The effect of elevated carbon dioxide and ozone on leaf- and branch-level photosynthesis and potential plant-level carbon gain in aspen

Two aspen (Populus tremuloides Michx.) clones, differing in O₃ tolerance, were grown in a free-air CO₂ enrichment (FACE) facility near Rhinelander, Wisconsin, and exposed to ambient air, elevated CO₂, elevated O₃ and elevated CO₂+O₃. Leaf instantaneous light-saturated photosynthesis (P_S) and leaf areas (A) were measured for all leaves of the current terminal, upper (current year) and the current-year increment of lower (1-year-old) lateral branches. An average, representative branch was chosen from each branch class. In addition, the average photosynthetic rate was estimated for the short-shoot leaves. A summing approach was used to estimate potential whole-plant C gain. The results of this method indicated that treatment differences were more pronounced at the plant- than at the leaf- or branch-level, because minor effects within modules accrued in scaling to plant level. The whole-plant response in C gain was determined by the counteracting changes in P_S and A. For example, in the O₃-sensitive clone (259), inhibition of P_S in elevated O₃ (at both ambient and elevated CO₂) was partially ameliorated by an increase in total A. For the O₃-tolerant clone (216), on the other hand, stimulation of photosynthetic rates in elevated CO₂ was nullified by decreased total A.     

The greater seedling high-light tolerance of <Emphasis Type="Italic">Quercus robur</Emphasis> over <Emphasis Type="Italic">Fagus sylvatica </Emphasis>is linked to a greater physiological plasticity

The responses of Quercus robur (oak) and Fagus sylvatica (beech) seedlings to four different light environments (full, 50%, 40% and 15% sunlight) and to a rapid increase in irradiance were explored during the summer, after 2 years of growth in a forest nursery at Nancy (France). Significant differences between the two species were found for most variables. Phenotypic plasticity for morphological variables (root-shoot ratio, leaf size, leaf weight ratio) was higher in beech than in oak, while the reverse was true for anatomical (stomatal density, epidermis thickness, exchange surface area of the palisade parenchyma) and physiological (maximum photosynthetic rate, stomatal conductance, Rubisco activity) variables. Predawn photochemical efficiency (F_v/F_m) was higher in oak than in beech in all light environments except in 15% sunlight. F_v/F_m was significantly lower in 100% sunlight than in the other light environments in beech but not in oak. Maximum photosynthetic rates (A_max) increased with increasing light availability in the two species but they were always higher in oak than in beech. Oak exhibited higher Rubisco activity than beech in full sunlight. The transfer of shade-adapted seedlings to the open caused a decrease of F_v/F_m, which was larger for beech than for oak. Transferred oak but not beech plants recovered gradually to the control F_v/F_m values. The decreased chlorophyll content and the increased non-photochemical quenching observed in high-light beech seedlings were not enough to avoid photoinhibition. The results suggest that a greater tolerance of strong irradiance is linked to an enhanced physiological plasticity (variables related to photosynthesis), while shade tolerance relies on an enhanced plasticity in light-harvesting variables (crown morphology and chlorophyll content).     

Gas exchange and chlorophyll fluorescence responses of <Emphasis Type="Italic">Pinus radiata</Emphasis> D. Don seedlings during and after several storage regimes and their effects on post-planting survival

Post-storage gas exchange parameters like CO₂ assimilation, stomatal conductance, transpiration, water use efficiency and intercellular CO₂ concentrations, together with several chlorophyll a fluorescence parameters: F_o, F_v, F_v/F_m, F_m/F_o and F_v/F_o were examined in radiata pine (Pinus radiata D. Don) seedlings that were stored for 1, 8 or 15 days at 4° or 10°C with or without soil around the roots. Results were analysed in relation to post-storage water potential and electrolyte leakage in order to forecast their vitality (root growth potential) following cold storage, and post-planting survival potential under optimal conditions. During storage at 4° and 10°C, photosynthesis was reduced, being more pronounced in bare-root seedlings than in seedlings with soil around the roots. The depletion of CO₂ assimilation seemed not to be solely a stomatal effect as effects on chloroplasts contributed to this photosynthetic inhibition. Thus, the fall in the ratios F_v/F_m, F_v/F_o and F_m/F_o indicated photochemical apparatus damage during storage. Photosynthetic rate was positively correlated with the root growth index and new root length showing that new root growth is dependent primarily on current photosynthesis. Pre-planting exposure of bare-root radiata pine seedlings to temperatures of 10°C for more than 24 h during transportation or storage is not recommended.     

大气臭氧浓度升高对水稻叶片膜脂过氧化及保护酶活性的影响

采用开顶式气室（open top chambers,OTCs）装置,以水稻品种“3694繁”(Oryza sativa L.)为材料,研究3种处理：过滤大气（CF,O₃浓度约为20 nl·L^-1）、环境大气（NF,O₃浓度约为40 nl·L^-1）和高浓度O₃（EO,O₃浓度约为75 nl·L^-1）下叶片可溶性蛋白质含量、膜脂过氧化程度与主要保护酶活性的变化.结果表明：过滤大气与环境大气处理之间各个指标无显著差异.与CF处理相比,高浓度O₃处理条件下水稻叶片中可溶性蛋白含量显著下降,而过氧化氢（H₂O₂）和抗坏血酸（ASA）含量显著增加;超氧化物歧化酶（SOD）、过氧化氢酶（CAT）、过氧化物酶（POD）活性升高,最大升高幅度分别为93.7%、39.9%和312.4%;抗坏血酸过氧化物酶（APX）活性呈先升高后下降趋势; 不同O₃处理条件下叶片中丙二醛（MDA)含量无显著变化.表明抗氧化系统可有效抑制水稻叶片中的O₃胁迫引起的膜质过氧化作用,说明水稻品种“3694繁”对O₃胁迫有一定抗性.     

Assessing the genetic relatedness of higher ozone sensitivity of modern wheat to its wild and cultivated progenitors/relatives

Modern wheat (Triticum aestivum L.) is one of the most ozone(O₃)-sensitive crops. However, little is known about its geneticbackground of O₃ sensitivity, which is fundamental for breedingO₃-resistant cultivars. Wild and cultivated species of winterwheat including donors of the A, B and D genomes of T. aestivumwere exposed to 100 ppb O₃ or charcoal-filtered air in opentop chambers for 21 d. Responses to O₃ were assessed by visibleO₃ injury, gas exchange, chlorophyll fluorescence, relativegrowth rate, and biomass accumulation. Ozone significantly decreasedlight-saturated net photosynthetic rate (–37%) and instantaneoustranspiration efficiency (–42%), but increased stomatalconductance (+11%) and intercellular CO₂ concentration (+11%).Elevated O₃ depressed ground fluorescence (–8%), maximumfluorescence (–26%), variable fluorescence (–31%),and maximum photochemical efficiency (–7%). Ozone alsodecreased relative growth rate and the allometric coefficient,which finally reduced total biomass accumulation (–54%),but to a greater extent in roots (–77%) than in the shoot(–44%). Winter wheat exhibited significant interspeciesvariation in the impacts of elevated O₃ on photosynthesis andgrowth. Primitive cultivated wheat demonstrated the highestrelative O₃ tolerance followed by modern wheat and wild wheatshowed the lowest. Among the genome donors of modern wheat,Aegilops tauschii (DD) behaved as the most O₃-sensitive followedby T. monococcum (AA) and Triticum turgidum ssp. durum (AABB)appeared to be the most O₃-tolerant. It was concluded that thehigher O₃ sensitivity of modern wheat was attributed to theincreased O₃ sensitivity of Aegilops tauschii (DD), but notto Triticum turgidum ssp. durum (AABB) during speciation. Key words: Biomass, Chl a fluorescence, genome, ozone sensitivity, relative growth rate, stomatal conductance, winter wheat Received 20 September 2007; Revised 30 November 2007 Accepted 16 January 2008     

Chlorophyll fluorescence and photosynthetic response to light in 1-year-old needles during spring and early summer in <Emphasis Type="Italic">Pinus leucodermis</Emphasis>

Ten-year-old trees from four Italian populations of Pinus leucodermis (populations A, B, C and D), which were collected from different sites at different altitudes, were grown near Florence, Italy. Needle CO₂ gas exchange and chlorophyll fluorescence response to increasing light intensities were evaluated; gas exchange and chlorophyll fluorescence variation between April and July were also monitored. Populations A, B and C showed a similar photosynthetic response to increasing photosynthetic photon flux density (PPFD) intensities, while at various light intensities population D, which originated from the highest altitude, showed the highest photosynthetic rates. In this population photosynthesis was saturated at PPFDs higher than 900 µmol m^-2s^-1 and a slow decrease of effective photosystem II quantum yield and F'_V/F'_M in response to increasing PPFDs were found. The same trees also showed a faster recovery in photosynthesis from limitations induced by winter temperatures than the other three populations. This work showed that photosynthetic response to light in population D was different from the other populations; trees from this population were probably naturally selected to prevent photoinhibition due to excess light.     

Photosynthesis, photoinhibition, and nitrogen use efficiency in native and invasive tree ferns in Hawaii

Photosynthetic gas exchange, chlorophyll fluorescence, nitrogen use efficiency, and related leaf traits of native Hawaiian tree ferns in the genus Cibotium were compared with those of the invasive Australian tree fern Sphaeropteris cooperi in an attempt to explain the higher growth rates of S. cooperi in Hawaii. Comparisons were made between mature sporophytes growing in the sun (gap or forest edge) and in shady understories at four sites at three different elevations. The invasive tree fern had 12-13 cm greater height increase per year and approximately 5 times larger total leaf surface area per plant compared to the native tree ferns. The maximum rates of photosynthesis of S. cooperi in the sun and shade were significantly higher than those of the native Cibotium spp (for example, 11.2 and 7.1 µmol m^-2 s^-1, and 5.8 and 3.6 µmol m^-2 s^-1 respectively for the invasive and natives at low elevation). The instantaneous photosynthetic nitrogen use efficiency of the invasive tree fern was significantly higher than that of the native tree ferns, but when integrated over the life span of the frond the differences were not significant. The fronds of the invasive tree fern species had a significantly shorter life span than the native tree ferns (approximately 6 months and 12 months, respectively), and significantly higher nitrogen content per unit leaf mass. The native tree ferns growing in both sun and shade exhibited greater photoinhibition than the invasive tree fern after being experimentally subjected to high light levels. The native tree ferns recovered only 78% of their dark-acclimated quantum yield (F_v/F_m), while the invasive tree fern recovered 90% and 86% of its dark-acclimated F_v/F_m when growing in sun and shade, respectively. Overall, the invasive tree fern appears to be more efficient at capturing and utilizing light than the native Cibotium species, particularly in high-light environments such as those associated with high levels of disturbance.     

Wine vinasses treatments by ozone and an activated sludge system in continuous reactors

In this work wine vinasses have been treated separately by means of a chemical ozonation and a biological aerobic degradation in an activated sludge system, and later by means of a combined process which consisted of an aerobic pretreatment followed by an ozonation treatment, in continuous reactors in all cases. In the ozonation experiments, the hydraulic retention time and the ozone partial pressure were varied leading to substrate removals in the range 4.4-16%, with increases in this removal when both operating variables were increased. A kinetic study, which combines mixed flow reactor model for the liquid phase and plug flow reactor model for the gas phase, allows to determine the rate constant for the ozone reaction and the consumption ratio, which are k_O3 = 3.6 l/(g COD · h) and b = 22.5 g COD degraded/mol O₃ consumed. The aerobic degradation experiments were conducted in the activated sludge system with variations in the retention time and influent organic substrate concentration in the wastewater. A modified Contois model applied to the experimental results leads to the determination of the kinetic parameters of that model: K₁ = 5.43 l/g VSS and q_max = 6.29 g COD/(g VSS · h). Finally, the combined process reveals an improvement in the efficiency of the ozonation stage due to the previous aerobic treatment with increases in the substrate removal reached and in the rate constant obtained, the last one being k_O3 = 5.6 l/(g COD · h).     

Is a short,sharp shock equivalent to long‐term punishment? Contrasting the spatial pattern of acute and chronic ozone damage to soybean leaves via chlorophyll fluorescence imaging

Experimental investigations of ozone (O₃) effects on plants have commonly used short, acute [O₃] exposure (>100 ppb, on the order of hours), while in field crops damage is more likely caused by chronic exposure (<100 ppb, on the order of weeks). How different are the O₃ effects induced by these two fumigation regimes? The leaf‐level photosynthetic response of soybean to acute [O₃] (400 ppb, 6 h) and chronic [O₃] (90 ppb, 8 h d^?1, 28 d) was contrasted via simultaneous in vivo measurements of chlorophyll a fluorescence imaging (CFI) and gas exchange. Both exposure regimes lowered leaf photosynthetic CO₂ uptake about 40% and photosystem II (PSII) efficiency (F_q′/F_m′) by 20% compared with controls, but this decrease was far more spatially heterogeneous in the acute treatment. Decline in F_q′/F_m′ in the acute treatment resulted equally from decreases in the maximum efficiency of PSII (F_v′/F_m′) and the proportion of open PSII centres (F_q′/F_v′), but in the chronic treatment decline in F_q′/F_m′ resulted only from decrease in F_q′/F_v′. Findings suggest that acute and chronic [O₃] exposures do not induce identical mechanisms of O₃ damage within the leaf, and using one fumigation method alone is not sufficient for understanding the full range of mechanisms of O₃ damage to photosynthetic production in the field.     

Responses of gas exchange,cellular membrane integrity,and antioxidant enzymes activities of salinity-stressed winter wheat to ozone pollution

A sand-culture experiment was conducted in open-top chambers which were constructed in a greenhouse to investigate the responses of salt-stressed wheat (Triticum aestivum L.) to O₃. Plant seeding of JN17 (a popular winter wheat cultivar) was grown in saltless (−S) and saline (+S, 100 mM NaCl) conditions combined with charcoal-filtered air (CF, < 5 ppb O₃) and elevated O₃ (+O₃, 80 ± 5 ppb, 8 h day⁻¹) for 30 d. O₃ significantly reduced net photosynthetic rate (PN), stomatal conductance, chlorophyll contents and plant biomass in -S treatment, but no considerable differences were noted in those parameters between +O₃+S and CF+S treatments. O₃-induced loss in cellular membrane integrity was significant in -S plants, but not in +S plants evidenced by significant elevations being measured in electrolyte leakage (EL) and malondialdehyde (MDA) content in -S plants, but not in +S plants. Both O₃ and salinity increased proline content and stimulated antioxidant enzymes activities. Soluble protein increased by salinity but decreased by O₃. Abscisic acid (ABA) was significantly elevated by O₃ in -S plants but not in +S plants. The results of this study suggested that the specificity of different agricultural environments should be considered in order to develop reliable prediction models on O₃ damage to wheat plants.     

Tissue structure and respiration of stems of <Emphasis Type="Italic">Betula pendula</Emphasis> under contrasting ozone exposure and nutrition

Tissue structure and respiration (Rs) of stems were analyzed in Betula pendula grown throughout the growing season in either filtered air (control) or 90/40 nl O₃ l^-1 (day/night). Both regimes were split into high and low nutrient supply. High nutrition increased tissue and cell sizes within the stem xylem, phloem and periderm, whereas ozone (O₃) tended to reduce tissue widths, inhibiting in particular the cambial activity of xylem growth in low-fertilized, O₃-exposed plants (O₃/LF). Callose deposition was enhanced in the phloem sieve plates and tannins tended to condense in vacuoles of parenchyma cells under O₃ stress. Decline occurred close to lenticels, related to O₃ impact during shoot differentiation and was probably exacerbated by the limited assimilate translocation. Radial stem growth ceased 4 weeks earlier than in control plants; however, the area-based Rs was enhanced during intense growth in high-fertilized, O₃-exposed plants. Photosynthetic CO₂ refixation of stems reached about 50% of their dark respiration rate and the relative growth rate (RGR) did not differ between treatments. At high nutrition, RGR enhanced Rs to levels twice as high as the maintenance level. Unit construction costs appeared to be similar in each treatment, although CO₂ release on a volume-increment basis was lowered by 45% in O₃/LF plants. This latter effect is ascribed to lowered maintenance demands of a xylem remaining reduced in width by 50%. The high respiratory costs in the carbon balance of O₃/LF plants result from an enhanced leaf rather than stem respiration, given the high demand for stress compensation in the foliage.     

In situ oxygen microelectrode measurements of bottom-ice algal production in McMurdo Sound, Antarctica

In-situ estimates of fast-ice algal productivity at Cape Evans, McMurdo Sound, in 1999 were lower than at the same site in previous years. Under-ice irradiance was between 0 and 8 µmol photons m^-2 s^-1; the ice was between 1.9 and 2.0 m thick and the algal biomass averaged 150 mg chl a m^-2, although values as high as 378 mg chl a m^-2 were recorded. Production on 11 and 12 November was between 0.053 and 1.474 mg C m^-2 h^-1. When the data from 11 November were fitted to a hyperbolic tangent function, a multilinear regression gave estimates for P_max of 0.571 nmol O₂ cm^-2 s^-1, an ! of 0.167 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1 and an E_k of 3.419 µmol photons m^-2 s^-1. A P_max of 2.674 nmol O₂ cm^-2 s^-1, an ! of 0.275 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1, r of 0.305 nmol O₂ cm^-2 s^-1 and an E_k of 9.724 µmol^-1 photons m^-2 s^-1 were estimated from the 12 November data. The sea-ice algal community was principally comprised of Nitzschia stellata, Entomoneis kjellmanii and Berkeleya adeliensis. Other taxa present included N. lecointei, Fragilariopsis spp., Navicula glaciei, Pleurosigma spp. and Amphora spp. Variations in the method for estimating the thickness of the diffusive boundary layer were not found to significantly affect the measurements of oxygen flux. However, the inability to accurately measure fine-scale variations in biomass is thought to contribute to the scatter of the P versus E data.     

Sensitivity of Metasequoia glyptostroboides to ozone stress

2-year-old seedlings of Metasequoia glyptostroboides were grown in open top chambers and exposed to four ozone concentrations [O₃] (charcoal-filtered air, CF; 50, 100, and 200 mm³ m⁻³) for 25 d. Measurements of growth, leaf chlorophyll (Chl) content, and gas exchange parameters were made before and/or after O₃ exposure. Leaf length, crown width, Chl a/b, net photosynthetic rate, stomatal conductance, and transpiration rate were significantly reduced at 100 and 200 mm³(O₃) m⁻³. A remarkable decrease in stomatal conductance also occurred at 50 mm³(O₃) m⁻³.     

Microbial community composition and function beneath temperate trees exposed to elevated atmospheric carbon dioxide and ozone

We hypothesized that changes in plant growth resulting from atmospheric CO₂ and O₃ enrichment would alter the flow of C through soil food webs and that this effect would vary with tree species. To test this idea, we traced the course of C through the soil microbial community using soils from the free-air CO₂ and O₃ enrichment site in Rhinelander, Wisconsin. We added either ¹³C-labeled cellobiose or ¹³C-labeled N-acetylglucosamine to soils collected beneath ecologically distinct temperate trees exposed for 3 years to factorial CO₂ (ambient and 200 µl l^-1 above ambient) and O₃ (ambient and 20 µl l^-1 above ambient) treatments. For both labeled substrates, recovery of ¹³C in microbial respiration increased beneath plants grown under elevated CO₂ by 29% compared to ambient; elevated O₃ eliminated this effect. Production of ¹³C-CO₂ from soils beneath aspen (Populus tremuloides Michx.) and aspen-birch (Betula papyrifera Marsh.) was greater than that beneath aspen-maple (Acer saccharum Marsh.). Phospholipid fatty acid analyses (¹³C-PLFAs) indicated that the microbial community beneath plants exposed to elevated CO₂ metabolized more ¹³C-cellobiose, compared to the microbial community beneath plants exposed to the ambient condition. Recovery of ¹³C in PLFAs was an order of magnitude greater for N-acetylglucosamine-amended soil compared to cellobiose-amended soil, indicating that substrate type influenced microbial metabolism and soil C cycling. We found that elevated CO₂ increased fungal activity and microbial metabolism of cellobiose, and that microbial processes under early-successional aspen and birch species were more strongly affected by CO₂ and O₃ enrichment than those under late-successional maple.     

Photosynthesis and crop growth of spring oilseed rape and broccoli under elevated tropospheric ozone

The impact of a season-long exposure to moderately elevated tropospheric O₃ concentrations on the canopy growth and photosynthetic capacity of two important Brassica crops, spring oilseed rape and broccoli, was studied during three consecutive growing seasons (2007–2009). Brassica napus L. cv. Ability and Brassica oleracea L. cv. Monaco were exposed to non-filtered ambient air (NF) and non-filtered air with addition of 20 (NF+) and/or 40 ppb O₃ (NF++) in open-top chambers. Light saturated CO₂ assimilation (A_sat), stomatal conductance (g_st), maximum and actual quantum yield of photosystem II (F_v/F_m, F_v′/F_m′), performance index (PI) and leaf area index (LAI) were monitored on a weekly basis from emergence or planting until harvest. Before flowering, elevated O₃ did not have an influence on LAI nor on the photosynthetic capacity of the upper canopy leaves of either crops. This corresponded with the absence of a reduction of aboveground biomass of oilseed rape at maximum leaf area (MLA) and of broccoli plants harvested before flowering. After flowering, which coincided with MLA, the oilseed rape canopy showed a faster decline of LAI and of the chlorophyll content in NF+ compared to NF. In the NF++ treatment, this effect was intensified with an additional decrease of A_sat, g_st, F_v/F_m, F_v′/F_m′ and PI. In broccoli these detrimental O₃ effects were only detected in the lower canopy leaves. The changes in canopy development and photosynthetic performance of the upper canopy leaves can unravel the underlying mechanisms leading to the contrasting yield effects of O₃ on broccoli and spring oilseed rape that were previously reported (De Bock et al., 2011).     

不同浓度NaCl和Na2CO3处理对菊芋幼苗光合及叶绿素荧光的影响

 以砂培菊芋(Helianthus tuberosus)幼苗作为试验材料,分别进行不同浓度NaCl (50、 100、150、200、250 mmol&;#8226;L^－1)和Na₂CO₃ (25、50、 75、100、125 mmol&;#8226;L^－1)胁迫处理,以1/2全营养液作为对照,处理7 d后研究NaCl和Na₂CO₃胁迫处理对菊芋幼苗叶片光合作用及叶绿素动力学 参数的影响。结果表明：1)在NaCl处理下,当浓度小于150 mmol&;#8226;L^－1时,增加了菊芋的叶绿素含量、净光合速率(Net photosynthetic rate, P_n)和气孔导度(Stomatal conductivity, G_s),对荧光参数PSⅡ的电子传递情况( F_m/F_o)、PSⅡ原初光能转换效率(F_v/F_m)、PSⅡ量子效率 (Actual quantum yield of PSⅡ under actinic irradiation,φPSⅡ)和光化学猝灭系数(Photochemical quenching coefficient, qP)和非 光化学猝灭系 数(Non-photochemical quenching coefficient, NPQ)没有显著影响,随着浓度的增加,各项生理指标与对照相比除了NPQ显著 增加,其余均显著降低;2)在Na₂CO₃胁迫处理下,随着Na₂CO₃浓度的增加,与对照相比菊芋幼苗叶绿素含量、P_n、G_s以及叶绿素a荧光诱导动力 学参数F_m/F_o、F_v/F_m、φPSⅡ和qP均显著降低,NPQ显著增加;3)就NaCl和Na₂CO₃相比而言,在相同Na⁺浓度情况下,处于Na₂CO₃胁迫下的菊芋 幼苗的叶绿素含量、P_n、G_s以及叶绿素a荧光诱导动力学参数F_m/F_o、F_v/F_m、φPSⅡ和qP下降幅度和NPQ的增加幅度均显著大于NaCl,这说明 NaCl和Na₂CO₃胁迫均对菊芋幼苗造成不同程度的伤害,但在相同Na⁺浓度情况下,Na₂CO₃的伤害程度大于NaCl。由此说明菊芋对盐的忍耐程度高 于碱。     

Elevated CO2 reduces O3 flux and O3-induced yield losses in soybeans: possible implications for elevated CO2 studies

Soybeans were grown for three seasons in open-top field chambersto determine (1) whether elevated CO₂ (360 versus 700 µmolmol^–1) alleviates some of the yield loss due to pollutantO₃, (2) whether the partial stomatal closure resulting fromchronic O₃ exposure (charcoal-filtered air versus 1.5 ambientconcentrations) is a cause or result of decreased photosynthesis,and (3) possible implications of CO₂/O₃ interactions to climatechange studies using elevated CO₂. Leaf conductance was reducedby elevated CO₂, regardless of O₃ level, or by exposure to O₃alone. As.a result of these effects on conductance, high CO₂reduced estimated midday O₃ flux into the leaf by an averageof 50% in charcoal-filtered air and 35% in the high O₃ treatment.However, while exposure to O₃ reduced seed yields by 41% atambient CO₂ levels, the yield reduction was completely amelioratedby elevated CO₂. The threshold midday O₃ flux for yield lossappears to be 20–30 nmol m^–2 s^–1 in this study.Although elevated CO₂ increased total biomass production, itdid not increase seed yields. A/C_i curves show a large reductionin the stomatal limitation to photosynthesis due to elevatedCO₂ but no effect of O₃. These data demonstrate that (1) reducedconductance due to O₃ is the result, and not the cause, of reducedphotosynthesis, (2) 700 µmol mol^–1 CO₂ can completelyameliorate yield losses due to O₃ within the limits of theseexperiments, and (3) some reports of increased yields underelevated CO₂ treatments may, at least in part, reflect the ameliorationof unrecognized suppression of yield by O₃ or other stresses. Key words: Stomatal limitation, elevated CO₂, O₃ flux, Glycine max, yield suppression     

不同灌溉处理对铁观音茶树光合作用的影响

以田间栽培的2年生铁观音茶树为试验材料,应用叶绿素荧光诱导动力学技术,以不灌溉为对照,分析不同灌溉间隔时间处理［5 d（T₁）、10 d (T₂)、15 d (T₃)、20 d (T₄) 和25 d (T₅)］对铁观音茶树叶片光合作用的影响.结果表明: 随着灌溉间隔期的延长,2年生铁观音茶树叶片水势和叶绿素含量降低; 净光合速率(P_n)先上升后降低,在T₂下达到最大(15.55 μmol·m^-2·s^-1); 光系统Ⅱ(PSⅡ)的原初光能转化效率(F_v/F_m)、可变荧光衰减(ΔF_v)和可变荧光淬灭速率(ΔF_v/F_o)均在T₂下达到最大值,分别为0.844、342.5和4.03.初始荧光(F_o)随着灌溉间隔期的延长而降低,而对照的F_o则呈上升趋势,表明干旱可对茶树叶片PSⅡ造成损害.灌溉间隔期为10 d处理有利于茶树叶片光合电子的传递和CO₂的同化,提高茶树的光合作用效率.     

Stem radius changes and their relation to stored water in stems of young Norway spruce trees

Changes in the stem radius of young Norway spruce [Picea abies (L.) Karst.] were related to changes in stem water content in order to investigate the relationship between diurnal stem size fluctuations and internally stored water. Experiments were performed on living trees and on cut stem segments. The defoliated stem segments were dried under room conditions and weight (W), volume (V), and xylem water potential (O_s) were continuously monitored for 95 h. Additionally, photos of cross-sections of fresh and air-dried stem segments were taken. For stem segments we found that the change in V was linearly correlated to the change in W as long as O_s was >-2.3ǂ.3 MPa (phase transition point). Stem contraction occurred almost solely in the elastic tissues of the bark (cambium, phloem, and parenchyma), and the stem radius changes were closely coupled to bark water content. For living trees, it is therefore possible to estimate the daily contribution of "bark water" to transpiration from knowledge of the stem size and continuous measurements of the stem radius fluctuations. When O_s reaches the phase-transition point, water is also withdrawn from the inelastic tissue of the stem (xylem), which - in the experiment with stem segments - was indicated by an increasing ratio between (V and (W. We assume that for O_s below the transition point, air is sucked into the tracheids (cavitation) and water is also withdrawn from the xylem. Due to the fact that in living P. abies O_s rarely falls below -2.3ǂ.3 MPa and the xylem size is almost unaffected by radius fluctuations, dendrometers are useful instruments with which to derive the diurnal changes in the bark water contents of Norway spruce trees.     

Raman spectroscopy of pigments and oxalates in situ within epilithic lichens: Acarospora from the Antarctic and Mediterranean

The effects of ambient and elevated ozone levels on growth and photosynthesis of beech ( Fagus sylvatica ) were studied by exposing seedlings in open-top chambers for one growing season to three treatments: charcoal-filtered (CF), non-filtered (NF) and non-filtered air with addition of ozone (30 ppb ozone) on clear days for 8–10 h d⁻¹ (NF +). Ambient levels were relatively low and accumulated to an AOT40 (accumulated exposure over a threshold of 40 ppb) of 4055 ppb h (for the period 23 Apr–30 Sept). The NF + chambers received an AOT40 of 8880 ppb h. Throughout the growing season we measured growth and photosynthetic properties. The treatments did not cause strong effects: measurements of gas exchange (light-saturated assimilation rate, CO₂ and light-response curves) and chlorophyll fluorescence showed slight and mostly non-significant reductions of several parameters. No significant differences were found for growth, though in the NF + treatment (AOT40 8880 ppb h) the relative growth rate for diameter increment was at times reduced by 12% compared with the control treatment.