Impact of elevated ozone concentration on growth,physiology, and yield of wheat (Triticum aestivum L.): a meta‐analysis

We quantitatively evaluated the effects of elevated concentration of ozone (O₃) on growth, leaf chemistry, gas exchange, grain yield, and grain quality relative to carbon‐filtered air (CF) by means of meta‐analysis of published data. Our database consisted of 53 peer‐reviewed studies published between 1980 and 2007, taking into account wheat type, O₃ fumigation method, rooting environment, O₃ concentration ([O₃]), developmental stage, and additional treatments such as drought and elevated carbon dioxide concentration ([CO₂]). The results suggested that elevated [O₃] decreased wheat grain yield by 29% (CI: 24–34%) and aboveground biomass by 18% (CI: 13–24%), where CI is the 95% confidence interval. Even in studies where the [O₃] range was between 31 and 59 ppb (average 43 ppb), there was a significant decrease in the grain yield (18%) and biomass (16%) relative to CF. Despite the increase in the grain protein content (6.8%), elevated [O₃] significantly decreased the grain protein yield (?18%). Relative to CF, elevated [O₃] significantly decreased photosynthetic rates (?20%), Rubisco activity (?19%), stomatal conductance (?22%), and chlorophyll content (?40%). For the whole plant, rising [O₃] induced a larger decrease in belowground (?27%) biomass than in aboveground (?18%) biomass. There was no significant response difference between spring wheat and winter wheat. Wheat grown in the field showed larger decreases in leaf photosynthesis parameters than wheat grown in < 5 L pots. Open‐top chamber fumigation induced a larger reduction than indoor growth chambers, when plants were exposed to elevated [O₃]. The detrimental effect was progressively greater as the average daily [O₃] increased, with very few exceptions. The impact of O₃ increased with developmental stages, with the largest detrimental impact during grain filling. Both drought and elevated [CO₂] significantly ameliorated the detrimental effects of elevated [O₃], which could be explained by a significant decrease in O₃ uptake resulting from decreased stomatal conductance.     

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.     

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.     

Has the sensitivity of soybean cultivars to ozone pollution increased with time? An analysis of published dose–response data

The rising trend in concentrations of ground‐level ozone (O₃) – a common air pollutant and phytotoxin – currently being experienced in some world regions represents a threat to agricultural yield. Soybean (Glycine max (L.) Merr.) is an O₃‐sensitive crop species and is experiencing increasing global demand as a dietary protein source and constituent of livestock feed. In this study, we collate O₃ exposure‐yield data for 49 soybean cultivars, from 28 experimental studies published between 1982 and 2014, to produce an updated dose–response function for soybean. Different cultivars were seen to vary considerably in their sensitivity to O₃, with estimated yield loss due to O₃ ranging from 13.3% for the least sensitive cultivar to 37.9% for the most sensitive, at a 7‐h mean O₃ concentration (M7) of 55 ppb – a level frequently observed in regions of the USA, India and China in recent years. The year of cultivar release, country of data collection and type of O₃ exposure used were all important explanatory variables in a multivariate regression model describing soybean yield response to O₃. The data show that the O₃ sensitivity of soybean cultivars increased by an average of 32.5% between 1960 and 2000, suggesting that selective breeding strategies targeting high yield and high stomatal conductance may have inadvertently selected for greater O₃ sensitivity over time. Higher sensitivity was observed in data from India and China compared to the USA, although it is difficult to determine whether this effect is the result of differential cultivar physiology, or related to local environmental factors such as co‐occurring pollutants. Gaining further understanding of the underlying mechanisms that govern the sensitivity of soybean cultivars to O₃ will be important in shaping future strategies for breeding O₃‐tolerant cultivars.     

Physiological acclimation dampens initial effects of elevated temperature and atmospheric CO2 concentration in mature boreal Norway spruce

Physiological processes of terrestrial plants regulate the land–atmosphere exchange of carbon, water, and energy, yet few studies have explored the acclimation responses of mature boreal conifer trees to climate change. Here we explored the acclimation responses of photosynthesis, respiration, and stomatal conductance to elevated temperature and/or CO₂ concentration ([CO₂]) in a 3‐year field experiment with mature boreal Norway spruce. We found that elevated [CO₂] decreased photosynthetic carboxylation capacity (?23% at 25 °C) and increased shoot respiration (+64% at 15 °C), while warming had no significant effects. Shoot respiration, but not photosynthetic capacity, exhibited seasonal acclimation. Stomatal conductance at light saturation and a vapour pressure deficit of 1 kPa was unaffected by elevated [CO₂] but significantly decreased (?27%) by warming, and the ratio of intercellular to ambient [CO₂] was enhanced (+17%) by elevated [CO₂] and decreased (?12%) by warming. Many of these responses differ from those typically observed in temperate tree species. Our results show that long‐term physiological acclimation dampens the initial stimulation of plant net carbon assimilation to elevated [CO₂], and of plant water use to warming. Models that do not account for these responses may thus overestimate the impacts of climate change on future boreal vegetation–atmosphere interactions.     

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     

Comparison of crop yield sensitivity to ozone between open‐top chamber and free‐air experiments

Assessments of the impacts of ozone (O₃) on regional and global food production are currently based on results from experiments using open‐top chambers (OTCs). However, there are concerns that these impact estimates might be biased due to the environmental artifacts imposed by this enclosure system. In this study, we collated O₃ exposure and yield data for three major crop species—wheat, rice, and soybean—for which O₃ experiments have been conducted with OTCs as well as the ecologically more realistic free‐air O₃ elevation (O₃‐FACE) exposure system; both within the same cultivation region and country. For all three crops, we found that the sensitivity of crop yield to the O₃ metric AOT40 (accumulated hourly O₃ exposure above a cut‐off threshold concentration of 40 ppb) significantly differed between OTC and O₃‐FACE experiments. In wheat and rice, O₃ sensitivity was higher in O₃‐FACE than OTC experiments, while the opposite was the case for soybean. In all three crops, these differences could be linked to factors influencing stomatal conductance (manipulation of water inputs, passive chamber warming, and cultivar differences in gas exchange). Our study thus highlights the importance of accounting for factors that control stomatal O₃ flux when applying experimental data to assess O₃ impacts on crops at large spatial scales.     

Importance of changing CO2, temperature, precipitation, and ozone on carbon and water cycles of an upland-oak forest: incorporating experimental results into model simulations

Observed responses of upland‐oak vegetation of the eastern deciduous hardwood forest to changing CO₂, temperature, precipitation and tropospheric ozone (O₃) were derived from field studies and interpreted with a stand‐level model for an 11‐year range of environmental variation upon which scenarios of future environmental change were imposed. Scenarios for the year 2100 included elevated [CO₂] and [O₃] (+385 ppm and +20 ppb, respectively), warming (+4°C), and increased winter precipitation (+20% November–March). Simulations were run with and without adjustments for experimentally observed physiological and biomass adjustments. Initial simplistic model runs for single‐factor changes in CO₂ and temperature predicted substantial increases (+191% or 508 g C m^?2 yr^?1) or decreases (?206% or ?549 g C m^?2 yr^?1), respectively, in mean annual net ecosystem carbon exchange (NEE_a≈266±23 g C m^?2 yr^?1 from 1993 to 2003). Conversely, single‐factor changes in precipitation or O₃ had comparatively small effects on NEE_a (0% and ?35%, respectively). The combined influence of all four environmental changes yielded a 29% reduction in mean annual NEE_a. These results suggested that future CO₂‐induced enhancements of gross photosynthesis would be largely offset by temperature‐induced increases in respiration, exacerbation of water deficits, and O₃‐induced reductions in photosynthesis. However, when experimentally observed physiological adjustments were included in the simulations (e.g. acclimation of leaf respiration to warming), the combined influence of the year 2100 scenario resulted in a 20% increase in NEE_a not a decrease. Consistent with the annual model's predictions, simulations with a forest succession model run for gradually changing conditions from 2000 to 2100 indicated an 11% increase in stand wood biomass in the future compared with current conditions. These model‐based analyses identify critical areas of uncertainty for multivariate predictions of future ecosystem response, and underscore the importance of long term field experiments for the evaluation of acclimation and growth under complex environmental scenarios.     

Intraspecific variation in sensitivity to ultraviolet-B radiation in endogenous hormones and photosynthetic characteristics of 10 wheat cultivars grown under field conditions

Field studies were conducted to determine the potential of altering endogenous hormones and photosynthetic characteristics and intraspecific variation in sensitivity of 10 wheat (Triticum aestivum) cultivars (four tolerant, two middle sensitive and four sensitive) to enhanced ultraviolet-B (UV-B, 280–315 nm) radiation under field conditions. The supplemental UV-B radiation was 5.00 kJ m⁻², simulating a depletion of 20% stratospheric ozone. Responses were cultivar-specific. Out of the 10 tested wheat cultivars, six showed significant decrease in IAA content. UV-B radiation significantly increased ZR content in two wheat cultivars and significantly decreased in five cultivars. ABA content of three wheat cultivars was increased significantly, while that of five cultivars was decreased significantly. UV-B radiation significantly increased the stomatal conductance of three cultivars, and significantly decreased that of four cultivars. Intercellular CO₂ concentrations were significantly increased in five cultivars and significantly decreased in one cultivar (Mianyang 20). Transpiration rate of three cultivars significantly increased, while that of three cultivars significantly decreased. UV-B radiation significantly decreased the net photosynthetic rate of six cultivars. Intraspecific differences were found for the different measured parameters. For seven measured parameters, UV-B radiation had significant effects on five wheat cultivars, while no effect on the others. Significant correlations were observed between net photosynthetic rate and stomatal conductance, intercellular CO₂ concentrations and transpiration rate in eight cultivars. UV-B radiation might change stomatal conductance, intercellular CO₂ concentrations and transpiration rate, thus resulting in changes in net photosynthetic rate.     

春小麦对不同灌水处理的气孔反应及其影响因子

选用3个春小麦品种（系）,采用大田试验方法,在冬灌1800 m³·hm^-2的基础上,在生育期设3次灌水处理（T₁）、2次灌水处理（T₂）和1次灌水处理（T₃）,每次灌水1050 m³·hm^-2,研究土壤水分对春小麦生育期气孔导度的影响及气孔导度与相关环境因素的关系.结果表明：灌水处理对春小麦生育期气孔导度的影响较大,气孔导度随着灌溉次数的减少逐渐降低,同时不同基因型间存在差异.从拔节期到开花期,不同处理春小麦气孔导度变化一致,都呈先升高后降低趋势, 在抽穗期达到峰值;开花期之后各处理出现差异,T₁各品种气孔导度先下降后上升,T₂品种间表现不同,T₃一直呈下降趋势.各环境因子中,大气相对湿度对春小麦气孔导度的影响最大,两者的相关系数在T₂和T₃中分别达显著(0.82^*)和极显著水平(0.92^**).春小麦适应水分亏缺的气孔调节机理为反馈式调节.     

Effects of elevated CO2 and/or O3 on growth, development and physiology of wheat (Triticum aestivum L.)

Two cultivars of spring wheat (Triticum aestivum L. cvs. Alexandria and Hanno) and three cultivars of winter wheat (cvs. Riband, Mercia and Haven) were grown at two concentrations of CO₂ [ambient (355 pmol mol^?1) and elevated (708 μmol mol^?1)] under two O₃ regimes [clean air (< 5 nmol mol^?1 O₃) and polluted air (15 nmol mol^?1 O₃ at night rising to a midday maximum of 75 nmol mol^?1)] in a phytotron at the University of Newcastle-upon-Tyne. Between the two-leaf stage and anthesis, measurements of leaf gas-exchange, non-structural carbohydrate content, visible O₃ damage, growth, dry matter partitioning, yield components and root development were made in order to examine responses to elevated CO₂ and/or O₃. Growth at elevated CO₂ resulted in a sustained increase in the rate of CO₂ assimilation, but after roughly 6 weeks' exposure there was evidence of a slight decline in the photosynthetic rate (c.-15%) measured under growth conditions which was most pronounced in the winter cultivars. Enhanced rates of CO₂ assimilation were accompanied by a decrease in stomatal conductance which improved the instantaneous water use efficiency of individual leaves. CO₂ enrichment stimulated shoot and root growth to an equivalent extent, and increased tillering and yield components, however, non-structural carbohydrates still accumulated in source leaves. In contrast, long-term exposure to O₃ resulted in a decreased CO₂ assimilation rate (c. -13%), partial stomatal closure, and the accumulation of fructan and starch in leaves in the light. These effects were manifested in decreased rates of shoot and root growth, with root growth more severely affected than shoot growth. In the combined treatment growth of O₃-treated plants was enhanced by elevated CO₂, but there was little evidence that CO₂ enrichment afforded additional protection against O₃ damage. The reduction in growth induced by O₃ at elevated CO₂ was similar to that induced by O₃ at ambient CO₂ despite additive effects of the individual gases on stomatal conductance that would be expected to reduce the O₃ flux by 20%, and also CO₂-induced increases in the provision of substrates for detoxification and repair processes. These observations suggest that CO₂ enrichment may render plants more susceptible to O₃ damage at the cellular level. Possible mechanisms are discussed.     

Effects of elevated O3 and CO2 concentrations on photosynthesis and stomatal conductance in Scots pine

Naturally regenerated Scots pines (Pinus sylvestris L.), aged 28–30 years old, were grown in open-top chambers and subjected in situ to three ozone (O₃) regimes, two concentrations of CO₂, and a combination of O₃ and CO₂ treatments From 15 April to 15 September for two growing seasons (1994 and 1995). The gas exchanges of current-year and 1-year-old shoots were measured, along with the nitrogen content of needles. In order to investigate the factors underlying modifications in photosynthesis, five parameters linked to photosynthetic performance and three to stomatal conductance were determined. Elevated O₃ concentrations led to a significant decline in the CO₂ compensation point (Г^*), maximum RuP₂-saturated rate of carboxylation (V_cmax), maximum rate of electron transport (J_max), maximum stomatal conductance (g_smax), and sensitivity of stomatal conductance to changes in leaf-to-air vapour pressure difference (?g_s/?D_v) in both shoot-age classes. However, the effect of elevated O₃ concentrations on the respiration rate in light (R_d) was dependent on shoot age. Elevated CO₂(700 μmol mol^?1) significantly decreased J_max and g_smax but increased R_d in 1-year-old shoots and the ?g_s/?D_v in both shoot-age classes. The interactive effects of O₃ and CO₂ on some key parameters (e.g. V_cmax and J_max) were significant. This may be closely related to regulation of the maximum stomatal conductance and stomatal sensitivity induced by elevated CO₂. As a consequence, the injury induced by O₃ was reduced through decreased ozone uptake in 1-year-old shoots, but not in the current-year shoots. Compared to ambient O₃ concentration, reduced O₃ concentrations (charcoal-filtered air) did not lead to significant changes in any of the measured parameters. Compared to the control treatment, calculations showed that elevated O₃ concentrations decreased the apparent quantum yield by 15% and by 18%, and the maximum rate of photosynthesis by 21% and by 29% in the current-year and 1-year-old shoots, respectively. Changes in the nitrogen content of needles resulting from the various treatments were associated with modifications in photosynthetic components.     

Effects of elevated ozone concentration on yield of four Chinese cultivars of winter wheat under fully open‐air field conditions

Four modern cultivars of winter wheat (Triticum aestivum L.) were grown under elevated ozone concentration (E‐O₃) in fully open‐air field conditions in China for three consecutive growth seasons from 2007 to 2009. Results indicated that a mean 25% enhancement above the ambient ozone concentration (A‐O₃, 45.7 p.p.b.) significantly reduced the grain yield by 20% with significant variation in the range from 10% to 35% among the combinations of cultivar and season. The varietal difference in the yield response to E‐O₃ became nonsignificant when the anova was done by omitting one cultivar which showed unstable response to E‐O₃ among the seasons. The reduction of individual grain mass accounted mostly for the yield loss by E‐O₃, and showed significant difference between the cultivars. The response of relative yield to E‐O₃ was not significantly different from those reported in China, Europe and India on the basis of experiments in open‐top chambers. Our results thus confirmed the rising threat of surface O₃ on wheat production worldwide in the near future. Various countermeasures are urgently needed against the crop losses due to O₃ such as mitigation of the increase in surface O₃ with stricter pollution control, and enhancement of the wheat tolerance against O₃ by breeding and management.     

A comprehensive analysis of the physiological and anatomical components involved in higher water loss rates after leaf development at high humidity

To better understand the poor regulation of water loss after leaf development at high relative air humidity (RH), the relative importance of the physiological and anatomical components was analyzed focusing on cultivars with a contrasting sensitivity to elevated RH. The stomatal responsiveness to three closing stimuli (desiccation, abscisic acid feeding, light/dark transition), as well as several stomatal features (density, index, size and pore dimensions) and the cuticular transpiration rate (CTR) were determined in four rose cultivars, grown under moderate (60%) and high (95%) RH. Moreover, the effects of changes in stomatal density and pore dimensions on the stomatal conductance (g_s) were quantified using a modified version of the Brown and Escombe equation. Higher water loss, as a result of plant growth at high RH, was primarily caused by an increase in residual g_s, and to a lesser extent due to higher CTR. It was estimated that in leaflets subjected to desiccation the enhanced g_s in high RH- as compared to moderate RH-grown plants was mostly due to poor stomatal functionality and to a lesser extent the combined result of higher stomatal density and longer pore length. It is concluded that the reduced degree and, specially, the reduced rate of stomatal closure are the primary causes of the large genotypic variation in the control of water loss in high RH-grown plants. Furthermore, it was found that although changes in stomatal length have no influence on stomatal functionality, changed anatomical features per se represent a significant and direct contribution to the increased water loss.     

Effects of elevated carbon dioxide, ozone and water availability on spring wheat growth and yield

Spring wheat (Triticum aestivum L. cv. Dragon) was exposed to elevated carbon dioxide (CO₂), alone (1995) or in combination with two levels of increased ozone (O₃) (1994) or increased irrigation (1996) during three successive growing seasons as part of the EU ESPACE‐wheat programme and conducted in open‐top chambers (OTCs) and ambient air (AA) plots at Östad, 50 km north‐east of Göteborg, Sweden. Doubling the CO₂ concentration had a positive effect on grain yield in all 3 years (+21, +7 and +11%, respectively), although only statistically significant in 1994. That year was characterised by a warm and dry summer in comparison with 1995 and 1996, in which the summers were more humid and typical for south‐west Sweden. In 1994, the CO₂‐induced increase in grain yield was associated with an increase in the duration of the green leaf area, a positive effect on straw yield and on the number of ears per square metre and a negative effect (?13%) on grain protein concentration. Harvest index was unaffected by the elevated CO₂ concentration. The only statistically significant effect of elevated CO₂ in 1995 was a decrease in the grain protein concentration (?11% in both CO₂ concentrations), and in 1996 an increase (+21%) in the straw yield. In 1996 the soil water potential was less negative in elevated CO₂, which is likely to reflect a lower water consumption of these plants. Addition of extra O₃ significantly affected the grain yield (?6 and ?10%, respectively) and the 1 000‐grain weight negatively (?3 and ?6%). Statistically significant interactions between CO₂ and O₃ were obtained for the number of ears per unit area and for the 1 000‐grain weight. The 1 000‐grain weight was negatively affected by O₃ in low CO₂, but remained unaffected in the high CO₂ treatment. There was a significant decrease (?6%) in the grain protein concentration induced by elevated irrigation. The chambers, compared with AA plots, had a positive effect on plant development and on grain yield in all 3 years.     

Foliar abscisic acid content underlies genotypic variation in stomatal responsiveness after growth at high relative air humidity

Background and Aims

Stomata formed at high relative air humidity (RH) respond less to abscisic acid (ABA), an effect that varies widely between cultivars. This study tested the hypotheses that this genotypic variation in stomatal responsiveness originates from differential impairment in intermediates of the ABA signalling pathway during closure and differences in leaf ABA concentration during growth.

Methods

Stomatal anatomical features and stomatal responsiveness to desiccation, feeding with ABA, three transduction elements of its signalling pathway (H₂O₂, NO, Ca²⁺) and elicitors of these elements were determined in four rose cultivars grown at moderate (60 %) and high (90 %) RH. Leaf ABA concentration was assessed throughout the photoperiod and following mild desiccation (10 % leaf weight loss).

Key Results

Stomatal responsiveness to desiccation and ABA feeding was little affected by high RH in two cultivars, whereas it was considerably attenuated in two other cultivars (thus termed sensitive). Leaf ABA concentration was lower in plants grown at high RH, an effect that was more pronounced in the sensitive cultivars. Mild desiccation triggered an increase in leaf ABA concentration and equalized differences between leaves grown at moderate and high RH. High RH impaired stomatal responses to all transduction elements, but cultivar differences were not observed.

Conclusions

High RH resulted in decreased leaf ABA concentration during growth as a result of lack of water deficit, since desiccation induced ABA accumulation. Sensitive cultivars underwent a larger decrease in leaf ABA concentration rather than having a higher ABA concentration threshold for inducing stomatal functioning. However, cultivar differences in stomatal closure following ABA feeding were not apparent in response to H₂O₂ and downstream elements, indicating that signalling events prior to H₂O₂ generation are involved in the observed genotypic variation.     

Ozone‐induced stomatal sluggishness changes stomatal parameters of Jarvis‐type model in white birch and deciduous oak

• Stomatal ozone flux is closely related to ozone injury to plants. Jarvis‐type multiplicative model has been recommended for estimating stomatal ozone flux in forest trees. Ozone can change stomatal conductance by both stomatal closure and less efficient stomatal control (stomatal sluggishness). However, current Jarvis‐type models do not account for these ozone effects on stomatal conductance in forest trees.
• We examined seasonal course of stomatal conductance in two common deciduous tree species native to northern Japan (white birch: Betula platyphylla var. japonica ; deciduous oak: Quercus mongolica var. crispula ) grown under free‐air ozone exposure. We innovatively considered stomatal sluggishness in the Jarvis‐type model using a simple parameter, s , relating to cumulative ozone uptake (defined as POD : phytotoxic ozone dose).
• We found that ozone decreased stomatal conductance of white birch leaves after full expansion (?28%). However, such a reduction of stomatal conductance by ozone fell in late summer (?10%). At the same time, ozone reduced stomatal sensitivity of white birch to VPD and increased stomatal conductance under low light conditions. In contrast, in deciduous oak, ozone did not clearly change the model parameters.
• The consideration of both ozone‐induced stomatal closure and stomatal sluggishness improved the model performance to estimate stomatal conductance and to explain the dose–response relationship on ozone‐induced decline of photosynthesis of white birch. Our results indicate that ozone effects on stomatal conductance (i.e . stomatal closure and stomatal sluggishness) are crucial for modelling studies to determine stomatal response in deciduous trees, especially in species sensitive to ozone.

     

Ozone sensitivity of four Pakchoi cultivars with different leaf colors: physiological and biochemical mechanisms

Ozone (O₃) is important air pollutant inducing severe losses of horticultural production. Cultivars of the same species, but with different leaf colors, may differ in their ozone sensitivity. However, it has not been clarified yet if different leaf coloration influences such a sensitivity. In this study, two purple-leafed and two green-leafed cultivars of Pakchoi were selected for ozone fumigation (240 ± 20 nmol mol^–1, 09:00–16:00 h). Elevated O₃ decreased chlorophyll content, increased anthocyanin (Ant) content, damaged cell membrane integrity, enhanced antioxidative enzyme activities, depressed photosynthetic rate (P _N) and stomatal conductance (g _s), inhibited maximal quantum yield (F_v/F_m) and effective quantum yield [Y_II] of PSII photochemistry, and caused visible injury. Purple-leafed cultivars with higher Ant contents were more tolerant than green-leafed cultivars as indicated by lower relative enhancement in malondialdehyde content and lower relative losses in P _N, g _s, F_v/F_m, and Y_II. The higher ability to synthesize Ant in the purple-leafed cultivars contributed to their higher photoprotective ability.     

Effects of elevated ozone concentration on CH4 and N2O emission from paddy soil under fully open‐air field conditions

We investigated the effects of elevated ozone concentration (E‐O₃) on CH₄ and N₂O emission from paddies with two rice cultivars: an inbred Indica cultivar Yangdao 6 (YD6) and a hybrid one II‐you 084 (IIY084), under fully open‐air field conditions in China. A mean 26.7% enhancement of ozone concentration above the ambient level (A‐O₃) significantly reduced CH₄ emission at tillering and flowering stages leading to a reduction of seasonal integral CH₄ emission by 29.6% on average across the two cultivars. The reduced CH₄ emission is associated with O₃‐induced reduction in the whole‐plant biomass (?13.2%), root biomass (?34.7%), and maximum tiller number (?10.3%), all of which curbed the carbon supply for belowground CH₄ production and its release from submerged soil to atmosphere. Although no significant difference was detected between the cultivars in the CH₄ emission response to E‐O₃, a larger decrease in CH₄ emission with IIY084 (?33.2%) than that with YD6 (?7.0%) was observed at tillering stage, which may be due to the larger reduction in tiller number in IIY084 by E‐O₃. Additionally, E‐O₃ reduced seasonal mean NO_x flux by 5.7% and 11.8% with IIY084 and YD6, respectively, but the effects were not significant statistically. We found that the relative response of CH₄ emission to E‐O₃ was not significantly different from those reported in open‐top chamber experiments. This study has thus confirmed that increasing ozone concentration would mitigate the global warming potential of CH₄ and suggested consideration of the feedback mechanism between ozone and its precursor emission into the projection of future ozone effects on terrestrial ecosystem.