^–2 s^–1)

There is abundant evidence that a reduction in mitochondrial respiration of plants occurs when atmospheric CO₂ (C_a) is increased. Recent reviews suggest that doubling the present C_a will reduce the respiration rate [per unit dry weight (DW)] by 15 to 18%. The effect has two components: an immediate, reversible effect observed in leaves, stems, and roots of plants as well as soil microbes, and an irreversible effect which occurs as a consequence of growth in elevated C_a and appears to be specific to C₃ species. The direct effect has been correlated with inhibition of certain respiratory enzymes, namely cytochrome-c-oxidase and succinate dehydrogenase, and the indirect or acclimation effect may be related to changes in tissue composition. Although no satisfactory mechanisms to explain these effects have been demonstrated, plausible mechanisms have been proposed and await experimental testing. These are carbamylation of proteins and direct inhibition of enzymes of respiration. A reduction of foliar respiration of 15% by doubling present ambient C_a would represent 3 Gt of carbon per annum in the global carbon budget.     

Does photosynthetic acclimation to elevated CO2 increase photosynthetic nitrogen-use efficiency? A study of three native UK grassland species in open-top chambers

1. The photosynthetic response to elevated CO₂ and nutrient stress was investigated in Agrostis capillaris, Lolium perenne and Trifolium repens grown in an open-top chamber facility for 2 years under two nutrient regimes. Acclimation was evaluated by measuring the response of light-saturated photosynthesis to changes in the substomatal CO₂ concentration.
2. Growth at elevated CO₂ resulted in reductions in apparent Rubisco activity in vivo in all three species, which were associated with reductions of total leaf nitrogen content on a unit area basis for A. capillaris and L. perenne . Despite this acclimation, photosynthesis was significantly higher at elevated CO₂ for T. repens and A. capillaris , the latter exhibiting the greatest increase of carbon uptake at the lowest nutrient supply.
3. The photosynthetic nitrogen-use efficiency (the rate of carbon assimilation per unit leaf nitrogen) increased at elevated CO₂, not purely owing to higher values of photosynthesis at elevated CO₂, but also as a result of lower leaf nitrogen contents.
4. Contrary to most previous studies, this investigation indicates that elevated CO₂ can stimulate photosynthesis under a severely limited nutrient supply. Changes in photosynthetic nitrogen-use efficiency may be a critical determinant of competition within low nutrient ecosystems and low input agricultural systems.     

Does soil CO2 efflux acclimatize to elevated temperature and CO2 during long-term treatment of Douglas-fir seedlings?

We investigated the effects of elevated soil temperature and atmospheric CO2 on soil CO2 efflux (SCE) during the third and fourth years of study. We hypothesized that elevated temperature would stimulate SCE, and elevated CO2 would also stimulate SCE with the stimulation being greater at higher temperatures. The study was conducted in sun-lit controlled-environment chambers using Douglas-fir (Pseudotsuga menziesii) seedlings grown in reconstructed litter-soil systems. We used a randomized design with two soil temperature and two atmospheric CO2 treatments. The SCE was measured every 4 wk for 18 months. Neither elevated temperature nor CO2 stimulated SCE. Elevated CO2 increased the temperature sensitivity of SCE. During the winter, the relationship between SCE and soil moisture was negative but it was positive during the summer. The seasonal patterns in SCE were associated with seasonal changes in photosynthesis and above-ground plant growth. SCE acclimatized in the high-temperature treatment, probably because of a loss of labile soil carbon. Elevated CO2 treatment increased the temperature sensitivity of SCE, probably through an increase in substrate availability.     

Does night-time transpiration provide any benefit to wheat (Triticum aestivum L.) plants which are exposed to salt stress?

The study aimed to test whether night-time transpiration provides any potential benefit to wheat plants which are subjected to salt stress. Hydroponically grown wheat plants were grown at four levels of salt stress (50, 100, 150, and 200 mM NaCl) for 5–8 days prior to harvest (day 14–18). Salt stress caused large decreases in transpiration and leaf elongation rates during day and night. The quantitative relation between the diurnal use of water for transpiration and leaf growth was comparatively little affected by salt. Night-time transpirational water loss occurred predominantly through stomata in support of respiration. Diurnal gas exchange and leaf growth were functionally linked to each other through the provision of resources (carbon, energy) and an increase in leaf surface area. Diurnal rates of water use associated with leaf cell expansive growth were highly correlated with the water potential of the xylem, which was dominated by the tension component. The tissue-specific expression level of nine candidate aquaporin genes in elongating and mature leaf tissue was little affected by salt stress or day/night changes. Growing plants under conditions of reduced night-time transpirational water loss by increasing the relative humidity (RH) during the night to 95% had little effect on the growth response to salt stress, nor was the accumulation of Na⁺ and Cl⁻ in shoot tissue altered. We conclude that night-time gas exchange supports the growth in leaf area over a 24 h day/night period. Night-time transpirational water loss neither decreases nor increases the tolerance to salt stress in wheat.     

Does down‐regulation of photosynthetic capacity by elevated CO2 depend on N supply in Dactylis glomerata?

Dactylis glomerata was grown hydroponically in a controlled environment at ambient (360 μl l⁻¹) or elevated (680 μl l⁻¹) CO₂ and four concentrations of nitrogen (0.15, 0.6, 1.5 and 6.0 m M NO₃⁻), to test the hypothesis that reduction of photosynthetic capacity at elevated [CO₂] is dependent on N availability and mediated by a build-up of non-structural carbohydrates. Photosynthetic capacity of the youngest fully expanded leaf (leaf 5, 2 days after full expansion) was reduced in CO₂-enriched plants at low, but not high N supply and so the stimulation of net photosynthesis by CO₂ enhancement was less at low than at high N supply. CO₂ enrichment resulted in a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content on a leaf area basis at 0.6 and 1.5 m M NO₃⁻, but not at 0.15 and 6.0 m M NO₃⁻, and had no effect on the total N content of the leaf on an area basis. However, decreases in Rubisco content could be primarily accounted for by a decrease in total N content of leaves, independent of [CO₂]. A doubling of the Rubisco content by increasing the N supply beyond 0.6 m M had only a marginal effect on the maximum carboxylation velocity in vivo, suggesting that the fraction of inactive Rubisco increased with increasing N supply. Although CO₂-enriched plants accumulated more non-structural carbohydrates in the leaf, the reduction of photosynthetic capacity at low N supply was not mediated simply by a build-up of carbohydrates. In D . glomerata , the photosynthetic capacity was mainly determined by the total N content of the leaf.     

Does a terminal drought tolerance QTL contribute to differences in ROS scavenging enzymes and photosynthetic pigments in pearl millet exposed to drought?

The control of reactive oxygen species (ROS) and the stability of photosynthetic pigments under stress conditions are hypothesized to contribute to drought tolerance. Here we studied how ascorbic peroxidase (APX), superoxide dismutase (SOD), catalase (CAT) isozyme activities and chlorophyll a, b (Chl a, b) and carotenoids (Car) contents responded to water stress and whether they related to presence of a terminal drought tolerance QTL in pearl millet. We used PRLT2/89-33 (QTL donor), H77/833-2 (sensitive), and near-isogenic lines (QTL-NILs) introgressed with the QTL in H77/833-2 background. Under water stress there was no significant change in the total APX activity; only the proportional APX5 activity increased, with higher band intensity in tolerant genotypes. There were no significant changes in total activities of CAT and SOD under water stress, with similar band intensities in all genotypes, and a new CAT isozyme was induced in all genotypes. The photosynthetic pigment content decreased under water stress, although not differently in any genotype. Under water stress, the activities of most APX, CAT and SOD isozymes were closely related to the total chlorophyll/carotenoids ratio. Overall, besides APX5, water stress did not lead to major changes in the profile of isoenzymes involved in ROS scavenging. Similarly, the pigment content under stress did not discriminate genotypes according to the presence/absence of the QTL. This absence of discrimination for the ROS scavenging enzymes and for the pigment content under stress suggests that these traits may not play a key role in terminal drought tolerance in pearl millet.     

Will elevated CO2 concentrations protect the yield of wheat from O3 damage?

This study investigated the interacting effects of carbon dioxide and ozone concentrations on the growth and yield of spring whet (Triticum aestivum L. cv. Wembley). Plants were exposed from time of sowing to harvest to reciprocal combinations of two carbon dioxide and two ozone treatments: [CO₂] at 350 or 700 μmol mol^?1, and [O₃] at < 5 or 60 nmol mol^?1. Records of leaf emergence, leaf duration and tillering were taken throughout leaf development. At harvest, biomass, yield and partitioning were analysed. Our data showed that elevated [CO₂] fully protected against the detrimental effect of elevated [O₃] on biomass, but not yield.     

How do aphids respond to elevated CO2?

The performance of herbivore insects is determined directly by the quality of host plants. Elevated CO₂ induced a decline in foliar nitrogen, which reduced the growth of chewing insects. Phloem-sucking insects (i.e. aphid), however, had species-specific responses to elevated CO₂ and were the only feeding guild to respond positively to elevated CO₂. Although many studies attempt to illuminate the interaction between aphids and plants under elevated CO₂, few studies can explain why some aphids are more successful than other chewing insects in elevated CO₂. Elevated CO₂ leads to a re-allocation of the carbon and nitrogen resources in plant tissue, which increases the thickness of the microscopic structures of leaves, reduces amino acids content of leaf phloem sap and increases the secondary metabolites. Considering the complexity of aphid–plant interactions, it is difficult and unreasonable to predict the general response of aphids to elevated CO₂ using a single plant component. Instead, it is more likely that aphids are able to overcome the disadvantages of the indirect effects of elevated CO₂ by reducing developmental times and increasing fecundity under elevated CO₂ conditions. Our results provide several clues to why some aphids are successful in elevated CO₂ conditions. We review recent studies of the effects of elevated CO₂ on aphids and discuss the effects of elevated CO₂ on aphid performance on crops using cotton and cereal aphids as examples.     

Does elevated atmospheric CO2 concentrations affect wood decomposition?

This study was conducted to test the hypothesis that wood tissues generated under elevated atmospheric [CO₂] have lower quality and subsequent reduced decomposition rates. Chemical composition and subsequent field decomposition rates were studied for beech (Fagus sylvatica L.) twigs grown under ambient and elevated [CO₂] in open top chambers. Elevated [CO₂] significantly affected the chemical composition of beech twigs, which had 38% lower N and 12% lower lignin concentrations than twigs grown under ambient [CO₂]. The strong decrease in N concentration resulted in a significant increase in the C/N and lignin/N ratios of the beech wood grown at elevated [CO₂]. However, the elevated [CO₂] treatment did not reduce the decomposition rates of twigs, neither were the dynamics of N and lignin in the decomposing beech wood affected by the [CO₂] treatment, despite initial changes in N and lignin concentrations between the ambient and elevated [CO₂] beech wood. This revised version was published online in June 2006 with corrections to the Cover Date.     

Does the growth response of woody plants to elevated CO2 increase with temperature? A model‐oriented meta‐analysis

The temperature dependence of the reaction kinetics of the Rubisco enzyme implies that, at the level of a chloroplast, the response of photosynthesis to rising atmospheric CO₂ concentration (C_a) will increase with increasing air temperature. Vegetation models incorporating this interaction predict that the response of net primary productivity (NPP) to elevated CO₂ (eC_a) will increase with rising temperature and will be substantially larger in warm tropical forests than in cold boreal forests. We tested these model predictions against evidence from eC_a experiments by carrying out two meta‐analyses. Firstly, we tested for an interaction effect on growth responses in factorial eC_a × temperature experiments. This analysis showed a positive, but nonsignificant interaction effect (95% CI for above‐ground biomass response = ?0.8, 18.0%) between eC_a and temperature. Secondly, we tested field‐based eC_a experiments on woody plants across the globe for a relationship between the eC_a effect on plant biomass and mean annual temperature (MAT). This second analysis showed a positive but nonsignificant correlation between the eC_a response and MAT. The magnitude of the interactions between CO₂ and temperature found in both meta‐analyses were consistent with model predictions, even though both analyses gave nonsignificant results. Thus, we conclude that it is not possible to distinguish between the competing hypotheses of no interaction vs. an interaction based on Rubisco kinetics from the available experimental database. Experiments in a wider range of temperature zones are required. Until such experimental data are available, model predictions should aim to incorporate uncertainty about this interaction.     

Do zwitterions contribute to the ionic strength of a solution?

Capillary electrophoresis has been used to determine whether zwitterions contribute to the ionic strength of a solution, by measuring the mobility of a double-stranded DNA oligomer in cacodylate-buffered solutions containing various concentrations of the ionic salt tetraethylammonium chloride (TEA(+)Cl(-)) or the zwitterion tricine(+/-). The mobility of the DNA decreased as the square root of ionic strength, as expected from the Debye-Hückel-Onsager theory of electrophoresis, when TEA(+)Cl(-) was added to the buffer. However, the mobility was independent of the concentration of added tricine(+/-). Hence, zwitterions do not contribute to the ionic strength of a solution.     

Does vertical transmission contribute to the prevalence of toxoplasmosis?

Toxoplasma gondii is a ubiquitous parasite with a widespread distribution both in terms of geographical and host range. Although the definitive host is the cat, it is also a major health hazard to domestic animals and humans. Three routes of transmission are recognised (infection from the cat, carnivory and congenital transmission). We aimed to assess the relative importance of congenital transmission, using sheep as a model system, due to the lack of carnivory. We report, using PCR as a diagnostic tool, that congenital transmission occurs with high frequency (69%). If transmission from oocysts was important in sheep, we would expect sheep reared under the same environmental conditions (i.e. a single farm) to have a random distribution of Toxoplasma infection. Using breeding records in conjunction with PCR, some families were found to have high Toxoplasma prevalence and abortion while others were free of Toxoplasma infection and abortion (P < 0.01). This supports the notion that Toxoplasma may be transmitted vertically. In humans, we conducted a similar study and showed that Toxoplasma was transmitted from mother to baby in 19.8% of cases. Vertical transmission in Toxoplasma may be more important than previously thought and this knowledge should be considered in any eradication strategies.     

Does growth under elevated CO2 moderate photoacclimation in rice?

Acclimation of plant photosynthesis to light irradiance (photoacclimation) involves adjustments in levels of pigments and proteins and larger scale changes in leaf morphology. To investigate the impact of rising atmospheric CO₂ on crop physiology, we hypothesize that elevated CO₂ interacts with photoacclimation in rice (Oryza sativa). Rice was grown under high light (HL: 700 µmol m^?2 s^?1), low light (LL: 200 µmol m^?2 s^?1), ambient CO₂ (400 µl l^?1) and elevated CO₂ (1000 µl l^?1). Leaf six was measured throughout. Obscuring meristem tissue during development did not alter leaf thickness indicating that mature leaves are responsible for sensing light during photoacclimation. Elevated CO₂ raised growth chamber photosynthesis and increased tiller formation at both light levels, while it increased leaf length under LL but not under HL. Elevated CO₂ always resulted in increased leaf growth rate and tiller production. Changes in leaf thickness, leaf area, Rubisco content, stem and leaf starch, sucrose and fructose content were all dominated by irradiance and unaffected by CO₂. However, stomata responded differently; they were significantly smaller in LL grown plants compared to HL but this effect was significantly suppressed under elevated CO₂. Stomatal density was lower under LL, but this required elevated CO₂ and the magnitude was adaxial or abaxial surface‐dependent. We conclude that photoacclimation in rice involves a systemic signal. Furthermore, extra carbohydrate produced under elevated CO₂ is utilized in enhancing leaf and tiller growth and does not enhance or inhibit any feature of photoacclimation with the exception of stomatal morphology.     

Does the different photosynthetic pathway of plants affect soil respiration in a subtropical wetland?

Plants with different photosynthetic pathways could produce different amounts and types of root exudates and debris which may affect soil respiration rates. Therefore, wetland vegetation succession between plants with different photosynthetic pathways may ultimately influence the wetland carbon budget. The middle and lower reaches of the Yangtze River has the largest floodplain wetland group in China. Tian'e Zhou wetland reserve (29°48'N, 112°33′E) is located in Shishou city, Hubei province and covers about 77.5 square kilometers. Hemathria altissima (C4) was found gradually being replaced by Carex argyi (C3) for several years in this place. An in situ experiment was conducted in Tian'e Zhou wetland to determine the change of soil respiration as the succession proceeds. Soil respiration, substrate‐induced respiration, and bacterial respiration of the C4 species was greater than those of the C3 species, but below‐ground biomass and fungal respiration of the C4 species was less than that of the C3 species. There were no significant differences in above‐ground biomass between the two species. Due to the higher photosynthesis capability, higher soil respiration and lower total plant biomass, we inferred that the C4 species, H. altissima, may transport more photosynthate below‐ground as a substrate for respiration. The photosynthetic pathway of plants might therefore play an important role in regulating soil respiration. As C. argyi replaces H. altissima, the larger plant biomass and lower soil respiration would indicate that the wetland in this area could fix more carbon in the soil than before.     

Does energy intensity contribute to CO2 emissions? A trivariate analysis in selected African countries

The present study investigates the dynamic relationship between energy intensity and CO₂ emissions by incorporating economic growth in environment CO₂ emissions function using data of Sub Saharan African countries. For this purpose, we applied panel cointegration to examine the long run relationship between the series. We employed the VECM Granger causality to test the direction of causality amid the variables.At panel level, our results validate the existence of cointegration among the series. The long run panel results show that energy intensity has positive and statistically significant impact on CO₂ emissions. There is also positive and negative link of non-linear and linear terms of real GDP per capita with CO₂ emissions supporting the presence of environmental Kuznets curve (EKC). The causality analysis reveals the bidirectional causality between economic growth and CO₂ emissions while energy intensity Granger causes economic growth and hence CO₂ emissions, while across the individual countries, the results differ. This paper opens up new insights for policy makers to design comprehensive economic, energy and environmental policy for sustainable long run economic growth.     

Does neuropeptide Y contribute to the anorectic action of amylin?

Neuropeptide Y (NPY) is a potent feeding stimulant acting at the level of the hypothalamus. Amylin, a peptide co-released with insulin from pancreatic beta cells, inhibits feeding following peripheral or central administration. However, the mechanism by which amylin exerts its anorectic effect is controversial. This study investigated the acute effect of amylin on food intake induced by NPY, and the effect of chronic amylin administration on food intake and body weight in male Sprague Dawley rats previously implanted with intracerebroventricular (icv) cannulae. Rats received 1 nmol NPY, followed by amylin (0.05, 0.1, 0.5 nmol) or 2 microl saline. Increasing doses of amylin resulted in a dose-dependent inhibition of NPY-induced feeding by 31%, 74% and 99%, respectively (P < 0.05). To determine the chronic effects of i.c.v. amylin administration on feeding, rats received 0.5 nmol amylin or saline daily, 30 min before dark phase, over 6 days. Amylin significantly reduced food intake at 1, 4, 16 and 24 hours; after 6 days, amylin-treated rats showed a significant reduction in body weight, having lost 17.3 +/- 6.1 g, while control animals gained 7.7 +/- 5.1 g (P < 0.05). Brain NPY concentrations were not elevated, despite the reduced food intake, suggesting amylin may regulate NPY production or release. Thus, amylin potently inhibits NPY-induced feeding and attenuates normal 24 hour food intake, leading to weight loss.