Synchronous records of pCO2 and Δ14C suggest rapid, ocean-derived pCO2 fluctuations at the onset of Younger Dryas

Just before the onset of the Younger Dryas (YD) cold event, several stomatal proxy-based pCO2 records have shown a sharp increase in atmospheric CO2 concentration (pCO2) of between ca 50 and 100 ppm, followed by a rapid decrease of similar or even larger magnitude. Here we compare one of these records, a high-resolution pCO2 record from southern Sweden, with the IntCal13 record of radiocarbon (Δ14C). The two records show broadly synchronous fluctuations at the YD onset. Specifically, the IntCal13 record documents decreasing Δ14C just before the YD onset when pCO2 peaks, consistent with a source of “old” CO2 from the deep ocean. We propose that this fluctuation occurred due to a major ocean flushing event. The cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor. We confirm that the earth system can produce such large multi-decadal timescale fluctuations in pCO2 through simulating an artificial ocean flushing event with the GENIE Earth System Model. We suggest that sharp transitions of pCO2 may have remained undetected so far in ice cores due to inter-firn gas exchange and time-averaging. The stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change.

Late-Glacial and early Holocene marine 14C reservoir ages off North Iceland (16-9 cal kyr BP)

Tephrochronological age models and 48 14C age determinations on molluscs and foraminifera (planktonic and benthic) are applied for the calculation of marine 14C reservoir age variability during a time period covering the Heinrich event H1 to early Holocene (16–9 cal kyr BP). Our data source consists of four high-resolution marine sediment cores (HM107-04, HM107-05, MD99-2271, MD99-2275) from the North Icelandic shelf. The marine reservoir age (ΔR) is found to be extremely variable, ranging from 385 to 1065 14C years. Extreme ΔR values occur at the end of H1, with values around 1000 14C years (~15 cal kyr BP), probably due to reduced northward flow of well-ventilated subtropical surface waters and a southward expansion of polar waters, as well as an expansion of sea ice limiting air-sea gas exchange. With the onset of the Bølling-Allerød interstadial, the ΔR values decrease towards 0 14C years suggesting a more vigorous North Atlantic Current and an active meridional overturning circulation system. During the Younger Dryas stadial, ΔR values are consistently around 700 14C years suggesting e renewed expansion of polar waters and a weakened meridional overtuning circulation. Interestingly, ΔR values remain high (~200 14C years) at the onset of the Holocene suggesting continued high influence of polar waters. Subsequently, ΔR values rapidly decrease to ~¬ 250 14C years around 11 cal kyr BP, indicating increased air-sea CO2 exchange with the coeval atmosphere. The ΔR values average around 0 14C years from around 10.5 to 9.0 cal kyr BP.

Non-invasive ventilation for cystic fibrosis.

Non-invasive ventilation may be a means to temporarily reverse or slow the progression of respiratory failure in cystic fibrosis. To compare the effect of non-invasive ventilation versus no non-invasive ventilation in people with cystic fibrosis. We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Trials Register comprising references identified from comprehensive electronic database searches, handsearching relevant journals and abstract books of conference proceedings. We searched the reference lists of each trial for additional publications possibly containing other trials.Most recent search: 22 February 2013. Randomised controlled trials comparing a form of pressure preset or volume preset non-invasive ventilation to no non-invasive ventilation in people with acute or chronic respiratory failure in cystic fibrosis. Three reviewers independently assessed trials for inclusion criteria and methodological quality, and extracted data. Fifteen trials were identified; seven trials met the inclusion criteria with a total of 106 participants. Six trials evaluated single treatment sessions and one evaluated a six-week intervention.Four trials (79 participants) evaluated non-invasive ventilation for airway clearance compared with an alternative chest physiotherapy method and showed that airway clearance may be easier with non-invasive ventilation and people with cystic fibrosis may prefer it. We were unable to find any evidence that NIV increases sputum expectoration, but it did improve some lung function parameters.Three trials (27 participants) evaluated non-invasive ventilation for overnight ventilatory support, measuring lung function, validated quality of life scores and nocturnal transcutaneous carbon dioxide. Due to the small numbers of participants and statistical issues, there were discrepancies in the results between the RevMan and the original trial analyses. No clear differences were found between non-invasive ventilation compared with oxygen or room air except for exercise performance, which significantly improved with non-invasive ventilation compared to room air over six weeks. Non-invasive ventilation may be a useful adjunct to other airway clearance techniques, particularly in people with cystic fibrosis who have difficulty expectorating sputum. Non-invasive ventilation, used in addition to oxygen, may improve gas exchange during sleep to a greater extent than oxygen therapy alone in moderate to severe disease. These benefits of non-invasive ventilation have largely been demonstrated in single treatment sessions with small numbers of participants. The impact of this therapy on pulmonary exacerbations and disease progression remain unclear. There is a need for long-term randomised controlled trials which are adequately powered to determine the clinical effects of non-invasive ventilation in cystic fibrosis airway clearance and exercise.

Aluminium toxicity in wheat and rye

Com o presente trabalho pretendeu-se determinar e compreender melhor quais os alvos do Alumínio (Al) nas plantas, e contribuir para um melhor entendimento dos mecanismos de tolerância presentes em genótipos com elevado grau de tolerância ao Al. O Al é um dos maiores constituintes do solo e torna-se biodisponível em solos com baixo pH. Nesses casos, a exposição ao Al afecta negativamente o crescimento das plantas conduzindo a uma diminuição da produção. Estes factos são especialmente visíveis nos cereais, sendo a exposição ao Al uma das principais causas das quebras de produção nestas espécies. O Capítulo I consiste numa revisão geral sobre a toxicidade do Al nas plantas, apontando os seus principais alvos. Apresenta também os mecanismos de resistência, que inclui Al-destoxificação externa e interna, em diferentes espécies. O Capítulo II aborda os estudos sobre a exposição de curto prazo ao Al em duas espécies de cereais: Triticum aestivum L. e Secale cereale L., tendo-se sempre utilizado um genótipo Al-tolerante e um Al-sensível para cada espécie. Este capítulo está dividido em três estudos: no Capítulo II.1 realça-se o efeito da exposição a 185 μM de Al no equilíbrio nutricional em trigo. Verificou-se que em ambos os genótipos (sensível e tolerante) o perfil de macro e micro nutrientes se alterou, tendo uma interferência negativa, sobretudo no nível de P, Mg e K. Além disso, registaram-se diferenças na diferenciação da endoderme consoante o grau de tolerância/sensibilidade do genótipo. No Capítulo II.2 apresenta-se uma visão mais abrangente dos efeitos da exposição a 185 μM de Al em trigo, incluindo parâmetros fisiológicos, estruturais, citológicos e genotóxicos. Demonstra-se, pela primeira vez, que a progressão do ciclo celular é diferentemente regulada, dependendo da tolerância/sensibilidade do genótipo e que, mesmo em zonas já diferenciadas da raiz a exposição ao Al leva à deposição de calose. O Capítulo II.3 aborda os efeitos da exposição de 1.1 mM de Al em centeio, numa perspectiva bastante alargada. Apresenta-se o desequilíbrio nutricional, sobretudo no genótipo sensível, assim como a translocação de Al para a parte aérea nesse mesmo genótipo. Analisa-se também o comportamento de ambos os genótipos no que se refere ao ciclo celular, diferenciação da endoderme, crescimento radicular, reservas de hidratos de carbono, entre outros. Os resultados apontam para estratégias bem definidas adoptadas pelo genótipo tolerante de forma a minimizar a acção do Al no sistema radicular. O Capítulo III compreende a exposição longa ao Al. Dois genótipos de centeio com diferentes graus de tolerância ao Al foram expostos a 1.11 mM e 1.85 mM de Al durante 21 dias, tendo sido usados dois pontos de amostragem (15 e 21 dias). Este capítulo está dividido em dois estudos: No Capítulo III. 1 analisamse os mecanismos antioxidantes (folhas e raízes) como resposta à exposição ao Al, dando-se especial atenção ao ciclo do ascorbato-glutationas. A exposição ao Al levou a stress oxidativo e a alterações na actividade de enzimas antioxidantes e no conteúdo de antioxidantes não-enzimáticos. Demonstra-se que os dois órgãos apresentam respostas diferentes à exposição ao Al e que a capacidade de sobreviver em ambientes ricos em Al depende da eficácia da resposta antioxidante. Para além disso, a resposta do ciclo ascorbato-glutationas parece estar dependente do tipo de órgão, grau de tolerância e do tempo de exposição ao Al. No Capítulo III. 2 analisam-se os efeitos da exposição ao Al na fotossíntese. Verificou-se que o Al afecta negativamente a taxa fotossintética em ambos os genótipos, embora as alterações que o Al provoca nas trocas gasosas e no Ciclo de Calvin sejam dependentes do genótipo. Verificou-se também que os danos no genótipo sensível surgem mais cedo do que no genótipo tolerante, mas que ambos apresentam susceptibilidade ao Al após exposição de longo termo. Por fim, no Capítulo IV são apresentadas as conclusões da Tese de Doutoramento.

Physiology and metabolic profile of Moringa olifera and Eucalyptus globulus plants subjected to climate changes

As alterações climáticas emergentes têm um grande impacto no crescimento e desenvolvimento de espécies florestais, nomeadamente em espécies de valor industrial e medicinal, como é o caso do eucalipto (Eucalyptus globulus) e da moringa (Moringa oleifera). Assim, é urgente conhecer as respostas fisiológicas e entender as variações que ocorrem nos perfis metabólicos de espécies vegetais. Neste trabalho, plantas jovens de Eucalyptus globulus foram expostas a radiação UVB (12kJ/m2) e foram avaliadas as respostas fisiológicas e o perfil metabólico, um e onze dias após a aplicação da radiação. A dose de UVB usada não afetou as reações fotoquímicas nem as trocas gasosas, contudo ao nível do metabolismo do carbono (AST e amido) e do conteúdo de pigmentos verificaram-se pequenas alterações (AST e pigmentos). Através da análise do perfil metabólico de E. globulus foram encontrados compostos voláteis e semi-voláteis pertencentes às famílias dos terpenos, sesquiterpenos e aldeídos. Em geral, os sesquiterpenos e os álcoois monoterpénicos apresentaram uma tendência para manter e, em alguns casos, diminuir com o stress, enquanto que o grupos dos aldeídos aumentou e os monoterpenos apresentaram um comportamento mais heterogéneo. O E. globulus mostrou ser uma espécie tolerante à aplicação da dose de UVB usada neste trabalho. Por outro lado, plantas jovens de M. oleifera foram expostas a défice hídrico (DH). Um grupo de plantas foi recolhido um dia após o final da exposição e o outro grupo após onze dias do final da exposição. Foi avaliado o perfil metabólico desta espécie através de GC/MS. Os dados cromatográficos indicaram que em condições de stress (DH(1) e DH(11)), as quantidades de compostos associados a vias primárias e secundárias de defesa (como os alcanos, álcoois, ácidos carboxílicos, esteróis, aminoácidos e açucares) sofreram algumas alterações. As plantas analisadas 11 dias após a remoção do stress mostraram maiores variações do perfil de metabolitos. No entanto, tanto um como onze dias após a remoção do stress, as plantas apresentaram a formação de novos rebentos. Apesar do perfil de metabolitos ter sofrido algumas alterações, por não se registarem casos de morte, conclui-se que as plantas de moringa mostraram ser tolerantes aos tratamentos aplicados.

Underway O2/Ar-estiment net community production across the Kuroshio Extension and impact on winter pCO2 drawdown

Senior thesis written for Oceanography 445

Metabolic and respiratory effects of infused sodium acetate in healthy human subjects.

The metabolic and respiratory effects of intravenous 0.5 M sodium acetate (at a rate of 2.5 mmol/min during 120 min) were studied in nine normal human subjects. O2 consumption (VO2) and CO2 production (VCO2) were measured continuously by open-circuit indirect calorimetry. VO2 increased from 251 +/- 9 to 281 +/- 9 ml/min (P < 0.001), energy expenditure increased from 4.95 +/- 0.17 kJ/min baseline to 5.58 +/- 0.16 kJ/min (P < 0.001), and VCO2 decreased nonsignificantly (211 +/- 7 ml/min vs. 202 +/- 7 ml/min, NS). The extrapulmonary CO2 loss (i.e., bicarbonate generation and excretion) was estimated at 48 +/- 5 ml/min. This observation is consistent with 1 mol of bicarbonate generated from 1 mol of acetate metabolized. Alveolar ventilation decreased from 3.5 +/- 0.2 l/min basal to 3.1 +/- 0.2 l/min (P < 0.001). The minute ventilation (VE) to VO2 ratio decreased from 22.9 +/- 1.3 to 17.6 +/- 0.9 l/l (P < 0.005), arterial PO2 decreased from 93.2 +/- 1.9 to 78.7 +/- 1.6 mmHg (P < 0.0001), arterial PCO2 increased from 39.2 +/- 0.7 to 42.1 +/- 1.1 mmHg (P < 0.0001), pH from 7.40 +/- 0.005 to 7.50 +/- 0.007 (P < 0.005), and arterial bicarbonate concentration from 24.2 +/- 0.7 to 32.9 +/- 1.1 (P < 0.0001). These observations indicate that sodium acetate infusion results in substantial extrapulmonary CO2 loss, which leads to a relative decrease of total and alveolar ventilation.

Measurement of 13CO2 in expired air as an index of compliance to a high carbohydrate diet naturally enriched in 13C.

The aim of this study was to determine whether breath 13CO2 measurements could be used to assess the compliance to a diet containing carbohydrates naturally enriched in 13C. The study was divided into two periods: Period 1 (baseline of 4 days) with low 13C/12C ratio carbohydrates. Period 2 (5 days) isocaloric diet with a high 13C/12C ratio (corn, cane sugar, pineapple, millet) carbohydrates. Measurements were made of respiratory gas exchange by indirect calorimetry, urinary nitrogen excretion and breath 13CO2 every morning in post-absorptive conditions, both in resting state and during a 45-min low intensity exercise (walking on a treadmill). The subjects were 10 healthy lean women (BMI 20.4 +/- 1.7 kg/m2, % body fat 24.4 +/- 1.3%), the 13C enrichment of oxidized carbohydrate and breath 13CO2 were compared to the enrichment of exogenous dietary carbohydrates. At rest the enrichment of oxidized carbohydrate increased significantly after one day of 13C carbohydrate enriched diet and reached a steady value (103 +/- 16%) similar to the enrichment of exogenous carbohydrates. During exercise, the 13C enrichment of oxidized carbohydrate remained significantly lower (68 +/- 17%) than that of dietary carbohydrates. The compliance to a diet with a high content of carbohydrates naturally enriched in 13C may be assessed from the measurement of breath 13CO2 enrichment combined with respiratory gas exchange in resting, postabsorptive conditions.

Transfer factor for carbon monoxide: a glance behind the scene.

The transfer factor for carbon monoxide (TLCO) is widely used in pulmonary function laboratories because it represents a unique non-invasive window on pulmonary microcirculation. The TLCO is the product of two primary measurements, the alveolar volume (VA) and the CO transfer coefficient (KCO). This test is most informative when VA and KCO are examined, together with their product TLCO. In a normal lung, a low VA due to incomplete expansion is associated with an elevated KCO, resulting in a mildly reduced TLCO. Thus, in case of low VA, a seemingly "normal KCO" must be interpreted as an abnormal gas transfer. The most common clinical conditions associated with an abnormal TLCO are characterised by a limited number of patterns for VA and KCO: incomplete lung expansion, discrete loss of alveolar units, diffuse loss of alveolar units, emphysema, pulmonary vascular disorders, high pulmonary blood volume, alveolar haemorrhage.

Importance of ENaC-mediated sodium transport in alveolar fluid clearance using genetically-engineered mice.

The lung possesses specific transport systems that intra- and extracellularly maintain salt and fluid balance necessary for its function. At birth, the lungs rapidly transform into a fluid (Na(+))-absorbing organ to enable efficient gas exchange. Alveolar fluid clearance, which mainly depends on sodium transport in alveolar epithelial cells, is an important mechanism by which excess water in the alveoli is reabsorbed during the resolution of pulmonary edema. In this review, we will focus and summarize on the role of ENaC in alveolar lung liquid clearance and discuss recent data from mouse models with altered activity of epithelial sodium channel function in the lung, and more specifically in alveolar fluid clearance. Recent data studying mice with hyperactivity of ENaC or mice with reduced ENaC activity clearly illustrate the impaired lung fluid clearance in these adult mice. Further understanding of the physiological role of ENaC and its regulatory proteins implicated in salt and water balance in the alveolar cells may therefore help to develop new therapeutic strategies to improve gas exchange in pulmonary edema.

Effect of diurnal cycling temperatures on cardiovascular- ventilatory function in statically-acclimated rainbow trout, Salmo gairdneri

Four groups of rainbow trout, Salmo gairdneri, were acclimated to 2°, 10°, and 18°e, and to a diurnal temperature cycle (100 ± 4°C). To evaluate the influence of cycling temperatures in terms of an immediate as opposed to acclimatory response various ventilatory-cardiovascular rate functions were observed for trout, either acclimated to cycling temperatures or acclimated to constant temperatures and exposed to a diurnal temperature cycle for the first time (10° ± 4°C for trout acclimated to 10°C; 18°+ 4°C for trout acclimated to l8°e). Gill resistance and the cardiac to ventilatory rate ratio were then calculated. Following a post preparatory recovery period of 36 hr, measurements were made over a 48 hour period with the first 24 hours being at constant temperature in the case of statically-acclimated fish followed by 24 hours under cyclic temperature conditions. Trout exhibited marked changes in oxygen consumption (Vo ) with temp- 2 erature both between acclimation groups, and in response to the diurnal temperature cycle. This increase in oxygen uptake appears to have been achieved by adjustment of ventilatory and, to some extent, cardiovascular activity. Trout exhibited significant changes in ventilatory rate (VR), stroke volume (Vsv), and flow (VG) in response to temperature. Marked changes in cardiac rate were also observed. These findings are discussed in relation to their importance in convective oxygen transport via water and blood at the gills and tissues. Trout also exhibited marked changes in pressure waveforms associated with the action of the resp; ratory pumps with temperature. Mean differenti a 1 pressure increased with temperature as did gill resistance and utilization. This data is discussed in relation to its importance in diffusive oxygen transport and the conditions for gas exchange at the gills. With one exception, rainbow trout were able to respond to changes in oxygen demand and availability associated with changes in temperature by means of adjustments in ventilation, and possibly pafusion, and the conditions for gas exchange at the gills. Trout acclimated to 18°C, however, and exposed to high cyclic temperatures, showed signs of the ventilatory and cardiovascular distress problems commonly associated with low circulating levels of oxygen in the blood. It appears these trout were unable to fully meet the oxygen requirements associated with c~ling temperatures above 18°C. These findings were discussed in relation to possible limitations in the cardiovascular-ventilatory response at high temperatures. The response of trout acclimated to cycling temperatures was generally similar to that for trout acclimated to constant temperatures and exposed to cycling temperatures for the first time. This result suggested that both groups of fish may have been acclimated to a similar thermal range, regardless of the acclimation regime employed. Such a phenomenon would allow trout of either acclimation group to respond equally well to the imposed temperature cycle. Rainbow trout showed no evidence of significant diurnal rhythm in any parameters observed at constant temperatures (2°, 10°, and 18° C), and under a 12/12 light-dark photoperiod regime. This was not taken to indicate an absence of circadian rhythms in these trout, but rather a deficiency in the recording methods used in the study.

Efectos de la ventilación mecánica con heliox en niños y adolescentes con patología bronquial obstructiva

Evaluar si el Heliox reduce la resistencia en la vía aérea en niños y adolescentes con patología bronquial obstructiva que requieren ventilación mecánica. Materiales y Métodos: Estudio prospectivo observacional descriptivo en niños y adolescentes con patología bronquial obstructiva y ventilación mecánica con Fi02 ≤ 0,5. Medición de variables: resistencia, presión pico, presión media de la vía aérea, presión meseta, volumen corriente, autoPEEP, distensibilidad, PetCO2, ventilación de espacio muerto antes de inicio de heliox y a los 30 minutos, 2, 4, 6, 12, 18 y 24 horas y diariamente hasta suspenderlo por extubación o FiO2 > 0,5. Resultados: Resultados parciales, incluyó 9 pacientes encontrando descenso significativo de resistencia espiratoria a los 30 minutos (51,2 vs 32,3; p=0,0008 ), 2 horas ( 51,2 vs 33,4; p=0,0019) y 4 horas (51,2 vs 30,7; p=0,0012) así como de la resistencia inspiratoria a la hora 2 (48,6 vs 36,2; p = 0,013) y hora 4 (48,6 vs 30 ; p=0,004). Se observó tendencia al descenso de la PetCO2 que no fue significativa (52,3 vs 34,3: p=0,06). No se evidenció cambios en las variables; autoPEEP, presión pico, presión media de la vía aérea, distensibilidad, ventilación de espacio muerto, presión meseta y volumen corriente antes y después del inicio del Heliox. Conclusión: La ventilación mecánica con Heliox en niños con patología bronquial obstructiva parece ser que reduce de manera significativa la resistencia de la vía aérea, con tendencia al descenso de la PetC02. Se necesitan estudios prospectivos al menos observacionales analíticos que corroboren estos hallazgos.

Efectos de un programa de rehabilitación pulmonar en un epoc severo

Se describe el caso de un varón de 59 años,con diagnóstico de enfermedad pulmonar obstructiva crónica (EPOC) severa, producto del consumo de cigarrillo. En la evaluación, presenta deficiencias en la capacidad aeróbica, en el desempeño muscular, en la ventilación e intercambio gaseoso, con alteración de sus volúmenes pulmonares, las cuales le ocasionaron limitación funcional y restricción en la realización de las actividades de la vida diaria. Se inició un plan de cuidado y acondicionamiento físico, consistente en incrementar la fuerza y resistencia de miembros inferiores y superiores, entrenamiento de músculos respiratorios y un plan de educación individual y grupal acerca del conocimiento y manejo de la enfermedad, además se le brindó soporte psicoterapéutico. Los resultados del programa mostraron efectos positivos en su condición de salud, en general, y en la disminución de su limitación funcional.

Efecto del Heliox sobre la resistencia en la vía aérea en niños con patología bronquial obstructiva que requieren ventilación mecánica

Efecto del heliox sobre la resistencia en la vía área en niños con patología bronquial obstructiva que requieren ventilación mecánica Contreras, MA; Angarita, D; Fernandez, J; Godoy, J; Mullet, H; Briceño, G. Resumen: Los pacientes con enfermedad bronquial obstructiva que requieren ventilación mecánica presentan un aumento en la resistencia de la vía aérea que produce efectos deletéreos sobre la mecánica respiratoria. El uso de Heliox ha sido propuesto como una terapia en patología bronquial obstructiva, ya que gracias a sus propiedades físicas disminuye la resistencia en la vía aérea, mejorando el intercambio gaseoso y reduciendo el tiempo de soporte ventilatorio. El objetivo de este estudio Cuasi-experimental, fue medir y comparar antes y después la resistencia de la vía aérea en niños con patología bronquial obstructiva en quienes se usó Heliox durante la ventilación mecánica. Métodos: Se incluyeron niños que requirieron ventilación mecánica, realizando mediciones de la resistencia inspiratoria y espiratoria de la vía aérea con el ventilador Hamilton G5. Las mediciones fueron realizadas antes y desde los 30 minutos del inicio de la ventilación mecánica, hasta lograr extubación. Para el análisis de las diferencias de todas las medidas se usó la prueba de comparaciones múltiples de Friedman y el coeficiente concordancia de Kendall. Resultados: En una muestra de 17 pacientes, se encontró una disminución con significancia estadística, en las mediciones de la resistencia inspiratoria y espiratoria, a partir de la segunda hora (p 0.0004), después del inicio de la ventilación mecánica con heliox, efecto que mantuvo hasta el momento de la extubación.

Acclimation of photosynthesis to elevated CO2 in onion (Allium cepa) grown at a range of temperatures

Onion (Allium cepa) was grown in the field within temperature gradient tunnels (providing about -2.5degreesC to +2.5degreesC from outside temperatures) maintained at either 374 or 532 mumol mol(-1) CO2. Plant leaf area was determined non-destructively at 7 day intervals until the time of bulbing in 12 combinations of temperature and CO2 concentration. Gas exchange was measured in each plot at the time of bulbing, and the carbohydrate content of the leaf (source) and bulb (sink) was determined. Maximum rate of leaf area expansion increased with mean temperature. Leaf area duration and maximum rate of leaf area expansion were not significantly affected by CO2. The light-saturated rates of leaf photosynthesis (A(sat)) were greater in plants grown at normal than at elevated CO2 concentrations at the same measurement CO2 concentration. Acclimation of photosynthesis decreased with an increase in growth temperature, and with an increase in leaf nitrogen content at elevated CO2. The ratio of intercellular to atmospheric CO2 (C-i/C-a ratio) was 7.4% less for plants grown at elevated compared with normal CO2. A(sat) in plants grown at elevated CO2 was less than in plants grown at normal CO2 when compared at the same C-i Hence, acclimation of photosynthesis was due both to stomatal acclimation and to limitations to biochemical CO2 fixation. Carbohydrate content of the onion bulbs was greater at elevated than at normal CO2. In contrast, carbohydrate content was less at elevated compared with normal CO2 in the leaf sections in which CO2 exchange was measured at the same developmental stage. Therefore, acclimation of photosynthesis in fully expanded onion leaves was detected despite the absence of localised carbohydrate accumulation in these field-grown crops.