Negative effects of tropospheric ozone on the feed value of rice straw are mitigated by an ozone tolerance QTL

High surface ozone concentrations are recognized as an emerging threat to food security in Asia. This study aimed at determining the effects of ozone on the nutritive quality of rice straw, a by‐product of rice grain production and a major feed resource for ruminant livestock. Further, the question was addressed whether negative effects of ozone can be mitigated through molecular breeding. Rice plants from three different genotypes were exposed to four different ozone treatments in fumigation chambers from transplanting to maturity. These genotypes were (i) IR64, one of the most wide spread indica varieties in the world, (ii) Nipponbare, a typical japonica variety, and (iii) SL41, an ozone tolerant breeding line that carried chromosomal inserts at the ozone tolerance QTL OzT9 in the genetic background of Nipponbare. The treatments consisted of (i) charcoal filtered air, (ii) simulated ambient ozone concentration, (iii) 2 × ambient ozone concentration, and (iv) 2.5 × ambient ozone concentration. The effects of ozone on the chemical composition of straw were clearly dependent of the ozone level, and were significant even at ambient ozone concentration. Increases in crude ash, lignin and phenolics concentration adversely affected the digestibility as demonstrated in incubation experiments simulating rumen digestion in vitro. Negative ozone effects included reductions in the rate and extent of gas production due to inhibition of microbial fermentation, reduced formation of short chain fatty acids (SCFA), and a decrease in the true organic matter digestibility. The ozone tolerant genotype SL41 was less responsive to ozone than its more susceptible recurrent parent Nipponbare in terms of lignin and phenolics formation, organic matter digestibility and SCFA production. These data demonstrate that the feed quality of rice straw is affected by ozone even at ambient concentration, and that these negative effects are mitigated by the ozone tolerance QTL OzT9.     

不同施肥措施臭氧浓度升高对不同水稻品种光合特性的影响

近地层臭氧(O₃)已严重威胁到作物生产,而施肥可以调节土壤的养分平衡,进而促进作物生长。以两个水稻品种(徽两优898和南粳9108)为研究对象,利用开顶式气室,设置2个O₃浓度处理(NF:环境大气为对照;NF40:环境大气+40 nmol/mol O₃),每个O₃处理下嵌套设置3个肥料处理(Ino:施无机肥处理,270 kg N hm^-2 a^-1;Red:减施无机肥30%处理,189 kg N hm^-2 a^-1;Com:有机无机肥配施处理,Red+有机肥鸡粪5000 kg hm^-2 a^-1),通过测定不同生育期水稻光合参数,探究不同肥料处理下O₃对水稻不同生育阶段光合生理的影响。结果表明,NF40对水稻营养生长阶段的饱和光合速率(A_sat)没有显著影响,而显著地降低了水稻灌浆期的A_sat。基于两个水稻品种的A_sat和相对叶绿素含量(SPAD)相对减少量与O₃累积剂量关系的斜率,发现杂交稻徽两优898(A_sat和SPAD的斜率:-1.55和-0.98)比常规稻南粳9108(A_sat和SPAD的斜率:-0.92和0.06)对O₃更敏感。此外,基于不同O₃处理下水稻的气孔导度(g_s)和胞间二氧化碳浓度(C_i),可以看出O₃造成南粳9108光合速率降低的主要是非气孔因素,而徽两优898光合的降低是由气孔因素和非气孔因素共同限制。与Ino处理相比,Red处理主要通过降低叶片SPAD进而显著地抑制两种水稻品种的A_sat,但Ino处理和Com处理间A_sat没有显著差异,说明有机无机肥配施能部分缓解减施无机肥造成水稻光合的降低。O₃和肥料处理对两个水稻的所有光合参数都没有显著的交互影响,表明短期有机无机肥配施并不能有效缓解O₃对作物造成的负面影响。在O₃污染背景下,研究结果可以为通过合理的农田氮肥管理措施减缓O₃造成的作物减产提供理论依据。     

Genotypic variation in tolerance to elevated ozone in rice: dissection of distinct genetic factors linked to tolerance mechanisms

Tropospheric ozone concentrations are increasing in many Asian countries and are expected to reach levels that adversely affect crop production. Developing ozone-tolerant rice (Oryza sativa L.) varieties is therefore essential to prevent yield losses in the future. The aims of this study were to assess genotypic variation for ozone tolerance in rice, to identify quantitative trait loci (QTL) conferring tolerance, and to relate QTLs to physiological tolerance mechanisms. The response of 23 varieties to elevated ozone (120 nl l(-1)) was assessed based on leaf bronzing and dry weight loss. The traditional variety 'Kasalath' was highly tolerant, whereas the modern variety 'Nipponbare' showed significant dry weight reductions. Using the Nipponbare/Kasalath/Nipponbare mapping population, six QTLs associated with tolerance to elevated ozone were identified, of which three were subsequently confirmed in Nipponbare/Kasalath substitution lines (SLs). Two QTLs associated with leaf bronzing were located on chromosomes three and nine. Kasalath alleles on chromosome three increased bronzing, while alleles on chromosome nine reduced bronzing. SLs carrying these contrasting QTLs differed significantly in leaf ascorbic acid (AsA) content when exposed to ozone, suggesting AsA as a principal antioxidant counteracting ozone-induced oxidative damage. A further confirmed QTL related to dry weight was located on chromosome eight, where the Kasalath allele increased relative dry weight. A SL carrying this QTL exhibited a less reduced net photosynthetic rate under ozone exposure compared with its recurrent parent Nipponbare. Although the effect of these QTLs on crop yield has not yet been established, their identification could be an important first step in developing ozone-tolerant rice varieties.     

Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2- and O3- enriched atmospheres

Two silver birch (Betula pendula Roth) clones K1659 and V5952 were grown in open‐top chambers over 3 years (age 7–9 years). The treatments were increased CO₂ concentration (+CO₂, 72 Pa), increased O₃ concentration (+O₃, 2 × ambient O₃ with seasonal AOT40 up to 28 p.p.m. h) and in combination (+CO₂ + O₃). Thirty‐seven photosynthetic parameters were measured in the laboratory immediately after excising leaves using a computer‐operated routine of gas exchange and optical measurements. In control leaves the photosynthetic parameters were close to the values widely used in a model (Farquhar, von Caemmerer and Berry, Planta 149, 78–90, 1980). The distribution of chlorophyll between photosystem II and photosystem I, intrinsic quantum yield of electron transport, uncoupled turnover rate of Cyt b₆f, Rubisco specificity and K_m (CO₂) were not influenced by treatments. Net photosynthetic rate responded to +CO₂ with a mean increase of 17% in both clones. Dry weight of leaves increased, whereas protein, especially Rubisco content and the related photosynthetic parameters decreased. Averaged over 3 years, eight and 17 mechanistically independent parameters were significantly influenced by the elevated CO₂ in clones K1659 and V5952, respectively. The elevated O₃ caused a significant decrease in the average photosynthetic rate of clone V5952, but not of clone K1659. The treatment caused changes in one parameter of clone K1659 and in 11 parameters of clone V5952. Results of the combined treatment indicated that +O₃ had less effect in the presence of +CO₂ than alone. Interestingly, changes in the same photosynthetic parameters were observed in chamberless grown trees of clone V5952 as under +O₃ treatment in chambers, but this was not observed for clone K1659. These results suggest that during chronic fumigation, at concentrations below the threshold of visible leaf injuries, ozone influenced the photosynthetic parameters as a general stress factor, in a similar manner to weather conditions that were more stressful outside the chambers. According to this hypothesis, the sensitivity of a species or a clone to ozone is expected to depend on the growth conditions: the plant is less sensitive to ozone if the conditions are close to optimal and it is more sensitive to ozone under conditions of stress.     

Differential responses in two varieties of winter wheat to elevated ozone concentration under fully open‐air field conditions

Two modern cultivars [Yangmai16 (Y16) and Yangfumai 2 (Y2)] of winter wheat (Triticum aestivum L.) with almost identical phenology were investigated to determine the impacts of elevated ozone concentration (E‐O₃) on physiological characters related to photosynthesis under fully open‐air field conditions in China. The plants were exposed from the initiation of tillering to final harvest, with E‐O₃ of 127% of the ambient ozone concentration (A‐O₃). Measurements of pigments, gas exchange rates, chlorophyll a fluorescence and lipid oxidation were made in three replicated plots throughout flag leaf development. In cultivar Y2, E‐O₃ significantly accelerated leaf senescence, as indicated by increased lipid oxidation as well as faster declines in pigment amounts and photosynthetic rates. The lower photosynthetic rates were mainly due to nonstomatal factors, e.g. lower maximum carboxylation capacity, electron transport rates and light energy distribution. In cultivar Y16, by contrast, the effects of E‐O₃ were observed only at the very last stage of flag leaf ageing. Since the two cultivars had almost identical phenology and very similar leaf stomatal conductance before senescence, the greater impacts of E‐O₃ on cultivars Y2 than Y16 cannot be explained by differential ozone uptake. Our findings will be useful for scientists to select O₃‐tolerant wheat cultivars against the rising surface [O₃] in East and South Asia.     

A quantitative trait locus for cold tolerance at the booting stage on rice chromosome 8

A quantitative trait locus (QTL) for cold tolerance at the booting stage of a cold-tolerant rice breeding line, Hokkai-PL9, was analyzed. A total of 487 simple sequence repeat (SSR) markers distributed throughout the genome were used to survey for polymorphism between Hokkai-PL9 and a cold-sensitive breeding line, Hokkai287, and 54 markers were polymorphic. Single marker analysis revealed that markers on chromosome 8 are associated with cold tolerance. By interval mapping using an F₂ population between Hokkai-PL9 and Hokkai287, a QTL for cold tolerance was detected on the short arm of chromosome 8. The QTL explains 26.6% of the phenotypic variance, and its additive effect is 11.4%. Substitution mapping suggested that the QTL is located in a 193-kb interval between SSR markers RM5647 and PLA61. We tentatively designated the QTL as qCTB8 (quantitative trait locus for cold tolerance at the booting stage on chromosome 8).     

Effects of chronic elevated ozone concentration on antioxidant capacity,photosynthesis and seed yield of 10 soybean cultivars

Crops losses to tropospheric ozone (O₃) in the United States are estimated to cost $1–3 billion annually. This challenge is expected to increase as O₃ concentrations ([O₃]) rise over the next half century. This study tested the hypothesis that there is cultivar variation in the antioxidant, photosynthetic and yield response of soybean to growth at elevated [O₃]. Ten cultivars of soybean were grown at elevated [O₃] from germination through maturity at the Soybean Free Air Concentration Enrichment facility in 2007 and six were grown in 2008. Photosynthetic gas exchange, leaf area index, chlorophyll content, fluorescence and antioxidant capacity were monitored during the growing seasons in order to determine if changes in these parameters could be used to predict the sensitivity of seed yield to elevated [O₃]. Doubling background [O₃] decreased soybean yields by 17%, but the variation in response among cultivars and years ranged from 8 to 37%. Chlorophyll content and photosynthetic parameters were positively correlated with seed yield, while antioxidant capacity was negatively correlated with photosynthesis and seed yield, suggesting a trade‐off between antioxidant metabolism and carbon gain. Exposure response curves indicate that there has not been a significant improvement in soybean tolerance to [O₃] in the past 30 years.     

Photosynthesis and yield responses of ozone-polluted winter wheat to drought

Winter wheat (Triticum aestivum L. cv. Jingdong 8) was exposed to short-term high ozone treatment after anthesis and then was either well irrigated with soil water content (SWC) of 80–85 % (O₃+W) or drought treated (SWC 35–40 %, O₃+D). Short-term ozone exposure significantly decreased irradiance-saturated net photosynthetic rate (P _N) of winter wheat. Under good SWC, P _N of the O₃-treated plant was similar to that of control on 2 d after O₃-exposure (6 DAA), but decreased significantly after 13 DAA, indicating that O₃ exposure accelerated leaf senescence. Meanwhile, green flag leaf area was reduced faster than that of control. As a result, grain yield of O₃+W was significantly decreased. P _N of O₃+D was further notably decreased and green flag leaf area was reduced more than that in O₃+W. Consequently, substantial yield loss of O₃+D was observed compared to that of O₃+W. Although P _N was significantly positively correlated with stomatal conductance, it also had notable positive correlation with the maximum photochemical efficiency in the dark adapted leaves (F_v/F_m), electron transport rate (ETR), photochemical quenching (q_P), as well as content of chlorophyll, suggesting that the depression of P _N was mainly caused by non-stomatal limitation. Hence optimal soil water condition should be considered in order to reduce the yield loss caused by O₃ pollution.     

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.     

Assessing the impact of ozone on photosynthesis of European beech (Fergus sylvatica L.) in environmental chambers

An exposure — response study with proportionalto-ambient ozone levels was conducted in closed chambers on 3-year-old European beech (Fagus sylvatica L.) of montane origin. The fumigation started in April 1990 and lasted for a single growing season. Climate data and ozone concentrations monitored at an experimental station of the Institute for Applied Plant Biology, Schönenbuch, Switzerland were simulated in the exposure chambers 12 days later (1*O₃). To test exposure-response relations three additional treatments were applied, subambient (0.2*O₃) and two proportionally increased ozone treatments (1.5*O₃ and 2*O₃). The photosynthetic behaviour of the trees in August revealed the light reactions to be less affected than parameters which are related to the dark reactions of photosynthesis. Assimilation (A₃₅₀), apparent carboxylation efficiency (CE), and maximum photosynthetic capacity (A₂₅₀₀) were reduced with increasing ozone concentration. For the ozone response of CE and A₂₅₀₀ Critical Levels were calculated.     

Cultivar Differences in Photosynthetic Tolerance to Photooxidation and Shading in Rice (Oryza Sativa L.)

Forty-four genotypes from the rice germplasm were identified under photoinhibition/photooxidation and shade conditions and divided into four basic types: (1) cultivars tolerant to both photooxidation and shading, (2) cultivars tolerant to photooxidation but sensitive to shading, (3) cultivars tolerant to shading but sensitive to photooxidation, and (4) cultivars sensitive to both photooxidation and shading. Photosynthetic characteristics of a cultivar tolerant (cv. Wuyugeng 3) and a cultivar sensitive (cv. Xiangxian) to photooxidation and shading were compared. The photochemical efficiency (F_v/F_m) of photosystem 2 (PS2) and the content of PS2-D1 protein in the tolerant cultivar Wuyugeng 3 decreased less under photooxidative conditions as compared with Xiangxian. Under similar conditions, superoxide dismutase was induced rapidly to a higher activity and the active oxygen (O^–) built up to a lower level in Wuyugeng 3 than in Xiangxian. Net photosynthetic rate (P _N) decreased by 23 % in Wuyugeng 3 vs. 64 % in Xiangxian. Shading (80 %) during the booting stage caused only small decreases (7–13 %) in ribulose-1,5-bisphosphate carboxylase activity and P _N in Wuyugeng 3 but severe decreases (57–64 %) were observed in Xiangxian which corresponded to the decreases in grain yield of the two cultivars (38 and 73 %, respectively). We described a simple and effective screening method and physiological basis for breeding crops for enhanced tolerance to both high and low irradiance.     

不同秧苗素质和移栽密度条件下臭氧胁迫对水稻光合作用、物质生产和产量的影响

近地层臭氧浓度升高使水稻生长受抑进而使产量下降,但这种影响是否因不同栽培条件而异尚不清楚。2011年依托先进的稻田臭氧FACE(Free Air gas Concentration Enrichment)技术平台,以汕优63为供试材料,臭氧设置大气臭氧浓度(Ambient)和高臭氧浓度(比Ambient高50%),秧苗素质设置弱苗(移栽时无分蘖)和壮苗(移栽时带两个分蘖),移栽密度设置低密度(16穴/m2)、中密度(24穴/m2)和高密度(32穴/m2),研究不同秧苗素质和移栽密度条件下臭氧胁迫对水稻生长和产量的影响。结果表明:高浓度臭氧使水稻结实期叶片SPAD值、净光合速率、气孔导度和蒸腾速率明显下降,但胞间CO2浓度和叶温无显著变化。高浓度臭氧对水稻拔节前物质生产量没有影响,但使拔节至抽穗期、抽穗至成熟期物质生产量平均分别降低13%和29%,进而使成熟期生物产量和籽粒产量均显著下降。方差分析表明,臭氧与秧苗素质间没有互作效应,但臭氧与移栽密度的互作对最终产量的影响达显著水平。以上结果表明,臭氧胁迫使水稻生长后期光合受阻,导致物质生产和产量显著下降;适当增加移栽密度可能会减少臭氧胁迫下水稻产量的损失。     

Effects of elevated ozone concentration on methane emission from a rice paddy in Yangtze River Delta,China

Few investigations have been made on the impact of elevated ozone (O₃) concentration on methane (CH₄) emission from rice paddies. Using open‐top chambers in situ with different O₃ treatments, CH₄ emissions were measured in a rice paddy in Yangtze River Delta, China in 2007 and 2008. There were four treatments applied: charcoal‐filtered air (CF), nonfiltered air (NF), and charcoal‐filtered air with different O₃ additions (O₃‐1 and O₃‐2). The mean O₃ concentrations during the O₃ fumigation were 19.7, 22.6, 69.6 and 118.6 ppb in 2007 and 7.0, 17.4, 82.2 and 138.3 ppb in 2008 for treatments CF, NF, O₃‐1 and O₃‐2, respectively. The rice yields, as compared with CF, were reduced by 32.8% and 37.1%, 58.3% and 52.1% in treatments O₃‐1 and O₃‐2 in 2007 and 2008, respectively. The diurnal patterns of CH₄ emission varied temporally with treatments and there was inconsistence in diurnal variations in CH₄ emissions from the paddy field. The daily mean CH₄ emissions were significantly lower in treatments O₃‐1 and O₃‐2 than those in treatments CF and NF. Compared with CF treatment, CH₄ emissions from the paddy field were decreased to 46.5% and 38.3%, 50.6% and 46.8% under treatments O₃‐1 and O₃‐2 in the whole growing seasons of 2007 and 2008, respectively. The seasonal mean CH₄ emissions were negatively related with AOT40 (accumulative O₃ concentration above 40 ppb; P < 0.01 in both years), but positively related to the relative rice yield (reference to CF; P < 0.01 in 2007 and P < 0.001 in 2008), aboveground biomass (P < 0.01 in both years) and underground biomass (P < 0.01 in 2007 and P < 0.05 in 2008). The decreased CH₄ emission from the rice paddy due to an increased O₃ exposure might partially mitigate the global warming potential induced by soil carbon loss under elevated O₃ concentrations.     

Effects of Oxygen Concentration on the Respiration of Excised Root-Tip Segments of Maize and Rice, and of Germinating Grains of Rice and Buckwheat

In view of the known tolerance of rice (Oryza sativa) to wet soils, in contrast to maize (Zea mays), a comparison of the respiratory responses of the two plants to different O₂ concen trations was made. It was found that if the O₂ concentration was raised to 100%, an increase was observed of the O₂ input and the CO₂ output in maize root-tip segments and in germinating grains of rice and also in buckwheat (Fagopyrum sp.). In marked contrast rice root-tip segments, when treated with 100% O₂, exhibited no such increase over the air controls. Increased O₂ concentrations decreased RQ values ranging from 2.0 (with 5% O₂) to 0.86 (with 100% O₂). Air-control RQ values were lowest with maize roots (0.9) and highest with germinating rice grains (1.0). Rice grains also exhibited the highest F/R ratio (0.85), and buckwheat grains the lowest (0.45). The addition of sucrose to maize root tips tended to promote fermentation (CO₂ output) rather than aerobic respiration (O₂ input).     

Photosynthetic characteristics of diploid honeysuckle (<Emphasis Type="Italic">Lonicera japonica</Emphasis> Thunb.) and its autotetraploid cultivar subjected to elevated ozone exposure

In order to investigate the effect of chromosome doubling on ozone tolerance, we compared the physiological responses of a diploid honeysuckle (Lonicera japonica Thunb.) and its autotetraploid cultivar to elevated ozone (O₃) exposure (70 ng g⁻¹, 7 h d⁻¹ for 31 d). Net photosynthetic rate (P _N) of both cultivars were drastically (P<0.01) impaired by O₃. Although there were significantly positive correlation between P _N and stomatal conductance (g _s) in both cultivars under each treatment, the decreased g _s in O₃ might be the result rather than the cause of decreased P _N as indicated by stable or increasing the ratio of intercellular to ambient CO₂ concentration(C _i/C _a). P _N under saturating CO₂ concentration (P _Nsat) and carboxylation efficiency (CE) significantly decreased under O₃ fumigation, which indicated the Calvin cycle was impaired. O₃ also inhibited the maximum efficiency of photosystem II (PSII) photochemistry in the dark-adapted state (F_v/F_m), actual quantum yield of PSII photochemistry (Φ_PSII), electron transport rate (ETR), photochemical quenching coefficient (qP), non-photochemical quenching (NPQ), the maximum in vivo rate of Rubisco carboxylation (V_cmax) and the maximal photosynthetic electron transport rate (J_max) which demonstrated that the decrease in P _N of the honeysuckle exposed to elevated O₃ was probably not only due to impairment of Calvin cycle but also with respect to the light-harvesting and electron transport processes. Compared to the diploid, the tetraploid had higher relative loss in transpiration rate (E), (g _s), (P _Nsat), V_cmax and J_max. This result indicated that the Calvin cycle and electron transport in tetraploid was damaged more seriously than in diploid. A barely nonsignificant (P=0.086) interaction between O₃ and cultivar on P _N suggested a higher photosynthetic sensitivity of the tetraploid cultivar.     

Response of photosynthesis and chlorophyll fluorescence to acute ozone stress in tomato (Solanum lycopersicum Mill.)

The crop sensitivity to ozone (O₃) is affected by the timing of the O₃ exposure, by the O₃ concentration, and by the crop age. To determine the physiological response to the acute ozone stress, tomato plants were exposed to O₃ at two growth stages. In Experiment I (Exp. I), O₃ (500 μg m^?3) was applied to 30-d-old plants (PL30). In Experiment II (Exp. II), three O₃ concentrations (200, 350, and 500 μg m^?3) were applied to 51-d-old plants (PL51). The time of the treatment was 4 h (7:30–11:30 h). Photosynthesis and chlorophyll fluorescence measurements were done 4 times (before the exposure; 20 min, 20 h, and 2–3 weeks after the end of the treatment) using a LI-COR 6400 photosynthesis meter. The stomatal pore area and stomatal conductance were reduced as the O₃ concentration increased. Ozone induced the decrease in the photosynthetic parameters of tomato regardless of the plant age. Both the photosystem (PS) II operating efficiency and the maximum quantum efficiency of PSII photochemistry declined under the ozone stress suggesting that the PSII activity was inhibited by O₃. The impaired PSII contributed to the reduced photosynthetic rate. The greater decline of photosynthetic parameters was found in the PL30 compared with the PL51. It proved the age-dependent ozone sensitivity of tomato, where the younger plants were more vulnerable. Ozone caused the degradation of photosynthetic apparatus, which affected the photosynthesis of tomato plants depending on the growth stage and the O₃ concentration.     

Ameliorating effect of UV-B radiation on the response of Norway spruce and Scots pine to ambient ozone concentrations

Elevated levels of both ozone and UV-B radiation are typical for high-altitude sites. Few studies have investigated their possible interaction on plants. This study reports interactive effects of O₃ and UV-B radiation in four-year-old Norway spruce and Scots pine trees. The trees were cultivated in controlled environmental facilities under simulated climatic conditions recorded on Mt Wank, an Alpine mountain in Bavaria, and were exposed for one growing season to simulated ambient or twice-ambient ozone regimes at either near ambient or near zero UV-B radiation levels. Chlorotic mottling and yellowing of current year needles became obvious under twice-ambient O₃ in both species at the onset of a high ozone episode in July. Development of chlorotic mottling in relation to accumulated ozone concentrations over a threshold of 40 nL L^–1 was more pronounced with near zero rather than ambient UV-B radiation levels. In Norway spruce, photosynthetic parameters at ambient CO₂ concentration, measured at the end of the experiment, were reduced in trees cultivated under twice-ambient O₃, irrespective of the UV-B treatment. Effects on photosynthetic capacity and carboxylation efficiency were restricted to trees exposed to near zero levels of UV-B radiation, and twice-ambient O₃. The data indicate that UV-B radiation, applied together with O₃, ameliorates the detrimental effects of O₃. The data also demonstrate that foliar symptoms develop more rapidly in Scots pine than in Norway spruce at higher accumulated ozone concentrations. Symbols and abbreviations: LSD, least significant difference; PAS300, UV-B irradiance weighted according to the plant action spectrum of Green et al. (1974) normalized at 300 (nm); AOT40, (AOT = accumulated over threshold) reflects the sum of hourly ozone concentrations above 40 nL L^–1 during daylight hours (> 50 Wm^–2) ( Kärenlampi & Skärby 1996 ); A₃₅₀, net photosynthesis at ambient CO₂; G₃₅₀, stomatal conductance for water vapour at ambient CO₂; A₂₅₀₀, net photosynthesis at saturating CO₂ (maximal potential photosynthetic activity); CE, carboxylation efficiency; ROS, reactive oxygen species; RuBP, ribulose 1,5-bisphosphate; Rubisco, ribulose 1,5-bisphosphate carboxylase/oxygenase; GLM, general linear model.     

Strigolactone and Methyl Jasmonate-Induced Antioxidant Defense and the Composition Alterations of Different Active Compounds in Dracocephalum kotschyi Boiss Under Drought Stress

Strigolactone (SL) and methyl jasmonate (MeJA) are one of the most important plant hormones that exert biological activity in plant responses to environmental stresses. Considering the undetected role of SL in drought tolerance and essential oil yield of medicinal plants as well as conceivable interaction among MeJA and SL, a factorial experiment was performed as a complete randomized design with three replications. Experimental factors including two irrigation regimes such as irrigation to 80% field capacity (control) and 40% field capacity (drought stress) and spraying treatments include MeJA (0 and 0.5 mM) and SL (0 and 10 μM) were applied. Treatment of plants with SL and MeJA resulted in higher tolerance to drought stress due to higher fresh and dry weights as well as lower electrolyte leakage, malondialdehyde, H₂O₂, total phenol content, total antioxidant activity and antioxidant power assay. The most important essential oil constituents of D. kotschyi included geranyl acetate (41.1–48.6%), α-pinene (16.2–18.9%), geranial (7.9–10.1%), limonene (5.5–7.0%), neral (3.5–4.1%), methyl geranate (2.3–3.3%) and geraniol (1–2.2%), the least of which was found in non-MeJA- and SL-treated plants under drought and the highest in MeJA- and SL-treated plants under drought stress. Drought tolerance of D. kotschyi became more intense and the amount of essential oil components of water stressed plants was the highest (99.2%) when these plant hormones were used together. These results suggest a cross-link between MeJA and SL in improving drought resistance and optimizing the production of essential oil of D. kotschyi.

     

Similar Effects of Ozone on Four Cultivars of Lettuce in Open Top Chambers During Winter

Ozone is the major phytotoxic air pollutant that reduces the yield of several agricultural crops in the Spanish Mediterranean area. We studied four lettuce cultivars (Lactuca sativa L.) for the effects of different O₃ concentrations during the winter on chlorophyll (Chl) a fluorescence, lipid peroxidation, and root length in outdoor open-top chambers. Under O₃ the photosynthetic quantum conversion declined while heat emissions increased in all cultivars; these results provide more evidence of non-filtered air with additional ozone (NFA+O₃) treatment compared with non-filtered air (NFA) and charcoal filtered ozone-free air (CFA). Changes in the Chl a fluorescence may be associated with an increase in membrane lipid peroxidation as well as with observed reduction of root length under O₃ stress.