Responses to ozone in 12 provenances of European beech (Fagus sylvatica): genotypic variation and chamber effects on photosynthesis and dry-matter partitioning

Seedlings of 12 provenances of European beech ( Fagus sylvatica ) were exposed to ambient, non-filtered air (NF) or NF+50 nl l⁻¹ ozone (NF50) for 8 h d⁻¹ in open-top chambers (OTCs), from 1 June to 4 October 1995. In 1996 exposure was continued from 31 May to 1 October at four levels: charcoal-filtered air (CF), NF, NF50 and NF+100 nl l⁻¹ ozone (NF100). Provenances were grown for both seasons in outside reference plots. All treatments were replicated. Ozone did not affect gas exchange in the provenances until late in the second season. NF100 reduced photosynthesis by 18% in August 1996 compared to CF. In September, photosynthesis was reduced by 22% in NF50 and by 29% in NF100. After two seasons, ozone reduced the root:shoot ratio by 24% when comparing CF and NF100; this was caused by reductions in the root biomass. Ozone did not affect height growth or stem diameter, and there were no ozone×provenance interactions for any growth parameter. There was, however, a significant ozone×provenance interaction for photosynthesis, showing northwest European provenances to be more sensitive to ozone than southeast European provenances when comparing dose–response estimates. This is interpreted in terms of genetic adaptation of the photosynthetic apparatus to regional growing conditions. Seedlings in the chambers grew 45% taller, and had 28% more shoot biomass and 29% smaller root biomass, resulting in a 44% reduction of root:shoot ratios compared to seedlings outside. Increased temperature and decreased PAR inside the chambers relative to the outside were probably the main causes for the differences. The magnitude of the chamber effects in OTCs raises doubts about conclusions drawn from ozone exposures in such chambers. This and previous ozone experiments with OTCs may have reached inaccurate conclusions concerning the size of ozone responses due to chamber effects.     

Impact of four years exposure to different levels of ozone, phosphorus and drought on chlorophyll, mineral nutrients, and stem volume of Norway spruce, Picea abies

Saplings of one clone of Norway spruce, Picea abies (L.) Karst, were planted in 120 l pots in 1991 and exposed to three levels of ozone, two levels of phosphorus and two levels of water supply in 42 open-top chambers (OTCs), during 1992–1996. The effects of pots and OTCs were also tested. Nutrient concentrations of the needles were not affected by ozone, while the low phosphorus supply (LP) and drought stress (D) treatments had significant effects on several mineral nutrients, e.g. phosphorus, calcium, magnesium, manganese, sulphur and boron. Ozone reduced the chlorophyll concentration in the 2- and 3-year-old needles in 1994 and 1995. The highest ozone concentration reduced the stem volumes (− 8%), as well as the stem lengths (− 5%), of the saplings in 1993 and 1994, after two and three years of exposure. After the fourth growing season this ozone-induced reduction in stem volume disappeared which might be caused by pot limitation. LP supply and D both caused large decreases in the stem volume and length. The needles from LP treatment had as high P concentration as 1.2–1.5 mg g⁻¹, implying a need for increasing the critical value for phosphorus. The OTC enclosure stimulated the stem volume growth significantly compared to saplings growing in ambient plots. This was suggested to be attributed to the slightly higher temperature in the OTCs. The overall result is that ozone in southern Sweden is likely to have negative effects on Norway spruce trees, although much less than other environmental factors, e.g. water and phosphorus.     

臭氧胁迫对水稻生长以及C、N、S元素分配的影响

采用开顶式气室(Open-top Chamber, OTC),对水稻"3694繁"(Oryza sativa L., 3694 Fan)在浙江嘉兴进行田间原位臭氧(O₃)熏气实验,研究不同臭氧浓度熏气对水稻生长以及C、N,S元素分配的影响。实验设置分4个水平:过滤大气组(CF,10 nL/L)、自然大气组(NF,40 nL/L)和两个不同浓度的臭氧处理组(O₃-1:100 nL/L; O₃-2:150 nL/L)。主要结果表明:(1)开始臭氧熏气时,各个处理组单茎水稻各组分生物量没有差异. 在熏气后期(水稻成熟期),臭氧处理使单茎水稻根、茎和穗生物量显著下降,根冠比降低,株高显著降低,表明臭氧胁迫增加水稻地上部分的干物质分配,且对株高的影响可能大于对地上生物量的影响;(2)臭氧处理使水稻根和茎C元素含量下降,叶C元素含量上升,表明臭氧胁迫提高了叶片中碳分配,而降低了根和茎的碳分配;(3)各个组分N元素含量上升和碳氮比下降;(4)茎、叶和穗S元素含量上升,可能会增强水稻抗氧化系统的作用,从而抵抗臭氧胁迫。所有实验结果表明臭氧浓度升高会对水稻生长产生严重不利影响,从而导致水稻各个组分的C、N、S元素分配格局发生改变。     

外源抗坏血酸对臭氧胁迫下水稻叶片膜保护系统的影响

在田间原位条件下,运用OTCs（open top chamber）装置研究了外源抗坏血酸（exogenous ascorbate acid,ExAsA）对臭氧（O3）胁迫下水稻（Oryza Sativa L.）叶片膜保护系统的影响.研究发现,O3胁迫下的水稻叶片经过ExAsA处理后叶绿素a含量显著升高,而叶绿素b含量变化不明显;相对于对照,经ExAsA处理后的水稻叶片过氧化氢（H2O2）和丙二醛（MDA）含量及相对电导率（REC）均降低,超氧化物岐化酶（SOD）和抗坏血酸过氧化物酶（APX）活性明显提高,抗氧化剂类胡萝卜素（Carotene）含量升高.这表明,ExAsA改善了O3胁迫下水稻叶片的抗氧化系统功能,减少了叶片中活性氧（activity oxygen species,AOS）的积累,抑制了脂质过氧化（lipid peroxidation,LP）,延迟了O3对水稻叶片的老化作用,提高了水稻叶片对O3危害的抗性.     

Growth responses of yellow-poplar(Liriodendron tulipifera L.) exposed to 5 years of O3 aloneor combined with elevated CO2

Field‐grown yellow‐poplar (Liriodendron tulipifera L.) werefumigated from May to October in 1992–96 within open‐topchambers to determine the impact of ozone (O₃) aloneor combined with elevated carbon dioxide (CO₂) on saplinggrowth. Treatments were replicated three times and included: charcoal‐filteredair (CF); 1 × ambient ozone (1 × O₃);1·5 × ambient ozone (1·5 × O₃);1·5 × ambient ozone plus 350 p.p.m.carbon dioxide (1·5 × O₃ + CO₂)(target of 700 p.p.m. CO₂); and open‐air chamberlessplot (OA). After five seasons, the total cumulative O₃ exposure (SUM₀₀ = sumof hourly O₃ concentrations during the study) rangedfrom 145 (CF) to 861 (1·5 × O₃) p.p.m. × h (partsper million hour). Ozone had no statistically significant effecton yellow‐poplar growth or biomass, even though total root biomasswas reduced by 13% in the 1·5 × O₃‐exposedsaplings relative to CF controls. Although exposure to 1·5 × O₃ + CO₂ hada stimulatory effect on yearly basal area growth increment aftertwo seasons, significant increases in shoot and root biomass (～ 60% increaserelative to all others) were not detected until the fifth season.After five seasons, the yearly basal area growth increment of saplingsexposed to 1·5 × O₃ + CO₂‐air increasedby 41% relative to all others. Based on this multi‐yearstudy, it appears that chronic O₃ effects on yellow‐poplargrowth are limited and slow to manifest, and are consistent withprevious studies that show yellow‐poplar growth is not highly responsiveto O₃ exposure. In addition, these results show thatenriched CO₂ may ameliorate the negative effects of elevatedO₃ on yellow‐poplar shoot growth and root biomass underfield conditions.     

High-throughput root phenotyping screens identify genetic loci associated with root architectural traits in Brassica napus under contrasting phosphate availabilities

Background and Aims

Phosphate (Pi) deficiency in soils is a major limiting factor for crop growth worldwide. Plant growth under low Pi conditions correlates with root architectural traits and it may therefore be possible to select these traits for crop improvement. The aim of this study was to characterize root architectural traits, and to test quantitative trait loci (QTL) associated with these traits, under low Pi (LP) and high Pi (HP) availability in Brassica napus.

Methods

Root architectural traits were characterized in seedlings of a double haploid (DH) mapping population (n = 190) of B. napus [‘Tapidor’ × ‘Ningyou 7’ (TNDH)] using high-throughput phenotyping methods. Primary root length (PRL), lateral root length (LRL), lateral root number (LRN), lateral root density (LRD) and biomass traits were measured 12 d post-germination in agar at LP and HP.

Key Results

In general, root and biomass traits were highly correlated under LP and HP conditions. ‘Ningyou 7’ had greater LRL, LRN and LRD than ‘Tapidor’, at both LP and HP availability, but smaller PRL. A cluster of highly significant QTL for LRN, LRD and biomass traits at LP availability were identified on chromosome A03; QTL for PRL were identified on chromosomes A07 and C06.

Conclusions

High-throughput phenotyping of Brassica can be used to identify root architectural traits which correlate with shoot biomass. It is feasible that these traits could be used in crop improvement strategies. The identification of QTL linked to root traits under LP and HP conditions provides further insights on the genetic basis of plant tolerance to P deficiency, and these QTL warrant further dissection.     

Survival and recovery of perennial forage grasses under prolonged Mediterranean drought: II. Water status, solute accumulation, abscisic acid concentration and accumulation of dehydrin transcripts in bases of immature leaves

Effects of ozone on spring wheat ( Triticum aestivum L. cv. Satu) were studied in an open-top chamber experiment during two growing seasons (1992–1993) at Jokioinen in south-west Finland. The wheat was exposed to filtered air (CF), non-filtered air (NF), non-filtered air+35 nl l⁻¹ ozone for 8 h d⁻¹ (NF⁺) and ambient air (AA). Each treatment was replicated five times. Two wk after anthesis, after 4 wk of ozone treatment (NF⁺, 45 nl l⁻¹ 1000–1800 hours, seasonal mean) the net CO₂ uptake of wheat flag leaves was decreased by c . 40% relative to CF and NF treatments, both initial and total activity of Rubisco and the quantity of protein-bound SH groups were decreased significantly. Added ozone also significantly accelerated flag leaf senescence recorded as a decrease in chloroplast size. The effect was significant 2 wk after anthesis, and senescence was complete after 4 wk. In the CF and NF treatments senescence was complete 5 wk after anthesis. The significant effect of ozone on the chloroplasts and net CO₂ uptake 2 wk after anthesis did not affect the grain filling rate. However, since the grain filling period was shorter for ozone fumigated plants, kernels were smaller. The decrease in 1000-grain weight explained most of the yield reduction in the plants under NF⁺ treatment. The results indicate that wheat plants are well buffered against substantial decrease in source activity, and that shortened flag leaf duration is the major factor causing ozone-induced yield loss.     

臭氧对水稻根系活力、可溶性蛋白含量与抗氧化系统的影响

采用旋转布气法开顶式气室 (Open top chambers, OTCs) 装置, 研究4种臭氧 (O3) 浓度水平 (过滤大气, O3浓度20nl·L-1;环境大气, O3浓度40nl·L-1;中等O3浓度处理, O3浓度为75nl·L-1;高浓度处理, O3浓度为150nl·L-1) 下水稻 (Oryzasativa) 根系中根系活力、可溶性蛋白含量、膜脂过氧化程度与抗氧化系统的变化差异。主要结果表明与过滤大气处理相比, O3浓度升高 (75和150nl·L-1) 使植株根系活力显著降低, 根系大幅度、过早地衰退;根系可溶性蛋白质含量显著下降;根系MDA含量显著升高, 膜脂过氧化程度加剧;SOD活性呈先升高后下降的变化趋势根系中H2O2含量大幅度显著上升, 并随着O3处理浓度升高和暴露时间延长变化幅度增大;CAT与POD活性则表现出升高趋势, 但处理后期升高幅度略微降低;整个处理期间根系ASA含量无显著变化。环境大气处理与过滤大气处理植株各个指标变化趋势基本一致并略微下降, 随着处理时间延长根系活力与蛋白质含量出现显著下降, 其他指标无显著差异。试验结果表明O3浓度升高会对植物地下部分根系产生影响;随着O3胁迫时间的延长, 植物将面临着缺乏强有力的根系生理代谢活力支持。     

Detection of QTL for phosphorus efficiency at vegetative stage in Brassica napus

Phosphorus (P) deficiency in soils is a major limiting factor for plant growth worldwide. Plants have developed adaptive strategies in response to P deficiency. The objective of this study was to map quantitative trait loci (QTL) for P efficiency using a recombinant inbred (RI) population consisting of 124 lines derived from a cross between Brassica napus P-inefficient cv. B104-2 and P-efficient cv. Eyou Changjia. Six traits (shoot dry weight, root dry weight, root/shoot ratio, P concentration, shoot P uptake and shoot P use efficiency) at vegetative stage were examined under high P (HP, 1 mM) and low P (LP, 5 ??M) conditions during three separate experimental trial periods. Their relative values (i.e., the ratio of a trait value under the LP condition to that under the HP condition) of these six traits were also determined. Eyou Changjia produced more biomass and acquired more P under the LP condition and, thus, had a higher relative dry weight and relative P uptake than B104-2, indicating Eyou Changjia was high P efficiency. A total of 71 QTL were detected on 13 linkage groups, including 28 QTL under the LP condition, 22 QTL under the HP condition and 21 QTL for relative traits. Nineteen and nine QTL were specific for the LP and HP conditions, respectively, suggesting that different mechanisms existed under the two P condition. Twelve of the twenty-one QTL for relative traits co-localized with QTL identified under the two P conditions. In addition, 18 orthologous genes involved in the P metabolic pathway of Arabidopsis were in silico mapped to the QTL confidence intervals identified in B. napus by comparative genomic analysis. These QTL and their corresponding candidate genes should be further investigated to better understand P efficiency in B. napus.     

Competitive interactions between Nardus stricta L. and Calluna vulgaris (L.) Hull: the effect of fertilizer and defoliation on above- and below-ground performance

1 The role of nutrient supply and defoliation on the competitive interactions between pot-grown Calluna vulgaris and Nardus stricta plants was investigated.WP leading adjustment
2 Young plants were grown alone and together in pots under a combination of fertilizer and defoliation treatments. After 18 months, parameters reflecting both above- and below-ground performance were measured, namely: total above-ground biomass, shoot nitrogen and phosphorus content, root length and the extent of mycorrhizal infection of the roots.
3 In the pots that received fertilizer, the shoot nutrient content and above-ground biomass of Nardus plants increased to a greater extent than those of Calluna plants; this effect was more marked for Nardus plants growing with Calluna plants than for those growing with other Nardus plants. In contrast , Calluna plants growing in competition with Nardus failed to respond to the addition of nutrients. However, in unfertilized pots, Calluna gained more above-ground biomass during the experimental period than Nardus.
4 Calluna had greater root length than Nardus , but Nardus had a higher proportion of its root length infected by mycorrhizal fungi. In both plants, the addition of fertilizer reduced the mycorrhizal infection and increased the root length. Nardus root length was decreased when grown in competition with Calluna only in pots where no nutrients were added. Defoliation decreased the extent of mycorrhizal infection in Calluna roots but not in those of Nardus; defoliation decreased the shoot nutrient content in Calluna plants, but not in Nardus plants.
5 These results suggest that the competitive balance between Nardus and Calluna may be altered by the addition of nutrients, and by defoliation, which may have serious implications for the future dominance of Calluna in heathland ecosystems, particularly those where nutrient inputs are increasing significantly or where grazing pressures are high.     

Effect of ambient ozone at the somma of Lake Mashu on growth and leaf gas exchange in Betula ermanii and Betula platyphylla var. japonica

We examined the effects of ambient ozone, at the somma of Lake Mashu in northern Japan, on the growth and photosynthetic traits of two common birch species in Japan (mountain birch and white birch). Seedlings of the two birch species were grown in open-top chambers and were exposed to charcoal-filtered ambient air (CF) or non-filtered ambient air (NF) at the somma of Lake Mashu during the growing season in 2009. For the mountain birch, ambient ozone significantly increased the ratio of aboveground dry mass to belowground dry mass (T/R ratio), although no difference in the whole-plant biomass was observed between the treatments. For the white birch, in contrast, ozone exposure at ambient level did not decrease in growth and photosynthesis. These results suggest that ambient O₃ at the somma of Lake Mashu may shift the allocation of biomass to above-ground rather than below-ground in the mountain birch.     

The influence of ozone on the stomatal and non-stomatal limitation of photosynthesis in Norway spruce,Picea abies (L.) Karst,exposed to soil moisture deficit

Summary Norway spruce, Picea abies (L.) Karst., was exposed to charcoal-filtered air (CF) and non-filtered air + ozone (NF+) and periods of soil moisture deficit from 1985 to 1988 in open-top chambers. Net photosynthesis, stomatal conductance, needle water potential and various shoot properties were measured on 1-year-old shoots during a period of soil moisture deficit. The gas exchange was measured at saturating photosynthetic photon flux density and across a range of CO₂ concentrations. The soil moisture deficit induced a mild drought stress in the plants, expressed by a pre-dawn needle water potential of approximately-0.9 MPa and a substantial reduction in net photosynthesis and gas phase conductance. In the CF treatment, intercellular CO₂ concentration was reduced, but was unaffected in the NF+ treatment. Furthermore, net photosynthesis declined more in response to the soil moisture deficit in the NF+ treatment than in the CF treatment. This is suggested to be attributed to the carboxylation efficiency at the operating point, which was decreased by 47% and 64% in shoots from the CF and the NF+ treatments, respectively. Stomatal limitation of net photosynthesis was increased by drought by 24–45% in the CF treatment, while it was unaffected in the NF+ treatment. Thus, our results imply that the coupling between the stomatal conductance and the photosynthetic rate was changed and that the marginal cost of water per given amount of carbon gain will increase in trees exposed to ozone, during periods of drought.     

Atmospheric CO(2) and mycorrhiza effects on biomass allocation and nutrient uptake of nodulated pea (Pisum sativum L.) plants

The effect of ambient and elevated atmospheric CO(2) on biomass partitioning and nutrient uptake of mycorrhizal and non-mycorrhizal pea plants grown in pots in a controlled environment was studied. The hypothesis tested was that mycorrhizae would increase C assimilation by increasing photosynthetic rates and reduce below-ground biomass allocation by improving nutrient uptake. This effect was expected to be more pronounced at elevated CO(2) where plant C supply and nutrient demand would be increased. The results showed that mycorrhizae did not interact with atmospheric CO(2) concentration in the variables measured. Mycorrhizae did not affect photosynthetic rates, had no effect on root weight or root length density and almost no effect on nutrient uptake, but still significantly increased shoot weight and reduced root/shoot ratio at harvest. Elevated CO(2) increased photosynthetic rates with no evidence for down-regulation, increased shoot weight and nutrient uptake, had no effect on root weight, and actually reduced root/shoot ratio at harvest. Non-mycorrhizal plants growing at both CO(2) concentrations had lower shoot weight than mycorrhizal plants with similar nutritional status and photosynthetic rates. It is suggested that the positive effect of mycorrhizal inoculation was caused by an enhanced C supply and C use in mycorrhizal plants than in non-mycorrhizal plants. The results indicate that plant growth was not limited by mineral nutrients, but partially source and sink limited for carbon. Mycorrhizal inoculation and elevated CO(2) might have removed such limitations and their effects on above-ground biomass were independent, positive and additive.     

Growth responses to ozone in plant species from wetlands

Ten wet grassland species were fumigated with four concentrations of ozone (charcoal-filtered air, non-filtered air and non-filtered air plus 25 or 50 nl l(-1) ozone) in open-top chambers during one growing season to investigate the long-term effect of this air pollutant on various growth variables. Only Eupatorium cannabinum showed ozone-related foliar injury, while five species reacted with significantly ozone-enhanced senescence. Premature senescence was paralleled by a significant ozone-induced reduction of green leaf area in Achillea ptarmica, E. cannabinum and Plantago lanceolata. At the intermediate harvest performed after 28 days shoot weights were significantly decreased by ozone in A. ptarmica and increased in Molinia caerulea. At the final harvest performed at the end of the growing season two other species, Cirsium dissectum and E. cannabinum had a significantly reduced shoot weight due to ozone. Root biomass was determined only at the intermediate harvest. The root:shoot ratio (RSR) was significantly reduced in C. dissectum, while it increased in M. caerulea. Seven of the species developed flowers during the experiment. While no significant ozone effects on flowering date and flower numbers were detected, flower weights were significantly reduced in E. cannabinum and P. lanceolata.     

Responses of alpine shrubs to simulated environmental change during three years in the mid-latitude mountain, northern Japan

Effects of artificial warming on phenology, individual leaf traits, vegetative growth, and reproduction of five alpine species (two deciduous and three evergreen shrubs) were investigated during three years in the mid-latitude alpine, northern Japan. Eleven open-top chambers (OTCs) were set up on a fellfield (1680 m a. s. l.) in the Taisetsu Mountains by which air temperature at plant height was increased by ca 2°C. Vaccinium uliginosum (deciduous shrub) showed earlier leaf emergence in every season and earlier flowering only in the first season in the OTCs. By contrast, acceleration of leaf emergence in the OTCs was not clear for other species, i.e. Arctous alpinus (deciduous shrub). Ledum palustre. V. vitis-idaea , and Empetrum nigrum (evergreen shrub). Both deciduous species showed longer leaf life-span in the OTCs every season. All evergreen species had higher leaf survival rates in the OTCs. indicating extension of leaf life-span. Leaf nitrogen concentration and leaf mass per unit leaf area (mg cm ⁻²) generally tended to decrease in the OTCs. Relationships between the individual leaf traits and cumulative air temperature during the leaf developing period were not clear. Total leaf production during the three seasons increased in the OTCs in A. alpinus. L. palustre. V. vitis-idaea , and E. nigrum. All evergreen shrubs showed larger shoot growth in the OTCs but both deciduous shrubs did not show significant changes. In contrast to the vegetative growth, deciduous shrubs produced more flowers in the OTCs. Fruit production was not influenced by the OTCs for all species. The extension of photosynthetic period in the OTCs may contribute to the larger vegetative growth or flower production.     

在控制条件下云南松幼苗根系对低磷胁迫的响应

磷是控制生命过程的重要元素,植物在生长过程中需要大量的磷,低磷常导致一些植物发生适应性变化。云南松(PinusyunnanensisFranch.)以云南高原为起源和分布中心,其对低磷土壤环境表现出了很强的适应能力,广泛分布并正常生长于贫瘠的低磷红壤上,研究云南松对低磷环境的适应机制,对人类探索高效利用有限的磷素资源的方法具有现实意义。本实验通过对不同磷处理水平下培养的云南松幼苗根系生物量和根冠比等的研究,分析了云南松幼苗根系对低磷胁迫的响应。实验所用云南松种子采集自云南省通海县秀山森林公园内的健壮云南松林。结果表明:当磷浓度下降到0.5mmol/L时,云南松幼苗主根长度开始随磷浓度的降低而增加,根冠比随磷浓度的降低而增大,而侧根发生数没有随磷浓度的降低而显示出显著的增减规律,根系生物量也没有随磷浓度的降低而呈现出有规律的增减,根系生物量始终保持在一定的水平。进一步的分析表明:低磷胁迫下,云南松幼苗保证了物质分配对根的优先地位,以维持其根的生物量在一定水平,进而维持整个生命;云南松幼苗主要是靠主根长度的增加而不是靠侧根数量的增加来适应低磷环境。     

Do chronic aboveground O3 exposure and belowground pathogen stress affect growth and belowground biomass partitioning of juvenile beech trees (Fagus sylvatica L.)?

The impact of chronic free air ozone (O₃) exposure and belowground pathogen stress on growth and total biomass development of young beech trees (Fagus sylvatica L.) was investigated in a lysimeter study. Plants were growing during four years under ambient or elevated atmospheric O₃ concentrations. Additionally, in the last vegetation period the root rot pathogen Phytophthora citricola was introduced to study the interaction of ozone exposure and pathogen stress in the soil-plant system. A complete harvest at the end of the experiment enabled for the first time the assessment of fine and coarse root biomass of individual trees with a high vertical resolution down to two meter depth. Plant growth was significantly reduced by elevated ozone but not affected by P. citricola. Biomass partitioning between fine and coarse roots as well as vertical root distribution were significantly affected by both factors, whereas changes in root/shoot biomass ratio were not observed.     

Root restriction as a factor in photosynthetic acclimation of cotton seedlings grown in elevated carbon dioxide

Interactive effects of root restriction and atmospheric CO₂ enrichment on plant growth, photosynthetic capacity, and carbohydrate partitioning were studied in cotton seedlings (Gossypium hirsutum L.) grown for 28 days in three atmospheric CO₂ partial pressures (270, 350, and 650 microbars) and two pot sizes (0.38 and 1.75 liters). Some plants were transplanted from small pots into large pots after 20 days. Reduction of root biomass resulting from growth in small pots was accompanied by decreased shoot biomass and leaf area. When root growth was less restricted, plants exposed to higher CO₂ partial pressures produced more shoot and root biomass than plants exposed to lower levels of CO₂. In small pots, whole plant biomass and leaf area of plants grown in 270 and 350 microbars of CO₂ were not significantly different. Plants grown in small pots in 650 microbars of CO₂ produced greater total biomass than plants grown in 350 microbars, but the dry weight gain was found to be primarily an accumulation of leaf starch. Reduced photosynthetic capacity of plants grown at elevated levels of CO₂ was clearly associated with inadequate rooting volume. Reductions in net photosynthesis were not associated with decreased stomatal conductance. Reduced carboxylation efficiency in response to CO₂ enrichment occurred only when root growth was restricted suggesting that ribulose-1,5-bisphosphate carboxylase/oxygenase activity may be responsive to plant source-sink balance rather than to CO₂ concentration as a single factor. When root-restricted plants were transplanted into large pots, carboxylation efficiency and ribulose-1,5-bisphosphate regeneration capacity increased indicating that acclimation of photosynthesis was reversible. Reductions in photosynthetic capacity as root growth was progressively restricted suggest sink-limited feedback inhibition as a possible mechanism for regulating net photosynthesis of plants grown in elevated CO₂.     

Growth and physiological responses to ozone and mild drought stress of tree species with different ecological requirements

An open-top chamber experiment was carried out in Curno (Northern Italy) in 2004 and 2005 on seedlings of Fagus sylvatica (FS), Quercus robur (QR), and an ozone-sensitive Populus (POP) clone, to investigate the role of two stress factors: tropospheric ozone and water shortage. Treatments were filtered air to achieve a 50% reduction in the environmental ozone concentrations (charcoal filtered, CF); and non-filtered air, with a 5% reduction in the environmental ozone concentrations (non-filtered, NF). Overall ozone exposure (AOT40) in open air (April–September) was 26,995 ppb h in 2004 and 25,166 ppb h in 2005. The plants were either watered (W) or not watered (dry, D). We investigated the above-ground biomass, tree-ring growth, stable carbon isotopes ratio, i.e. δ¹³C of tree rings, and the photosynthetic parameter Driving forces (DF_ABS), derived from chlorophyll a fluorescence analysis. Ozone-induced growth reduction (in terms of biomass) in POP, and that reduction was more pronounced in D plots. A synergistic effect of ozone and drought stress was evidenced by DF_ABS in POP and QR, but not in FS. The water availability was revealed as the main factor influencing the isotopic ratio δ¹³C. In drought-stressed seedlings, the increase in δ¹³C value was accompanied by the reduction in stomatal conductance and increased DF_ABS. Fast-growing plant species with high water requirements are more susceptible to ozone and drought stress.     

The impact of ozone on juvenile maize (Zea mays L.) plant photosynthesis: effects on vegetative biomass, pigmentation, and carboxylases (PEPc and Rubisco)

The impact of ozone on crops was more studied in C (3) than in C (4) species. In C (3) plants, ozone is known to induce a photosynthesis impairment that can result in significant depressions in biomass and crop yields. To investigate the impact of O (3) on C (4) plant species, maize seedlings ( ZEA MAYS L. cv. Chambord) were exposed to 5 atmospheres in open-top chambers: non-filtered air (NF, 48 nL L (-1) O (3)) and NF supplied with 20 (+ 20), 40 (+ 40), 60 (+ 60), and 80 (+ 80) nL L (-1) ozone. An unchambered plot was also available. Leaf area, vegetative biomass, and leaf dry mass per unit leaf area (LMA) were evaluated 33 days after seedling emergence in OTCs. At the same time, photosynthetic pigments as well as carboxylase (PEPc and Rubisco) activities and amounts were also examined in the 5th leaf. Ozone enhanced visible symptoms characterizing foliar senescence. Across NF, + 20, + 40, and + 60 atmospheres, both chlorophylls and carotenoids were found to be linearly decreased against increasing AOT40 ( CA. - 50 % in + 60). No supplementary decrease was observed between + 60 and + 80. Total above-ground biomass was reduced by 26 % in + 80 atmosphere; leaf dry matter being more depressed by ozone than leaf area. In some cases, LMA index was consistent to reflect low negative effects caused by a moderate increase in ozone concentration. PEPc and Rubisco were less sensitive to ozone than pigments: only the two highest external ozone doses reduced their activities by about 20 - 30 %. These changes might be connected to losses in PEPc and Rubisco proteins that were decreased by about one-third. The underlying mechanisms for these results were discussed with special reference to C (3) species. To conclude, we showed that both light and dark reactions of C (4) photosynthesis can be impaired by realistic ozone doses.