The effect of ammonium and nitrate on carbondioxide compensation point and enzymes associated with carbondioxide exchange in wheat

The carbondioxide compensation point (), dry matter production, and the activities of nitrate reductase (NR), glycolate oxidase (GO), ribulose 1,5-bisphosphate carboxylase (RuBPC) and phosphoenolpyruvate carboxylase (PEPC) were measured in wheat, grown on media, containing nitrate or ammonium. Significantly higher and lower dry matter was observed in plants supplied with ammonium-nitrogen (NH₄-N), as compared to those supplied with nitrate-nitrogen (NO₃-N). The activities of NR and PEPC were higher in plants grown on NO₃-N than to those grown on NH₄-N. There were no significant differences in the activities of GO and RuBPC irrespective of whether NO₃-N or NH₄-N was supplied. None of the enzymes was found to be associated directly with the .PEPC activity accounted the measured differences in the and biomass production between NH₄-N and NO₃-N supplied plants. The relationship between PEPC and the is discussed.     

Effect of withdrawal of phosphorus on nitrate assimilation and PEP carboxylase activity in tomato

Tomato plants (Lycopersicon esculentum) grown in a complete nutrient solution for 8 days were transferred to a P-free solution of pH 6.0. Within 2 days of transfer the rate of alkalinization of the nutrient solution declined and by 4 days the solution had become acid. Nitrate transferred from roots to leaves was depressed over this period, and the rate of nitrate reductase activity in the leaves (the main site of assimilation of nitrate in tomato) had declined by 60% within 5 days of transfer. The activity of PEP carboxylase in the leaves of the P-deficient plants increased after 3 days, eventually becoming 3 times greater than in the leaves of plants adequately supplied with P. The PEP carboxylase activity in the roots of the P-deficient plants increased within 2 days, becoming 4 times greater after 8 days' growth. These results are discussed in relation to mechanisms for enhancement of P acquisition and maintenance of cation and anion uptake during P-deficiency.     

Effects of phosphorus nutrition on the response of photosynthesis to CO2 and O2, activation of ribulose bisphosphate carboxylase and amounts of ribulose bisphosphate and 3-phosphoglycerate in spinach leaves

Phosphorus-deficient spinach plants were grown by transferring them to nutrient solutions without PO₄. Photosynthetic rates were measured at a range of intercellular CO₂ partial pressures from 50–500 bar and then the leaves were freeze-clamped in situ to measure ribulose bisphosphate carboxylase (Rubisco) activity and metabolite concentrations. Compared with control leaves, deficient leaves had significantly lower photosynthetic rates, percentage activation of Rubisco, and amounts of ribulose bisphosphate and 3-phosphoglycerate at all CO₂ partial pressures. After feeding 10 mM PO₄ to the petioles of detached deficient leaves, all these measurements increased within 2 hours. At atmospheric CO₂ partial pressure the photosynthetic rate was stimulated in 19 mbar O₂ compared with 200 mbar. At higher CO₂ partial pressures this stimulation was less but the percentage stimulation in deficient leaves was no different from controls in either CO₂ partial pressure. It was concluded that phosphorus deficiency affects both Rubisco activity and the capacity for ribulose bisphosphate regeneration, and possible causes are discussed.Abbreviations A CO₂ assimilation rate - C_i intercellular CO₂ partial pressure - PGA 3-phosphoglycerate - RuP₂ ribulose 1,5-bisphosphate - Rubisco RuP₂ carboxylase/oxygenase     

开放式空气二氧化碳浓度增高对小麦物质生产与分配的影响

2001—2003年,利用农田开放式空气CO₂浓度增高 (FACE) 技术平台,以冬小麦宁麦9号为供试材料,研究开放式条件下CO₂浓度增高对小麦整个生育期干物质生产与分配的影响.结果表明：与对照相比,FACE处理使小麦播种-越冬始期的干物质生产量略有增加（10.8%),使越冬始期-拔节期、拔节期-孕穗期、孕穗期-抽穗期显著增加,分别增加了31.6%、40.5%、27.2%,使抽穗期-成熟期略有减少(-5.5%),使成熟期生物产量显著增加(13.6%);FACE处理对小麦播种-越冬始期的平均叶面积系数(LAI)和净同化率(NAR)均无显著影响,但使越冬始期-抽穗期LAI显著增加,NAR稍有增加,使抽穗期-抽穗后20 d NAR显著下降;FACE处理使不同生育时期叶片占全株质量的比例下降,而使茎鞘占全株质量的比例增加;FACE小麦抽穗期和成熟期茎鞘可溶性糖和淀粉含量及总量均明显增加.     

Ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxylase activities of photoautotrophic callus of Platycerium coronarium (Koenig ex O.F. Muell.) Desv. under CO2 enrichment

The in vitro activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) were measured in cell-free extracts of Platycerium coronarium callus cultured for up to 42 days under photoautotrophic conditions with CO₂ enrichment. With an increase in CO₂ in the culture environment to 10% (v/v) at low light, the apparent photoautotrophic fixation of CO₂ by Rubisco declined, whereas the non-photoautotrophic CO₂ fixation by PEPC activity was enhanced. Hence, photosynthesis appears to play a lesser role in providing carbon skeletons and energy with prolonged culture in a CO₂-enriched environment. Instead, the anaplerotic supply of C-skeletons by PEPC may be important under such a situation. Short-term H¹⁴CO₃-fixation experiments indicated that photoautotrophic callus cultured for 3 weeks with 10% CO₂ enrichment assimilated less ¹⁴CO₂ than the control (0.03% CO₂). Analyses of ¹⁴C-metabolites indicated that about 50% of the total soluble ¹⁴CO₂ fixed was in the organic acid fraction and 35% in the amino acid fraction. Despite the changes in the in vitro Rubisco/PEPC activity-ratio, no significant change in the ¹⁴C distribution pattern was apparent in response to increasing sucrose or CO₂ concentrations. The suppression of Rubisco activity and total chlorophyll content in high sucrose or elevated CO₂ concentrations suggests an inhibition of the capacity for photoautotrophic callus growth under these conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil drying and its effect on leaf conductance and CO2 assimilation of Vigna unguiculata (L.) Walp

Summary Well watered plants of Vigna unguiculata (L.) Walp cv. California Blackeye No. 5 had maximum photosynthetic rates of 16 mol m^-2 s^-1 (at ambient CO₂ concentration and environmental parameters optimal for high CO₂ uptake). Leaf conductance declined with increasing water vapour concentration difference between leaf and air (w), but it increased with increasing leaf temperature at a constant small w. When light was varied, CO₂ assimilation and leaf conductance were correlated linearly. We tested the hypothesis that g was controlled by photosynthesis via intercellular CO₂ concentration (c _i). No unique relationship between (1) c _i, (2) the difference between ambient CO₂ concentration (c _a) and c _i, namely c _a-c _i, or (3) the c _i/c _a ratio and g was found. g and A appeared to respond to environmental factors fairly independently of each other. The effects of different rates of soil drying on leaf gas exchange were studied. At unchanged air humidity, different rates of soil drying were produced by using (a) different soils, (b) different irrigation schemes and (c) different soil volumes per plant. Although the soil dried to wilting point the relative leaf water content was little affected. Different soil drying rates always resulted in the same response of photosynthetic capacity (A _max) and corresponding leaf conductance (g(_Amax)) when plotted against percent relative plant-extractable soil water content (W _e %) but the relationship with relative soil water content (W _e ) was less clear. Above a range of W _e of 15%–25%, A _max and g(_Amax) were both high and responded little to decreasing W _e . As soon as W _e fell below this range, A _max and g(_Amax) declined. The data suggest root-to-leaf communication not mediated via relative leaf water content. However, g(_Amax) was initially more affected than A _max.List of abbreviations A CO₂ assimilation - A _max photosynthetic capacity at favourable ambient conditions - c _a CO₂ concentration of the air in the leaf chamber - c _i intercellular - CO₂ concentration - E transpiration - g leaf conductance - g(_Amax) leaf conductance corresponding to photosynthetic capacity - I photon flux rate - T _l leaf temperature - W _e relative plant-extractable soil water content - W _e absolute plant-extractable soil water content - W _l relative leaf water content - W _s relative soil water content - w difference in water vapour mole fraction between leaf and air - leaf water potential     

A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species

Various aspects of the biochemistry of photosynthetic carbon assimilation in C₃ plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylase-oxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO₂(p(CO₂)) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O₂(p(O₂)), the dependence of net CO₂ assimilation rate on p(CO₂) and irradiance, and the influence of p(CO₂) and irradiance on the temperature dependence of assimilation rate.Abbreviations RuP₂ ribulose bisphosphate - PGA 3-phosphoglycerate - C=p(CO₂) partial pressure of CO₂ - O=p(O₂) partial pressure of O₂ - PCR photosynthetic carbon reduction - PCO photorespiratory carbon oxidation     

Effect of temperature on the CO2/O2 specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase and the rate of respiration in the light

Responses of the rate of net CO₂ assimilation (A) to the intercellular partial pressure of CO₂ (p _i ) were measured on intact spinach (Spinacia oleracea L.) leaves at different irradiances. These responses were analysed to find the value of p _i at which the rate of photosynthetic CO₂ uptake equalled that of photorespiratory CO₂ evolution. At this CO₂ partial pressure (denoted ), net rate of CO₂ assimilation was negative, indicating that there was non-photorespiratory CO₂ evolution in the light. Hence was lower than the CO₂ compensation point, . Estimates of were obtained at leaf temperatures from 15 to 30°C, and the CO₂/O₂ specificity of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase (E.C. 4.1.1.39) was calculated from these data, taking into account changes in CO₂ and O₂ solubilities with temperature. The CO₂/O₂ specificity decreased with increasing temperature. Therefore we concluded that temperature effects on the ratio of photorespiration to photosynthesis were not solely the consequence of differential effects of temperature on the solubilities of CO₂ and O₂. Our estimates of the CO₂/O₂ specificity of RuBP carboxylase/oxygenase are compared with in-vitro measurements by other authors. The rate of nonphotorespiratory CO₂ evolution in the light (R _d ) was obtained from the value of A at . At this low CO₂ partial pressure, R _d was always less than the rate of CO₂ evolution in darkness and appeared to decrease with increasing irradiance. The decline was most marked up to about 100 mol quanta m^-2 s^-1 and less marked at higher irradiances. At one particular irradiance, however, R _d as a proportion of the rate of CO₂ evolution in darkness was similar in different leaves and this proportion was unaffected by leaf temperature or by [O₂] (ambient and greater). After conditions of high [CO₂] and high irradiance for several hours, the rate of CO₂ evolution in darkness increased and R _d also increased.Abbreviations and symbols A rate of net CO₂-assimilation - CO₂ compensation point - CO₂ compensation point in the absence of R _d - p _i intercellular partial pressure of CO₂ - R _d (day respiration) rate of non-photorespiratory CO₂ evolution in the light - R _n (night respiration) rate of CO₂ evolution in darkness - RuBP ribulose-1,5-bisphosphate - Rubisco RuBP carboxylase/oxygenase     

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄ ⁺ at a higher rate than NO₃ ^–. Maximum NO₃ ^– uptake by the whole plant occurred at 120 mg L^–1 NO₃ ^–-N, whereas NH₄ ⁺ absorption was saturated at 240 mg L^–1 NH₄ ⁺-N. ¹⁵NH₄ ⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃ ^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄ ⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃ ^– uptake. Maximum inhibition of ¹⁵NO₃ ^– uptake was about 55% at 2.14 mM NH₄ ⁺ (30 mg L^–1 NH₄ ⁺-N) and it did not increase any further at higher NH₄ ⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃ ^– or NH₄ ⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄ ⁺ versus NO₃ ^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃ ^–-N:NH₄ ⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃ ^– or NH₄ ⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃ ^–-N/NH₄ ⁺-N ratio. With increasing proportions of NH₄ ⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄ ⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃ ^–-N:NH₄ ⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄ ⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃ ^–:NH₄ ⁺ ratio.     

Microplankton respiratory activity and CO2 production rates in the Otranto Strait (Mediterranean Sea)

Respiratory activity and metabolic CO₂production of the microplankton in the Otranto Strait (Mediterranean Sea) were determined by monitoring the Electron Transport System activity. Ten stations were repeatedly investigated during two oceanographic surveys in February–March and August 1994. Respiratory activity and CO₂ production, estimated from the surface to the bottom, were higher in the euphotic layers (0-200 m) during summer (mean values: Winter = 0.024 μg C h⁻¹ dm⁻³; Summer = 0.042 μg C h⁻¹ dm⁻³); in the aphotic zone (deeper than 200 m), the rates were similar throughout different seasons (0.013 and 0.014 μg C h⁻¹ dm⁻³, respectively). A comparison with data collected by other authors from the euphotic layers of the Mediterranean Sea was made. Respiratory activities decreased from Western to Eastern Mediterranean Basins. The values of CO₂ production, integrated between 200 and 1000 m in the Otranto Strait (mean value 237.7 mg C m⁻² d⁻¹), were compared with other data collected from the Mediterranean Sea as well as from the Pacific, Atlantic and Indian Oceans. The comparison showed the Otranto Strait to be a site of organic matter oxidation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Combined effects of CO2 concentration and nutrient status on the biomass production and nutrient uptake of birch seedlings (Betula pendula)

Birch seedlings (Betula pendula) were grown for four months in a greenhouse at three nutrient levels (fertilization of 0, 100 and 500 kg ha^-1 monthy) and at four CO₂ concentrations (350, 700, 1050 and 1400 ppm). The effect of CO₂ concentration on the biomass production depended on the nutrient status. When mineralization of the soil material was the only source of nutrients (0 kg ha^-1), CO₂ enhancement reduced the biomass production slightly, whereas the highest production increase occurred at a fertilization of 100 kg ha^-1, being over 100% between 350 and 700 ppm CO₂. At 500 kg ha^-1 the production increase was smaller, and the production decreased beyond a CO₂ concentration of 700 ppm. The CO₂ concentration had a slight effect on the biomass distribution, the leaves accounting for the highest proportion at the lowest CO₂ concentration (350 ppm). An increase in nutrient status led to a longer growth period and increased the nutrient concentrations in the plants, but the CO₂ concentration had no effect on the growth rhythm and higher CO₂ reduced the nutrient concentrations.     

Impact of bimetallic combinations of Cu,Ni and Fe on growth rate,uptake of nitrate and ammonium,14C02 fixation,nitrate reductase and urease activity ofChlorella vulgaris

Summary The toxicity of Cu, Ni and Fe individually, as well as in combination (Cu + Ni, Cu + Fe, Ni + Fe), on growth-rate depression, uptake of NO₃ ^– and NH₄ ⁺, photosynthesis, nitrate reductase and urease activity ofChlorella vulgaris has been studied. All the test metals when used individually showed pronounced toxicity on all the parameters studied. However, their interactive effect was mostly antagonistic except for Cu + Ni (synergism). Pre-addition of Fe offered more protection to the cells against copper and nickel toxicity. The data of statistical analysis reconfirmed that¹⁴C0₂ uptake is the most sensitive parameter (significant atP<0.005, both for time and treatment) than others in metal toxicity assessment. However, these results suggest further that exposure time and sequence of metal addition are very important in biomonitoring of heavy metal toxicity.     

Hydrogen-stimulated CO2 fixation and coordinate induction of hydrogenase and ribulosebiphosphate carboxylase in a H2-uptake positive strain of Rhizobium japonicum

H₂-uptake positive strains (122 DES and SR) and H₂-uptake negative strains SR2 and SR3 of Rhizobium japonicum were examined for ribulosebisphosphate (RuBP) carboxylase and H₂-uptake activities during growth conditions which induced formation of the hydrogenase system. The rate of ¹⁴CO₂ uptake by hydrogenase-derepressed cells was about 6-times greater in the presence than in the absence of H₂. RuBP carboxylase activity was observed in free-living R. japonicum strains 122 DES or SR only when the cells were derepressed for their hydrogenase system. Hydrogenase and RuBP carboxylase activities were coordinately induced by H₂ and both were repressed by added succinate. Hydrogenase-negative mutant strains SR2 and SR3 derived from R. japonicum SR showed no detecyable RuBP carboxylase activities under hydrogenase derepression conditions. No detectable RuBP carboxylase was observed in bacteroids formed by H₂-uptake positive strains R. japonicum 122 DES or SR. Propionyl CoA carboxylase activity was consistently observed in extracts of cells from free-living cultures of R. japonicum but activity was not appreciably influenced by the addition of H₂. Neither phosphoenolpyruvate carboxylase nor phosphoenolpyruvate carboxykinase activity was detected in extracts of R. japonicum.Abbreviations RuBP Ribulose 1,5-bisphosphate - (Na₂EDTA) (Ethylenedinitrilo)-tetraacetic acid, disodium salt - (propionyl CoA) Propionyl coenzyme A - (PEP) Phosphoenolpyruvate - (GSH) Reduced glutathione - (Tricine) N-tris(hydroxymethyl)-methylglycine     

Chloroflexus aurantiacus secretes 3-hydroxypropionate,a possible intermediate in the assimilation of CO2 and acetate

Chloroflexus aurantiacus OK-70 fl secreted 3-hydroxypropionate (3HP) during phototrophic growth. The greatest amounts were secreted by cells grown on propionate (0.35 mM 3HP) while the lowest levels were found in autotrophically grown cultures (1.5 M). Large amounts of 2-fluoro,3-hydroxypropionate were formed by autotrophically grown cells exposed to fluoroacetate (FAc). Increased levels of 3HP were observed in these cultures when incubated with acctate. The secretion of 3HP was further stimulated by 0.2 mM KCN, an inhibitor of CO₂ fixation, but only in the presence of acetate. The pathway of 3HP formation was studied by using ¹³C-labelled substrates and NMR. The 3HP formed in the presence of C1-labelled acetate and FAc was labelled at C3 and somewhat less at C2 while with C2-labelled acetate as the tracer 3HP was labelled predominantly at C2. The carboxyl group was derived from CO₂. The 3HP formed by cells grown on propionate and ¹³CO₂ was labelled at all carbon atoms, the label content of C2 and C3 was about 25 and 65% of that of C1 respectively. It is suggested that 3HP is an intermediate in a pathway for acetate assimilation and in a new reductive carboxylic acid cycle for autotrophic CO₂ fixation.Abbreviations 3HP 3-hydroxypropionate - 2F3HP 2,fluoro,3-hydroxypropionate - FAc fluoroacetate - GC gas chromatography - MS mass spectrometry - NMR nuclear magnetic resonance     

Differing substomatal and chloroplastic CO2 concentrations in water-stressed wheat

Gas exchanges of wheat (Triticum aestivum L. cv. Courtot) shoots were measured before and during a water stress. While photosynthesis, transpiration and dark respiration decreased because of the stress, photorespiration increased initially, up to a maximum of 50% above its initial value. The CO₂ concentration in the intercellular space was calculated from gas-diffusion resistances, and remained approximately constant before and during the stress. On the other hand, the CO₂ concentration in the chloroplast, in the vicinity of Ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco), was evaluated from the ratio of CO₂ to O₂ uptake, using the known kinetic constants of the oxygenation and carboxylation reactions which compete for Rubisco. In the well-watered plants, the calculated chloroplastic concentration was slightly smaller than the substomatal concentration. During water stress, this concentration decreased while the substomatal CO₂ concentration remained constant. Hypotheses to explain this difference between substomatal and chloroplastic CO₂ concentrations are discussed.     

Hormonal effects on tube elongation, 14CO2 fixation and phosphoenolpyruvate carboxylase activity in Amaryllis pollen: Promotion by abscisic acid

Abscisic acid promoted tube elongation, ¹⁴CO₂ fixation and phosphoenolpyruvate carboxylase activity during in vitro germination of Amaryllis vittata Ait. pollen. Other growth regulators tested viz. indol-3yl-acetic acid, gibberellic acid, N⁶-benzyladenine and ethephon did not affect pollen tube elongation at 1 g/ml concentration but inhibited it at 10 g/ml. The case for abscisic acid as a growth promoter is argued.     

The fate of labelled15N urea and ammonium nitrate applied to a winter wheat crop

Labelled urea or ammonium nitrate was applied to winter wheat growing on a loamy soil in Northern France. Two applications of fertilizer were given: 50 kg N ha^–1 at tillering (early March) and 110 kg N ha^–1 at the beginning of stem elongation (mid-April). The kinetics of urea hydrolysis, nitrification of ammonium and the disappearance of inorganic nitrogen were followed at frequent intervals. Inorganic nitrogen soon disappeared, mainly immobilized by soil microflora and absorbed by the crop. Net immobilization of fertilizer N occured at a very similar rate for urea and ammonium nitrate. Maximum immobilization (16 kg N ha¹) was found at harvest for the first dressing and at anthesis for the second dressing (23 kg N ha¹). During the nitrification period, the labelled ammonium pool was immobilized two to three times faster than the labelled nitrate pool. No significant net¹⁵N remineralization was found during the growth cycle.The actual denitrification and volatilization losses were probably more important than indicated from calculations made by extrapolation of fluxes measured over short intervals. However microbial immobilization was the most important of the processes which compete with plant uptake for nitrogen.     

In vitro enzyme activities and products of 14CO2 assimilation in flag leaf and ear parts of wheat (Triticum aestivum L.)

Activities of key enzymes of Calvin cycle and C₄ metabolism, rate of ¹⁴CO₂ fixation in light and dark and the initial products of photosynthetic ¹⁴CO₂ fixation were determined in flag leaf and different ear parts of wheat viz. pericarp, awn and glumes. Compared to the activities of RuBP carboxylase and other Calvin cycle enzymes viz. NADP-glyceraldehyde-3-phosphate dehydrogenase, NAD-glyceraldehyde-3-phosphate dehydrogenase and ribulose-5-phosphate kinase, the levels of PEP carboxylase and other enzymes of C₄ metabolism viz. NADP-malate dehydrogenase, NAD-malate dehydrogenase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase genase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase, were generally greater in ear parts than in the flag leaf. In contrast to CO₂ fixation in light, the various ear parts incorporated CO₂ in darkness at much higher rates than flag leaf. In short term assimilation of ¹⁴CO₂ by illuminated ear parts, most of the ¹⁴C was in malate with less in 3-phosphoglyceric acid, whereas flag leaves incorporated most into 3-phosphoglyceric acid. It seems likely that ear parts have the capability of assimilating CO₂ by the C₄ pathway of photosynthesis and utilise PEP carboxylase for recapturing the respired CO₂.     

Impact of elevated CO2 on soil organic matter dynamics as related to changes in aggregate turnover and residue quality

Increasing global atmospheric CO₂ concentration can potentially affect C cycling in terrestrial ecosystems. This study was conducted to assess the impact of elevated CO₂ concentration on soil organic matter and aggregate dynamics in Lolium perenne and Trifolium repens pastures. Soil samples from a 6 year old `free air CO₂ enrichment' (FACE) experiment were separated in four aggregate size classes (<53, 53–250, 250–2000, and > 2000 m). Free light fraction (i.e. particulate organic matter (POM) outside of aggregates; free LF) and intra-aggregate-POM (i.e. POM occluded within the aggregate structure; iPOM) were isolated. The distinct ¹³C-signature of the CO₂ used to raise the ambient CO₂ concentration in FACE allowed us to calculate proportions of recently incorporated C (< 6 yr) in the physically defined soil fractions. The proportion of new C increased with increasing aggregate size class, except the two largest aggregate size classes had a similar proportion of new C; this indicates a faster turnover of macroaggregates compared to microaggregates. In addition, higher proportions of new C in macroaggregates under T. repens compared to L. perenne indicate a faster macroaggregate turnover under T. repens. This faster macroaggregate turnover is hypothesized to be a result of the higher residue quality (C:N ratio) of T. repens compared to L. perenne and reduces the potential of sequestering C under elevated CO₂. In the L. perenne soil, elevated CO₂ did not significantly increase total C, but led to: (1) a 54% increase in aggregation and (2) a 40% increase in total iPOM-C. It is hypothesized that the sequestration of iPOM-C induced by elevated CO₂ in the low residue quality, L. perenne treatment, resulted from an increase in the proportion of large macroaggregates with a slow turnover.