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.     

Effect of rice plants on CH4 production,transport, oxidation and emission in rice paddy soil

To understand the integrated effects of rice plants (variety Wuyugeng 2) on CH₄ emission during the typical rice growth stage, the production, oxidation and emission of methane related to rice plants were investigated simultaneously through laboratory and greenhouse experiments. CH₄ emission was significantly higher from the rice planted treatment than from the unplanted treatment. In the rice planted treatment, CH₄ emission was higher at tillering stage than at panicle initiation stage. An average of 36.3% and 54.7% of CH₄ produced was oxidized in the rhizosphere at rice tillering stage and panicle initiation stage, respectively, measured by using methyl fluoride (MF) technique. In the meantime, CH₄ production in the planted treatments incubated under O₂-free N₂ condition was reduced by 44.9 and 22.3%, respectively, compared to unplanted treatment. On the contrary, the presence of rice plants strongly stimulated CH₄ production by approximately 72.3% at rice ripening stage. CH₄ emission through rice plants averaged 95% at the tillering stage and 89% at the panicle initiation stage. Based on these results, conclusions are drawn that higher CH₄ emission from the planted treatment than from unplanted treatment could be attributed to the function of rice plants for transporting CH₄ from belowground to the atmosphere at tillering and panicle initiation stage, and that a higher CH₄ emission at tillering stage than at panicle initiation stage is due to the lower rhizospheric CH₄ oxidation and more effective transport mediated by rice plants.     

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 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.     

Interaction of drought and ozone exposure on isoprene emission from extensively cultivated poplar

The combined effects of ozone (O₃) and drought on isoprene emission were studied for the first time. Young hybrid poplars (clone 546, Populus deltoides cv. 55/56 x P. deltoides cv. Imperial) were exposed to O₃ (charcoal‐filtered air, CF, and non‐filtered air +40 ppb, E‐O₃) and soil water stress (well‐watered, WW, and mild drought, MD, one‐third irrigation) for 96 days. Consistent with light‐saturated photosynthesis (A_sat), intercellular CO₂ concentration (C_i) and chlorophyll content, isoprene emission depended on drought, O₃, leaf position and sampling time. Drought stimulated emission (+38.4%), and O₃ decreased it (?40.4%). Ozone increased the carbon cost per unit of isoprene emission. Ozone and drought effects were stronger in middle leaves (13th–15th from the apex) than in upper leaves (6th–8th). Only A_sat showed a significant interaction between O₃ and drought. When the responses were up‐scaled to the entire‐plant level, however, drought effects on total leaf area translated into around twice higher emission from WW plants in clean air than in E‐O₃. Our results suggest that direct effects on plant emission rates and changes in total leaf area may affect isoprene emission from intensively cultivated hybrid poplar under combined MD and O₃ exposure, with important feedbacks for air quality.     

Influence of rice cultivar on methane emission from paddy fields

Influence of rice cultivars on CH₄ emissions from a paddy field was studied using four Japonica types, two Indica types, and two Japonica/Indica F₁ hybrids. In addition, the suppression of CH₄ emission by interrupting irrigation at the flowering stage was investigated. Patterns of seasonal variation in CH₄ emission rates were similar among the eight cultivars. Two of the Japonica types showed the maximum and minimum CH₄ emissions among the cultivars investigated. Neither the number of tillers, shoot length, shoot weight, and root weight correlated with the CH₄ emission rates at the tillering and reproductive growth stages. Following temporary interruption of irrigation at the flowering stage, CH₄ emission rates decreased drastically and remained at very low levels until the harvesting stage, indicating its great effectiveness for the suppression of CH₄ emission from rice paddies.     

CH4 emission with differences in atmospheric CO2 enrichment and rice cultivars in a Japanese paddy soil

A pot experiment was conducted to investigate CH₄ emissions from a sandy paddy soil as influenced by rice cultivars and atmospheric CO₂ elevation. The experiment with two CO₂ levels, 370 μL L⁻¹ (ambient) and 570 μL L⁻¹ (elevated), was performed in a climatron, located at the National Institute for Agro‐Environmental Sciences, Tsukuba, Japan. Four rice cultivars were tested in this experiment, including IR65598, IR72, Dular and Koshihikari. Tiller number, root length and grain yield were clearly larger under elevated CO₂ than under ambient CO₂. IR72 and Dular showed significantly higher tiller number, root length and grain yield than Koshihikari and IR65598. Average daily CH₄ fluxes under elevated CO₂ were significantly larger by 10.9–23.8% than those under ambient CO₂, and varied with the cultivars in the sequence Dular ≧ IR72>IR65598 ≧ Koshihikari. Dissolved organic C (DOC) content in the soil was obviously higher under elevated CO₂ than under ambient CO₂ and differed among the cultivars, in the sequence IR72>Dular>Koshihikari>IR65598. The differences in average daily CH₄ fluxes between CO₂ levels and among the cultivars were related to different root exudation as DOC content, root length and tiller number. This study indicated that Koshihikari should be a potential cultivar for mitigating CH₄ emission and simultaneously keeping stable grain yield, because this cultivar emitted lowest CH₄ emission and produced medium grain yield.     

Increasing global agricultural production by reducing ozone damages via methane emission controls and ozone‐resistant cultivar selection

Meeting the projected 50% increase in global grain demand by 2030 without further environmental degradation poses a major challenge for agricultural production. Because surface ozone (O₃) has a significant negative impact on crop yields, one way to increase future production is to reduce O₃‐induced agricultural losses. We present two strategies whereby O₃ damage to crops may be reduced. We first examine the potential benefits of an O₃ mitigation strategy motivated by climate change goals: gradual emission reductions of methane (CH₄), an important greenhouse gas and tropospheric O₃ precursor that has not yet been targeted for O₃ pollution abatement. Our second strategy focuses on adapting crops to O₃ exposure by selecting cultivars with demonstrated O₃ resistance. We find that the CH₄ reductions considered would increase global production of soybean, maize, and wheat by 23–102 Mt in 2030 – the equivalent of a ~2–8% increase in year 2000 production worth $3.5–15 billion worldwide (USD₂₀₀₀), increasing the cost effectiveness of this CH₄ mitigation policy. Choosing crop varieties with O₃ resistance (relative to median‐sensitivity cultivars) could improve global agricultural production in 2030 by over 140 Mt, the equivalent of a 12% increase in 2000 production worth ~$22 billion. Benefits are dominated by improvements for wheat in South Asia, where O₃‐induced crop losses would otherwise be severe. Combining the two strategies generates benefits that are less than fully additive, given the nature of O₃ effects on crops. Our results demonstrate the significant potential to sustainably improve global agricultural production by decreasing O₃‐induced reductions in crop yields.     

Higher yields and lower methane emissions with new rice cultivars

Breeding high‐yielding rice cultivars through increasing biomass is a key strategy to meet rising global food demands. Yet, increasing rice growth can stimulate methane (CH₄) emissions, exacerbating global climate change, as rice cultivation is a major source of this powerful greenhouse gas. Here, we show in a series of experiments that high‐yielding rice cultivars actually reduce CH₄ emissions from typical paddy soils. Averaged across 33 rice cultivars, a biomass increase of 10% resulted in a 10.3% decrease in CH₄ emissions in a soil with a high carbon (C) content. Compared to a low‐yielding cultivar, a high‐yielding cultivar significantly increased root porosity and the abundance of methane‐consuming microorganisms, suggesting that the larger and more porous root systems of high‐yielding cultivars facilitated CH₄ oxidation by promoting O₂ transport to soils. Our results were further supported by a meta‐analysis, showing that high‐yielding rice cultivars strongly decrease CH₄ emissions from paddy soils with high organic C contents. Based on our results, increasing rice biomass by 10% could reduce annual CH₄ emissions from Chinese rice agriculture by 7.1%. Our findings suggest that modern rice breeding strategies for high‐yielding cultivars can substantially mitigate paddy CH₄ emission in China and other rice growing regions.     

Options for mitigating methane emission from a permanently flooded rice field

Permanently flooded rice fields, widely distributed in south and south‐west China, emit more CH₄ than those drained in the winter crop season. For understanding CH₄ emissions from permanently flooded rice fields and developing mitigation options, CH₄ emission was measured year‐round for 6 years from 1995 to 2000, in a permanently flooded rice field in Chongqing, China, where two cultivations with four treatments were prepared as follows: plain‐cultivation, summer rice crop and winter fallow with floodwater layer annually (convention, Ch‐FF), and winter upland crop under drained conditions (Ch‐Wheat); ridge‐cultivation without tillage, summer rice and winter fallow with floodwater layer annually (Ch‐FFR), and winter upland crop under drained conditions (Ch‐RW), respectively. On a 6‐year average, compared to the treatments with floodwater in the winter crop season, the CH₄ flux during rice‐growing period from the treatments draining floodwater and planting winter crop was reduced by 42% in plain‐cultivation and by 13% in ridge‐cultivation (P < 0.05), respectively. The reduction of annual CH₄ emission reached 68 and 48%, respectively. Compared to plain‐cultivation (Ch‐FF), ridge‐cultivation (Ch‐FFR) reduced annual CH₄ emission by 33%, and which was mainly occurred in the winter crop season. These results indicate that draining floodwater layer for winter upland crop growth was not only able to prevent CH₄ emission from permanently flooded paddy soils directly in the winter crop season, but also to reduce CH₄ emission substantially during the following rice‐growing period. As an alternative to the completely drainage of floodwater layer in the winter crop season, ridge‐cultivation could also significantly mitigate CH₄ emissions from permanently flooded rice fields.     

Genotypic differences in leaf biochemical, physiological and growth responses to ozone in 20 winter wheat cultivars released over the past 60 years

Ozone (O₃) concentrations in periurban areas in East Asia are sufficiently high to decrease crop yield. However, little is known about the genotypic differences in O₃ sensitivity in winter wheat in relation to year of cultivar release. This paper reports genotypic variations in O₃ sensitivity in 20 winter wheat cultivars released over the past 60 years in China highlighting O₃‐induced mechanisms. Wheat plants were exposed to elevated O₃ (82 ppb O₃, 7 h day^?1) or charcoal‐filtered air (<5 ppb O₃) for 21 days in open top chambers. Responses to O₃ were assessed by the levels of antioxidative activities, protein alteration, membrane lipid peroxidation, gas exchange, leaf chlorophyll, dark respiration and growth. We found that O₃ significantly reduced foliar ascorbate (?14%) and soluble protein (?22%), but increased peroxidase activity (+46%) and malondialdehyde (+38%). Elevated O₃ depressed light saturated net photosynthetic rate (?24%), stomatal conductance (?8%) and total chlorophyll (?11%), while stimulated dark respiration (+28%) and intercellular CO₂ concentration (+39%). O₃ also reduced overall plant growth, but to a greater extent in root (?32%) than in shoot (?17%) biomass. There was significant genotypic variation in potential sensitivity to O₃ that did not correlate to observed O₃ tolerance. Sensitivity to O₃ in cultivars of winter wheat progressed with year of release and correlated with stomatal conductance and dark respiration in O₃‐exposed plants. O₃‐induced loss in photosynthetic rate was attributed primarily to impaired activity of mesophyll cells and loss of integrity of cellular membrane as evidenced by increased intercellular CO₂ concentration and lipid peroxidation. Our findings demonstrated that higher sensitivity to O₃ in the more recently released cultivars was induced by higher stomatal conductance, larger reduction in antioxidative capacity and lower levels of dark respiration leading to higher oxidative damage to proteins and integrity of cellular membranes.     

Effects of free-air CO2 enrichment (FACE) on CH4 emission from a rice paddy field

Methane (CH₄) is a particularly potent greenhouse gas with a radiative forcing 23 times that of CO₂ on a per mass basis. Flooded rice paddies are a major source of CH₄ emissions to the Earth's atmosphere. A free‐air CO₂ enrichment (FACE) experiment was conducted to evaluate changes in crop productivity and the crop ecosystem under enriched CO₂ conditions during three rice growth seasons from 1998 to 2000 in a rice paddy at Shizukuishi, Iwate, Japan. To understand the influence of elevated atmospheric CO₂ concentrations on CH₄ emission, we measured methane flux from FACE rice fields and rice fields with ambient levels of CO₂ during the 1999 and 2000 growing seasons. Methane production and oxidation potentials of soil samples collected when the rice was at the tillering and flowering stages in 2000 were measured in the laboratory by the anaerobic incubation and alternative propylene substrates methods, respectively. The average tiller number and root dry biomass were clearly larger in the plots with elevated CO₂ during all rice growth stages. No difference in methane oxidation potential between FACE and ambient treatments was found, but the methane production potential of soils during the flowering stage was significantly greater under FACE than under ambient conditions. When free‐air CO₂ was enriched to 550 ppmv, the CH₄ emissions from the rice paddy field increased significantly, by 38% in 1999 and 51% in 2000. The increased CH₄ emissions were attributed to accelerated CH₄ production potential as a result of more root exudates and root autolysis products and to increased plant‐mediated CH₄ emissions because of the larger rice tiller numbers under FACE conditions.     

UV-B辐射增强对抗除草剂转基因水稻 CH4排放的影响

在大田条件下,研究了UV-B(ultraviolet-B)辐射增强对抗除草剂转基因水稻及亲本常规水稻甲烷(CH4)排放的影响。UV-B辐射设2水平,即对照(CK,自然光),增强(Elevated,14.4 kJ·m-·2d-1),相当于南京地区大气臭氧耗损25%的辐射剂量。结果表明,UV-B辐射增强并没有改变稻田CH4排放通量的季节性变化规律。与对照相比,UV-B辐射增强显著提高CH4排放通量和累积排放量。水稻分蘖期CH4累积排放量最高,占全生育累积排放量的51.55%—61.01%;其次是拔节至孕穗期,占20.00%—26.64%。抗除草剂转基因水稻的CH4排放通量和累积排放量显著低于亲本常规水稻。研究说明,UV-B辐射增强下种植抗除草剂转基因水稻对于减缓稻田甲烷排放有积极意义。     

Leaf surface wax is a source of plant methane formation under UV radiation and in the presence of oxygen

The terrestrial vegetation is a source of UV radiation‐induced aerobic methane (CH₄) release to the atmosphere. Hitherto pectin, a plant structural component, has been considered as the most likely precursor for this CH₄ release. However, most of the leaf pectin is situated below the surface wax layer, and UV transmittance of the cuticle differs among plant species. In some species, the cuticle effectively absorbs and/or reflects UV radiation. Thus, pectin may not necessarily contribute substantially to the UV radiation‐induced CH₄ emission measured at surface level in all species. Here, we investigated the potential of the leaf surface wax itself as a source of UV radiation‐induced leaf aerobic CH₄ formation. Isolated leaf surface wax emitted CH₄ at substantial rates in response to UV radiation. This discovery has implications for how the phenomenon should be scaled to global levels. In relation to this, we demonstrated that the UV radiation‐induced CH₄ emission is independent of leaf area index above unity. Further, we observed that the presence of O₂ in the atmosphere was necessary for achieving the highest rates of CH₄ emission. Methane formation from leaf surface wax is supposedly a two‐step process initiated by a photolytic rearrangement reaction of the major component followed by an α‐cleavage of the generated ketone.     

不同施肥处理对土壤活性有机碳和甲烷排放的影响

通过采集田间试验区连续3a施入有机肥的稻田耕层土壤,分析土壤中微生物量碳(MBC)、水溶性有机碳(DOC)、易氧化有机碳(ROC)和可矿化有机碳(readily mineralizable carbon,RMC)等活性有机碳的含量,稻田甲烷(CH_4)的排放通量,探讨施用有机肥的土壤活性有机碳变化及与CH_4排放的关系。研究结果显示:(1)施有机肥对土壤中的活性有机碳均有一定的促进作用。3a不同施肥处理土壤中DOC、ROC、MBC和RMC的平均含量分别为383.6、2501.2、640.4 mg/kg和291.7 mg/kg。3a施猪粪(猪粪+化肥,PM)、鸡粪(鸡粪+化肥,CM)和稻草(稻草+化肥,RS)的DOC的含量分别比化肥(CF)处理增加5.6%、6.7%和19.3%,ROC的含量分别比CF增加6.6%、8.4%和9.8%;MBC含量分别比CF增加5.1%、14.8%和21.5%,RMC增加6.8%、22.0%和33.9%。不同施肥处理的稻田土壤活性有机碳为分蘖期高于成熟期。(2)施肥处理显著增加稻田CH_4排放,CH_4分蘖期的排放通量是成熟期的143倍,3a PM、CM和RS处理的CH_4排放分别比CF处理增加37.0%(P0.05)、92.7%(P0.05)和99.4%(P0.05)。(3)不同施肥处理的DOC、ROC、MBC和RMC含量与CH_4排放通量均存在显著正相关关系,ROC与CH_4排放的相关系数最高,为0.754(P0.01),且4种有机碳间关系密切。稻田分蘖期土壤中的活性有机碳与稻田CH_4排放呈显著正相关关系。(4)综合分析,在4种有机碳中,土壤中ROC和MBC的含量直接影响CH_4排放。     

Revising a process-based biogeochemistry model (DNDC) to simulate methane emission from rice paddy fields under various residue management and fertilizer regimes

A comprehensive biogeochemistry model, DNDC, was revised to simulate crop growth and soil processes more explicitly and improve its ability to estimate methane (CH₄) emission from rice paddy fields under a wide range of climatic and agronomic conditions. The revised model simulates rice growth by tracking photosynthesis, respiration, C allocation, tillering, and release of organic C and O₂ from roots. For anaerobic soil processes, it quantifies the production of electron donors [H₂ and dissolved organic carbon (DOC)] by decomposition and rice root exudation, and simulates CH₄ production and other reductive reactions based on the availability of electron donors and acceptors (NO₃^?, Mn⁴⁺, Fe³⁺, and SO₄^2?). Methane emission through rice is simulated by a diffusion routine based on the conductance of tillers and the CH₄ concentration in soil water. The revised DNDC was tested against observations at three rice paddy sites in Japan and China with varying rice residue management and fertilization, and produced estimates consistent with observations for the variation in CH₄ emission as a function of residue management. It also successfully predicted the negative effect of (NH₄)₂SO₄ on CH₄ emission, which the current model missed. Predicted CH₄ emission was highly sensitive to the content of reducible soil Fe³⁺, which is the dominant electron acceptor in anaerobic soils. The revised DNDC generally gave acceptable predictions of seasonal CH₄ emission, but not of daily CH₄ fluxes, suggesting the model's immaturity in describing soil heterogeneity or rice cultivar‐specific characteristics of CH₄ transport. It also overestimated CH₄ emission at one site in a year with low temperatures, suggesting uncertainty in root biomass estimates due to the model's failure to consider the temperature dependence of leaf area development. Nevertheless, the revised DNDC explicitly reflects the effects of soil electron donors and acceptors, and can be used to quantitatively estimate CH₄ emissions from rice fields under a range of conditions.     

Pattern and Amount of Aerenchyma Relate to Variable Methane Transport Capacity of Different Rice Cultivars

Abstract: Aerenchyma, developed in both root and aboveground parts of rice plants, is predominantly responsible for plant‐mediated transfer of methane (CH₄) from the soil to the atmosphere. To clarify the pathways of CH₄ transport through the rice plant and find differences that may determine the large¡variation in the patterns of methane transport capacity (MTC) of rice cultivars, we examined the appearance, the distribution pattern, and the density of aerenchyma in different parts of rice¡plants of three widely varying rice cultivars during panicle initiation, flowering, and maturity stages. The data on the amount and density of small (> 1 ¡Á 10³? 5 ¡Á 10³Ìm²), medium (> 5 ¡Á 10³? 20 ¡Á 10³Ìm²) and large aerenchyma lacunae (> 20 ¡Á 10³Ìm²) were collected using a computer assisted image‐analyzing system. The brightfield optical microscopy of roots of all tested rice plants demonstrated the continuity of aerenchyma channels in the roots that function as conduits for bi‐directional transport of gases. The aerenchyma channels of primary roots showed direct connection with those of culms. Intercalary meristems were found at the transition zone of rootaCculm aerenchyma connections. Well‐developed aerenchyma lacunae present in the internodal region of the culm base were uniformly distributed in the peripheral cortical zone. The nodal region had relatively fewer and smaller aerenchyma lacunae that showed a non‐uniform distribution pattern. As a result, few aerenchyma channels continued from the internodal region through to the nodal region. The aerenchyma in the cortex zone of the culm expanded along with the growing secondary tiller, developing continuity between the culm and the secondary tiller. The micrographs of longitudinal sections of different specimens of culmaCleaf sheath intersection showed the continuity of aerenchyma channels from the culm to the leaf. The amount of medium and large aerenchyma lacunae in the leaf sheath was respectively 2 and 33 times greater as compared to those of the tiller. The proportion of the large lacunae in the total amount of aerenchyma in leaf sheath was 75 % as compared to only 8 % in the tiller, revealing higher number and larger size of aerenchyma in the former. There were significant differences in amount and density of aerenchyma between individual cultivars at a given growth stage, as well as in the development patterns. While the amount and density of medium and small aerenchyma lacunae in the internodal region of the culm base did not show any relationship with MTC of rice cultivars, large aerenchyma lacunae exhibited highly significant correlations with MTC of different cultivars, suggesting that the wide variation in MTC of rice plants during different growth stages are related to these structural features.     

Diurnal variation in methane efflux at different growth stages of tropical rice

Diurnal variation in CH₄ efflux from continuously flooded fields planted to rice (Oryza sativa L. cv. IR-36) was examined at different crop growth stages using a closed chamber method during the wet season. CH₄ emission showed a distinct diurnal pattern especially at tillering, panicle initiation and maturity stages of a field-grown rice crop, with maximum emission in the early afternoon (12.00 to 15.00) followed by a decline to a minimum around midnight. Among several variables (ambient temperature, flood water temperature, redox potential, soil pH, and root oxidase activity), a significant negative correlation existed between oxidase activity of the root base and diurnal fluctuations in CH₄ efflux at tillering stage. Evidence also suggested that redox status in the rhizosphere region and atmospheric, soil, and water temperatures influenced CH₄ emission from rice fields probably by their contrasting effects on CH₄ production and oxidation.     

Quantifying the effects of ozone on plant reproductive growth and development

Tropospheric ozone (O₃) is a harmful air pollutant that can negatively impact plant growth and development. Current O₃ concentrations ([O₃]) decrease forest productivity and crop yields and future [O₃] will likely increase if current emission rates continue. However, the specific effects of elevated [O₃] on reproductive development, a critical stage in the plant's lifecycle, have not been quantitatively reviewed. Data from 128 peer‐reviewed articles published from 1968 to 2010 describing the effects of O₃ on reproductive growth and development were analysed using meta‐analytic techniques. Studies were categorized based on experimental conditions, photosynthetic type, lifecycle, growth habit and flowering class. Current ambient [O₃] significantly decreased seed number (?16%), fruit number (?9%) and fruit weight (?22%) compared to charcoal‐filtered air. In addition, pollen germination and tube growth were decreased by elevated [O₃] compared to charcoal‐filtered air. Relative to ambient air, fumigation with [O₃] between 70 and 100 ppb decreased yield by 27% and individual seed weight by 18%. Reproductive development of both C₃ and C₄ plants was sensitive to elevated [O₃], and lifecycle, flowering class and reproductive growth habit did not significantly affect a plant's response to elevated [O₃] for many components of reproductive development. However, elevated [O₃] decreased fruit weight and fruit number significantly in indeterminate plants, and had no effect on these parameters in determinate plants. While gaps in knowledge remain about the effects of O₃ on plants with different growth habits, reproductive strategies and photosynthetic types, the evidence strongly suggests that detrimental effects of O₃ on reproductive growth and development are compromising current crop yields and the fitness of native plant species.     

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.