Integration of chlorophyll a fluorescence and photorespirometry techniques to understand production dynamics in macroaglal communities

Global declines of macroalgal beds in coastal waters have prompted a plethora of studies attempting to understand the drivers of change within dynamic nearshore ecosystems. Photosynthetic measurements are good tools for assessing the consequences of numerous stressors of macroalgae, but there is somewhat of a disconnection between studies that focus on organism‐specific ecophysiological responses and those that address causes and consequences of shifts in macroalgal productivity. Our goal is to highlight the applications of two complementary tools for measuring photosynthesis—variable chlorophyll a fluorescence and photorespirometry—and provide guidance for the integration of physiology and ecology to understand the drivers of change in macroalgal communities. Photorespirometry can provide an integrated measure of whole‐community metabolism, including an estimate of the physiological costs associated with stressors, while fluorescence‐based techniques provide point measures of the efficiency of the photosynthetic apparatus within communities. Variable chlorophyll a fluorescence does not provide an estimate of carbon balance or integrated photosynthesis across either whole plants or whole communities but can be used to estimate the contribution of individual community components in the dynamic subcanopy environment to help us understand the mechanisms underlying observed responses. We highlight the importance of the highly dynamic light environment within macroalgal communities and call for better integration of physiological techniques in an ecological context to enhance our understanding of the responses of whole communities to local and global stressors.     

Responses of Photosynthetic and Defence Systems to High Temperature Stress in Quercus suber L Seedlings Grown under Elevated CO2

Abstract: Growth in elevated CO₂ led to an increase in biomass production per plant as a result of enhanced carbon uptake and lower rates of respiration, compared to ambient CO₂-grown plants. No down-regulation of photosynthesis was found after six months of growth under elevated CO₂. Photosynthetic rates at 15°C or 35 °C were also higher in elevated than in ambient CO₂-grown plants, when measured at their respective CO₂ growth condition. Stomata of elevated CO₂-grown plants were less responsive to temperature as compared to ambient CO₂ plants. The after effect of a heat-shock treatment (4 h at 45 °C in a chamber with 80% of relative humidity and 800–1000 tmol m^-2 s^-1 photon flux density) on A_max was less in elevated than in ambient CO₂-grown plants. At the photochemical level, the negative effect of the heat-shock treatment was slightly more pronounced in ambient than in elevated CO₂-grown plants. A greater tolerance to oxidative stress caused by high temperatures in elevated CO₂-grown plants, in comparison to ambient CO₂ plants, is suggested by the increase in superoxide dismutase activity, after 1 h at 45 °C, as well as its relatively high activity after 2 and 4 h of the heat shock in the elevated CO₂-grown plants in contrast with the decrease to residual levels of superoxide dismutase activity in ambient CO₂-grown plants immediately after 1 h at 45 °C. The observed increase in catalase after 1 h at 45 °C in both ambient and elevated CO₂-grown plants, can be ascribed to the higher rates of photorespiration and respiration under this high temperature.     

不同氮营养水平下草莓叶片光合作用对高CO2浓度的适应

研究了不同氮素水平（12mmol／L,4mmol／L,0、4mmol／L）下生长的‘丰香’草莓在富C02（700μL/L）和大气CO（390μL/L）下的光合作用。结果表明,高氮（12mmol／L）下,在富CO2环境中生长的‘丰香’草莓叶片未出现光合作用下调,富CO2下草莓叶片的净光合速率、最大羧化速率（Vc.max）、最大电子传递速率（Jmax）、碳同化的电子传递速率（Jc）和光化学猝灭系数（qp）等均显著提高;而在中氮（4mmol／L）、低氮（0．4mmol／L）下,富CO2下生长的草莓叶片的上述参数均出现不同程度的下降。富CO2下,无论氮素水平如何,草莓叶片的光呼吸电子传递速率（Jo）均降低高氮草莓叶片的非光化学猝灭系数（qN或NPQ）降低,光抑制降低,而低氮则相反。上述结果说明,氮素供应不足时草莓叶片在富CO2下光合作用出现下调,因此生产上进行CO2施肥时应适度增加氮素的供应。     

Photosynthetic responses of two eucalypts to industrial‐age changes in atmospheric [CO2] and temperature

The unabated rise in atmospheric [CO₂] is associated with increased air temperature. Yet, few CO₂‐enrichment studies have considered pre‐industrial [CO₂] or warming. Consequently, we quantified the interactive effects of growth [CO₂] and temperature on photosynthesis of faster‐growing Eucalyptus saligna and slower‐growing E. sideroxylon. Well‐watered and ‐fertilized tree seedlings were grown in a glasshouse at three atmospheric [CO₂] (290, 400, and 650 µL L^?1), and ambient (26/18 °C, day/night) and high (ambient + 4 °C) air temperature. Despite differences in growth rate, both eucalypts responded similarly to [CO₂] and temperature treatments with few interactive effects. Light‐saturated photosynthesis (A_sat) and light‐ and [CO₂]‐saturated photosynthesis (A_max) increased by ～50% and ～10%, respectively, with each step‐increase in growth [CO₂], underpinned by a corresponding 6–11% up‐regulation of maximal electron transport rate (J_max). Maximal carboxylation rate (V_cmax) was not affected by growth [CO₂]. Thermal photosynthetic acclimation occurred such that A_sat and A_max were similar in ambient‐ and high‐temperature‐grown plants. At high temperature, the thermal optimum of A_sat increased by 2–7 °C across [CO₂] treatments. These results are the first to suggest that photosynthesis of well‐watered and ‐fertilized eucalypt seedlings will remain strongly responsive to increasing atmospheric [CO₂] in a future, warmer climate.     

3种落叶松幼苗对CO2升高的光合生理响应

采用Li-6400便携式光合作用测定系统研究CO2升高对长白落叶松(Larix olgensisHerry.),日本落叶松(Larix kaempferiCarr.)和兴安落叶松(LarixgmeliniRupr.)当年生和1年生幼苗的光合特性的影响。结果表明:CO2升高使3种落叶松光饱和光合速率(Pmax)和呼吸速率均有不同程度的增加,其中,长白落叶松当年生和1年生幼苗Pmax分别比对照提高了91%和83%,日本落叶松当年生和1年生幼苗Pmax分别比对照提高了71%和94%,而兴安落叶松当年生和1年生幼苗Pmax分别比对照提高了32%和106%。除兴安落叶松外,CO2升高使所有落叶松当年生幼苗的光补偿点(LCP)下降,说明当年生幼苗对CO2浓度升高的反应更敏感。CO2升高使日本落叶松当年生和1年生幼苗光饱和点(LSP)都升高,反映了其光合作用提高的潜力较大。CO2升高条件下,除1年生兴安落叶松外,其他处理的落叶松最大量子效率(AQYmax)均增加。比较分析表明,在未来大气CO2浓度升高条件下,日本落叶松的生长潜能可能最大,具有较强的生态优势,长白落叶松次之,兴安落叶松最小。     

不同尺度上植物叶气孔导度对升高CO2的响应

植物叶气孔导度对大气CO2浓度升高的响应可表现在以下几个层面：在叶水平上,叶气孔导度和气孔密度下降;在植物个体水平上,单位叶面积蒸腾下降,植株的水分利用率升高;在生态系统水平上,蒸散降低,土壤泾流和土壤水分含量增加;在全球尺度上,扩大了温室气体的增温效应,同时也降低了全球降雨量增加的趋势。正是因为植物叶气孔导度的变化会影响全球水循环,所以它在全球变化中起着非常重要的作用。但目前的研究结果还不能外推到更大的尺度上去。     

厚壁毛竹光合作用对CO2浓度倍增的短期响应

采用Li-6400P光合测定仪对比测定了大气CO2浓度和短期CO2浓度倍增下不同季节厚壁毛竹的光合特性,结果表明:CO2浓度加倍促使最大净光合速率、净光合速率、水分利用率、光合量子效率和光饱和点升高,年平均增幅分别为62.79%、48.74%、94.41%、8.70%和16.67%;CO2浓度加倍促使蒸腾速率、暗呼吸速率和光补偿点下降,年平均降幅分别为17.60%、37.25%和40.50%。不同季节厚壁毛竹光合生理特性参数在CO2浓度加倍后的增加幅度或降低幅度与叶片生理活性和气候变化密切相关。CO2浓度的倍增并未明显改变厚壁毛竹光合特性的季节变化规律,除光补偿点外,其它光合参数的季节大小顺序仍与大气CO2浓度下的相同。厚壁毛竹光合作用对短期CO2浓度升高的响应特征与C3植物光合作用对短期CO2浓度升高响应的普遍规律相符。     

高CO2浓度对大型海藻光合作用及有关过程的影响

大气CO2浓度升高对陆生植物的影响引起广泛的关注,相对来讲,人们对大型海藻类与CO2浓度升高关系的研究则要薄弱得多,但近年来在这方面仍有很大进展。高CO2浓度下,许多大型海藻表现出光合能力下降,对海水中HCO3^-利用能力下降,并使得光合量子产额下降;而另一些大型海藻则没有上述光合作用的下调现象。另外,高CO2浓度下,大型海藻的其它许多生理过程（如生长、呼吸作用、营养盐代谢、生化组成以及钙化作用等）,均发生了相应的变化。文章分析了影响高CO2浓度对大型海藻效应的主要因素,并对今后的研究作了展望。     

Photosynthetic response to globally increasing CO2 of co‐occurring temperate seagrass species

Photosynthesis of most seagrass species seems to be limited by present concentrations of dissolved inorganic carbon (DIC). Therefore, the ongoing increase in atmospheric CO₂ could enhance seagrass photosynthesis and internal O₂ supply, and potentially change species competition through differential responses to increasing CO₂ availability among species. We used short‐term photosynthetic responses of nine seagrass species from the south‐west of Australia to test species‐specific responses to enhanced CO₂ and changes in HCO₃^?. Net photosynthesis of all species except Zostera polychlamys were limited at pre‐industrial compared to saturating CO₂ levels at light saturation, suggesting that enhanced CO₂ availability will enhance seagrass performance. Seven out of the nine species were efficient HCO₃^? users through acidification of diffusive boundary layers, production of extracellular carbonic anhydrase, or uptake and internal conversion of HCO₃^?. Species responded differently to near saturating CO₂ implying that increasing atmospheric CO₂ may change competition among seagrass species if co‐occurring in mixed beds. Increasing CO₂ availability also enhanced internal aeration in the one species assessed. We expect that future increases in atmospheric CO₂ will have the strongest impact on seagrass recruits and sparsely vegetated beds, because densely vegetated seagrass beds are most often limited by light and not by inorganic carbon.     

Photosynthetic acclimation in trees to rising atmospheric CO2: A broader perspective

Analysis of leaf-level photosynthetic responses of 39 tree species grown in elevated concentrations of atmospheric CO₂ indicated an average photosynthetic enhancement of 44% when measured at the growth [CO₂]. When photosynthesis was measured at a common ambient [CO₂], photosynthesis of plants grown at elevated [CO₂] was reduced, on average, 21% relative to ambient-grown trees, but variability was high. The evidence linking photosynthetic acclimation in trees with changes at the biochemical level is examined, along with anatomical and morphological changes in trees that impact leaf- and canopy-level photosynthetic response to CO₂ enrichment. Nutrient limitations and variations in sink strength appear to influence photosynthetic acclimation, but the evidence in trees for one predominant factor controlling acclimation is lacking. Regardless of the mechanisms that underlie photosynthetic acclimation, it is doubtful that this response will be complete. A new focus on adjustments to rising [CO₂] at canopy, stand, and forest scales is needed to predict ecosystem response to a changing environment.Abbreviations A/C_i photosynthesis as a function of internal [CO₂] - J_max maximum rate of electron transport - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - Vc_max maximum rate of carboxylation The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Responses of the chloroplast glyoxalase system to high CO2 concentrations

ABSTRACT

Sugar metabolism pathways such as photosynthesis produce dicarbonyls, e.g. methylglyoxal (MG), which can cause cellular damage. The glyoxalase (GLX) system comprises two enzymes GLX1 and GLX2, and detoxifies MG; however, this system is poorly understood in the chloroplast, compared with the cytosol. In the present study, we determined GLX1 and GLX2 activities in spinach chloroplasts, which constituted 40% and 10%, respectively, of the total leaf glyoxalase activity. In Arabidopsis thaliana, five GFP-fusion GLXs were present in the chloroplasts. Under high CO₂ concentrations, where increased photosynthesis promotes the MG production, GLX1 and GLX2 activities in A. thaliana increased and the expression of AtGLX1-2 and AtGLX2-5 was enhanced. On the basis of these findings and the phylogeny of GLX in oxygenic phototrophs, we propose that the GLX system scavenges MG produced in chloroplasts during photosynthesis.     

亚高山林线优势种形态结构和竞争力对CO2浓度和温度升高的响应

利用封顶式生长室模拟未来变化的气候条件,研究了亚高山林线优势物种岷江冷杉(Abies faxoniana)和4种草本植物形态与竞争指标对CO2浓度和温度升高的响应.结果表明:处理2个生长季后,高CO2浓度条件下,岷江冷杉冠体积增加42％,比叶面积、比冠体积和比根长分别增加17％、65％和19％;温度升高使岷江冷杉冠形更纵向生长,冠体积增加22％,根冠比和比根长均比对照增加17％;二者同时升高使岷江冷杉冠体积增加79％,比叶面积、比冠体积和比根长分别增加17％、197％和18％.CO2浓度升高处理下糙野青茅(Deyeuxia scabrescen)的株高、基茎和每株叶片数增加,但比叶面积降低;甘肃苔草(Carexkansuensis)、东方草莓(Fragaria orientali)和紫花碎米荠(Cardamine tangutorum)的各项指标变化与青茅相反.温度升高下青茅、苔草、草莓株高、基茎和根冠比下降.二者同时升高条件下4种草本植物的基茎和每株叶片数增加,但比叶面积和根冠比降低.这表明,在CO2浓度和温度升高处理下,岷江冷杉形成有利于生长的冠层结构且单位质量的竞争力增加,而4种草本植物的形态结构和竞争力均受到不同程度的负面影响.     

Effects of nitrogen supply on the acclimation of photosynthesis to elevated CO2

A common observation in plants grown in elevated CO₂ concentration is that the rate of photosynthesis is lower than expected from the dependence of photosynthesis upon CO₂ concentration in single leaves of plants grown at present CO₂ concentration. Furthermore, it has been suggested that this apparent down regulation of photosynthesis may be larger in leaves of plants at low nitrogen supply than at higher nitrogen supply. However, the available data are rather limited and contradictory. In this paper, particular attention is drawn to the way in which whole plant growth response to N supply constitutes a variable sink strength for carbohydrate usage and how this may affect photosynthesis. The need for further studies of the acclimation of photosynthesis at elevated CO₂ in leaves of plants whose N supply has resulted in well-defined growth rate and sink activity is emphasised, and brief consideration is made of how this might be achieved.Abbreviations A rate of CO₂ assimilation - C_i internal CO₂ concentration - PCR photosynthetic carbon reduction - Rubisco Ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate     

Photosynthetic response of Tempranillo grapevine to climate change scenarios

Photosynthesis in C₃ plants is CO₂ limited and therefore any increase in Rubisco carboxylation substrate may increase net CO₂ fixation, unless plants experience acclimation or other limitations. These aspects are largely unexplored in grapevine. Photosynthesis analysis was used to assess the stomatal, mesophyll, photochemical and biochemical contributions to the decreasing photosynthesis observed in Tempranillo grapevines (Vitis vinifera) from veraison to ripeness, modulated by CO₂, temperature and water availability. Photosynthesis and photosystem II photochemistry decreased from veraison to ripeness. The elevated CO₂ and temperature increased photosynthesis, but transiently, in both well irrigated (WI) and water‐stressed plants. Photosynthetic rates were maxima 1 week after the start of elevated CO₂ and temperature treatments, but differences with treatments of ambient conditions disappeared with time. There were not marked changes in leaf water status, leaf chlorophyll or leaf protein that could limit photosynthesis at ripeness. Leaf total soluble sugars remained at ripeness as high as 2 weeks after the start of treatments. On the other hand, and as expected, CO₂ diffusional limitations impaired photosynthesis in grapevine plants grown under water scarcity, stomatal and mesophyll conductances to CO₂ decreased and in turn low chloroplastic CO₂ concentrations limited photosynthetic CO₂ fixation. In summary, photochemistry and photosynthesis from veraison to ripeness in Tempranillo grapevine were dominated by a developmental‐related decreasing trend that was only transiently influenced by elevated CO₂ concentrations.     

Responses of C4 grasses to atmospheric CO2 enrichment

Summary The growth and photosynethetic responses to atmospheric CO₂ enrichment of 4 species of C₄ grasses grown at two levels of irradiance were studied. We sought to determine whether CO₂ enrichment would yield proportionally greater growth enhancement in the C₄ grasses when they were grown at low irradiance than when grown at high irradiance. The species studied were Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350, 675 and 1,000 l 1^-1 CO₂ and 1,000 or 150 mol m^-2 s^-1 photosynthetic photon flux density (PPFD). An increase in CO₂ concentration and PPFD significantly affected net photosynthesis and total biomass production of all plants. Plants grown at low PPFD had significantly lower rates of photosynthesis, produced less biomass, and had reduced responses to increases in CO₂. Plants grown in CO₂-enriched atmosphere had lower photosynthetic capacity relative to the low CO₂ grown plants when exposed to lower CO₂ concentration at the time of measurement, but had greater rate of photosynthesis when exposed to increasing PPFD. The light level under which the plants were growing did not influence the CO₂ compensation point for photosynthesis.     

Effect of elevated CO2 concentration on photosynthetic characteristics of hyperaccumulator Sedum alfredii under cadmium stress

The combined effects of elevated CO_2 and cadmium(Cd) on photosynthetic rate,chlorophyll fluorescence and Cd accumulation in hyperaccumulator Sedum alfredii Hance were investigated to predict plant growth under Cd stress with rising atmospheric CO_2 concentration.Both pot and hydroponic experiments were conducted and the plants were grown under ambient(350 μL L~(-1)) or elevated(800 μL L~(-1))CO_2.Elevated CO_2 significantly(P 0.05) increased Pn(105%-149%),Pn_(max)(38.8%-63.0%) and AQY(20.0%-34.8%) of S.alfredii in all the Cd treatments,but reduced chlorophyll concentration,dark respiration and photorespiration.After 10 days growth in medium with 50μM Cd under elevated CO_2,PSII activities were significantly enhanced(P 0.05) with Pm,Fv/Fm,Φ(Ⅱ) and qP increased by 66.1%,7.5%,19.5%and 16.4%,respectively,as compared with ambient-grown plants.Total Cd uptake in shoot of S.alfredii grown under elevated CO_2 was increased by 44.1%-48.5%,which was positively correlated with the increase in Pn.These results indicate that elevated CO_2 promoted the growth of 5.alfredii due to increased photosynthetic carbon uptake rate and photosynthetic lightuse efficiency,and showed great potential to improve the phytoextraction of Cd by S.alfredii.     

The Photosynthetic Response to Elevated CO2 in High Altitude Potato Species (Solanum curtilobum)

Plants of Solanum curtilobum (from high altitude) and Solanum tuberosum (from low altitude) were grown in open-top chambers in a greenhouse at either ambient (AC, 360 µmol mol^–1) or ca. twice ambient (EC, 720 µmol mol^–1) CO₂ concentrations for 30 d. CO₂ treatments started at the reproductive stage of the plants. There were similar patterns in the physiological response to CO₂ enrichment in the two species. Stomatal conductance was reduced by 59 % in S. tuberosum and by 55 % in S. curtilobum, but such a reduction did not limit the net photosynthetic rate (P _N), which was increased by approximately 56 % in S. curtilobum and 53 % in S. tuberosum. The transpiration rate was reduced by 16 % in both potato species while instantaneous transpiration efficiency increased by 80 % in S. tuberosum and 90 % in S. curtilobum. Plants grown under EC showed 36 and 66 % increment in total dry biomass, whereas yields (dry mass of tubers) were increased by 40 and 85 % in S. tuberosum and S. curtilobum, respectively. EC promoted productivity by increasing P _N. Thus S. tuberosum, cultivated around the world at low altitudes, and S. curtilobum, endemic of the highland Andes, respond positively to EC during the tuberisation stage.     

Photosynthetic performance of deep-water macroalgae (Phaeophyta,Dictyotales) off Bermuda

Photosynthetic performance and dark respiration rates were determined in situ for abundant macroalgae occurring between 27–49 m depths off Bermuda. Brown algae, particularly members of the order Dictyotales, predominated at all deep-water sites, and Stypopodium zonale was the most abundant species. Species showed net photosynthesis at very low ambient irradiances (<0.01 maximum I_o). Lobophora variegata, a species with a highly decumbent growth form, had low productivity across all irradiances. In contrast, Dictyota spp. (D. bartayresii, D. dichotoma, D. divaricata) and S. zonale had high photosynthetic capacity (ca. 400 µmol O₂ gdw^–1 h^–1), and light saturation was not evidenced at the highest ambient irradiance level (300 µE m^–1 s^–1) for species with thin, flat thalli. Light-harvesting pigment concentrations reflected tissue-nitrogen levels. C:N atom ratios, except for L. variegata and D. divaricata, were within the ratio for balanced growth. The repeated occurrence and photosynthetic efficiency of these Dictyotalean species in subtropical and tropical deep-water habitats emphasize their successful adaptation to low-light, nutrient-poor environments.     

Plant Molecular and Genomic Responses to Stresses in Projected Future CO2 Environment

The Earth has been undergoing climatic changes for centuries, driven by increasing concentration of atmospheric carbon dioxide (CO₂). The atmospheric CO₂ concentration has been predicted to reach 550–750 μmol mol^?1 by 2050, or twice as high as the current level. Much of the research in the last 20–30 years concerning elevated CO₂ (eCO₂) has been about how plants would respond to the eCO₂ at physiological levels. As eCO₂ can lead to more frequent drought or extreme high or low temperature, increasingly more research has focused on the interactions between eCO₂ and other abiotic stresses. How stresses may affect plant growth and development and productivity, as well as how agricultural practices may be altered to cope with these changes must be determined. These concerns have been the subject of numerous reviews. However, it is only in the last several years that data at the “omics” levels has been available to explore how necessary physiological changes may be brought about in a future complex environment. The systems biology approaches provide us an insight into the mechanism of plant responses to climatic changes at the genomics level. In this review, we present an overview of physiological effects of eCO₂ on plants, but focus on the interactions of eCO₂ with drought, high temperature, O₃, and multiple abiotic stresses, with particular emphasis at the molecular and genomics levels. We also provide perspectives on future research and emphasize the importance of integrated research on eCO₂ and multiple environmental stresses using systems biology approaches.     

Photosynthetic acclimation to long-term exposure to elevated CO2 concentration in Pinus radiata D. Don. is related to age of needles

The effects of CO₂ enrichment on photosynthesis and ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco) in current year and 1-year-old needles on the same branch were studied on Pinus radiata D. Don. trees growing for 4 years in large, open-top chambers at ambient (36 Pa) and elevated (65 Pa) CO₂ partial pressures. At this age trees were 3·5–4 m tall. Measurements made late in the growing cycle (March) showed that photosynthetic rates at the growth CO₂ concentration [(pCO₂)_a] were lower in 1-year-old needles of trees grown at elevated CO₂ concentrations than in those of trees grown at ambient (pCO₂)_a. At elevated CO₂ concentrations V_cmax (maximum carboxylation rate) was reduced by 13% and J_max (RuBP regeneration capacity mediated by maximum electron transport rate) by 17%. This corresponded with photosynthetic rates at the growth (pCO₂)_a of 4·68 ± 0·41 μmol m^–2 s^–1 and 6·15 ± 0·46 μmol m^–2 s^–1 at 36 and 65 Pa, respectively (an enhancement of 31%). In current year needles photosynthetic rates at the growth (pCO₂)_a were 6·2 ± 0·72 μmol m^–2 s^–1 at 36 Pa and 10·15 ± 0·64 μmol m^–2 s^–1 at 65 Pa (an enhancement of 63%). The smaller enhancement of photosynthesis in 1-year-old needles at 65 Pa was accompanied by a reduction in Rubisco activity (39%) and content (40%) compared with that at 36 Pa. Starch and sugar concentrations in 1-year-old needles were not significantly different in the CO₂ treatments. There was no evidence in biochemical parameters for down-regulation at elevated (pCO₂)_a in fully fexpanded needles of the current year cohort. These data show that enhancement of photosynthesis continues to occur in needles after 4 years’ exposure to elevated CO₂ concentrations. Photosynthetic acclimation reduces the degree of this enhancement, but only in needles after 1 year of growth. Thus, responses to elevated CO₂ concentration change during the lifetime of needles, and acclimation may not be apparent in current year needles. This transitory effect is most probably attributable to the effects of developmental stage and proximity to actively growing shoots on sink strength for carbohydrates. The implications of such age-dependent responses are that older trees, in which the contribution of older needles to the photosynthetic biomass is greater than in younger trees, may become progressively more acclimated to elevated CO₂ concentration.