Charcoal-filtered Ambient Air Research Articles

Whole-plant compensatory responses of isoprene emission from hybrid poplar seedlings exposed to elevated ozone

It is still unclear whether the responses of isoprene (ISO) emission to elevated O3 vary with biological organization level (i.e. leaf and whole-plant). To study such responses and the possible reasons explaining their variation, we investigated the effect of O3 (CF: charcoal-filtered ambient air; E-O3: non-filtered ambient air enriched with O3) on ISO emission rate (ISOrate), net photosynthetic rate (Pn), leaf nitrogen and carbon contents, and leaf growth traits in poplar seedlings (Populus deltoides cv. 55/56 × P. deltoides cv. Imperial) during one growing season. Opposite effects of E-O3 on Pn were found between upper leaves (positive effect) and lower leaves (negative effect). Compared to CF, E-O3 significantly decreased leaf mass per area, number of leaves, and leaf biomass, but increased leaf nitrogen content and individual leaf size. In the framework of such compensatory responses, poplar seedlings further increased ISOrate in upper leaves and decreased ISOrate in lower leaves, thus preventing significant decrease in the overall whole-plant ISOrate by E-O3. The measured whole-plant ISOrate also showed that the simplistic estimation approaches based on the linear regression between chlorophyll content indicated by soil plant analysis development meter (SPAD value) and leaf-level ISOrate could not accurately reflect the true response of whole plant to elevated O3. For more accurate predictions, the potential ISO compensatory response to increasing O3 concentration should be incorporated into the climate biogeochemical models related to ISO emission.

Response of isoprene emission from poplar saplings to ozone pollution and nitrogen deposition depends on leaf position along the vertical canopy profile

We investigated isoprene (ISO) emission and gas exchange in leaves from different positions along the vertical canopy profile of poplar saplings (Populus euramericana cv. ‘74/76’). For a growing season, plants were subjected to four N treatments, control (NC, no N addition), low N (LN, 50 kg N ha−1year−1), middle N (MN, 100 kg N ha−1year−1), high N (HN, 200 kg N ha−1year−1) and three O3 treatments (CF, charcoal-filtered ambient air; NF, non-filtered ambient air; NF + O3, NF + 40 ppb O3). Our results showed the effects of O3 and/or N on standardized ISO rate (ISOrate) and photosynthetic parameters differed along with the leaf position, with larger negative effects of O3 and positive effects of N on ISOrate and photosynthetic parameters in the older leaves. Expanded young leaves were insensitive to both treatments even at very high O3 concentration (67 ppb as 10-h average) and HN treatment. Significant O3 × N interactions were only found in middle and lower leaves, where ISOrate declined by O3 just when N was limited (NC and LN). With increasing light-saturated photosynthesis and chlorophyll content, ISOrate was reduced in the upper leaves but on the contrary increased in middle and lower leaves. The responses of ISOrate to AOT40 (accumulated exposure to hourly O3 concentrations > 40 ppb) and PODY (accumulative stomatal uptake of O3 > Y nmol O3 m−2 PLA s−1) were not significant in upper leaves, but ISOrate significantly decreased with increasing AOT40 or PODY under limited N supply in middle leaves but at all N levels in lower leaves. Overall, ISOrate changed along the vertical canopy profile in response to combined O3 and N exposure, a behavior that should be incorporated into multi-layer canopy models. Our results are relevant for modelling regional isoprene emissions under current and future O3 pollution and N deposition scenarios.

Molecular response of poplar to single and combined ozone and drought

High concentration in ground-level ozone (O3) and water deficit affect forest ecosystems service. Previously we found intercellular CO2 concentration and isoprene emission were affected by the combination of O3 and drought, but the molecular mechanisms controlling these phenotypes are still open questions. In this study, we investigated the stomatal conductance (gs) and transcriptome changes in an O3-sensitive hybrid poplar exposed to two O3 levels [charcoal-filtered ambient air (CF) and non-filtered ambient air plus 40 ppb (NF40)] and two water conditions [well-watered (W) and moderate drought (D)]. NF40 reduced the gs more under D than W. We identified the differentially expressed genes (DEGs) from pairwise comparisons and found the poplar's molecular response to drought was counteracted by elevated O3. From nine clusters obtained through K-means clustering, 12 core transcription factors were identified. DEGs involved in isoprene biosynthesis and phytohormones signal pathways indicate the molecular response and stomatal closure of poplar under O3 and/or drought might be through MEP/DOXP and ABA-dependent pathways. In addition, 102 Helitrons capturing DEGs were involved in response to O3 and/or drought and related with ABA-dependent pathway. This integrated analysis provides multi-dimensional insights to understand the molecular response to the combination of O3 and drought.

Concentration- and flux-based dose-responses of isoprene emission from poplar leaves and plants exposed to an ozone concentration gradient.

Concentration- and flux-based O3 dose-responses of isoprene emission from single leaves and whole plants were developed. Two poplar clones differing in O3 sensitivity were exposed to five O3 levels in open-top chambers for 97d: charcoal-filtered ambient air (CF), non-filtered ambient air (NF) and NF plus 20ppb (NF+20), 40ppb (NF+40) and 60ppb (NF+60). At both leaf and plant level, isoprene emission was significantly decreased by NF+40 and NF+60 for both clones. Although intra-specific variability was found when the emissions were up-scaled to the whole plant, both leaf- and plant-level emissions decreased linearly with increasing concentration-based (AOT40, cumulative exposure to hourly O3 concentrations >40ppb) and flux-based indices (PODY , cumulative stomatal uptake of O3 >Y nmol O3 m-2 PLA s-1 ). AOT40- and POD7 -based dose-responses performed equally well. The two clones responded differently to AOT40 and similarly to PODY (with a slightly higher R2 for POD7 ) when the emission was expressed as change relative to clean air. We thus recommend POD7 as a large-scale risk assessment metric to estimate isoprene emission responses to O3 in poplar.

Water stress mitigates the negative effects of ozone on photosynthesis and biomass in poplar plants

Tropospheric ozone (O3) pollution frequently overlaps with drought episodes but the combined effects are not yet understood. We investigated the physiological and biomass responses of an O3 sensitive hybrid poplar clone (‘546’) under three O3 levels (charcoal-filtered ambient air, non-filtered ambient air (NF), and NF plus 40 ppb) and two watering regimes (well-watered (WW) and reduced watering (RW), i.e. 40% irrigation) for one growing season. Water stress increased chlorophyll and carotenoid contents, protecting leaves from pigment degradation by O3. Impairment of photosynthesis by O3 was also reduced by stomatal closure due to water stress, which preserved light-saturated CO2 assimilation rate, and the maximum carboxylation efficiency. Water stress increased water use efficiency of the leaves while O3 decreased it, showing significant interactions. Effects were more evident in older leaves than in younger leaves. Water stress reduced biomass production, but the negative effects of O3 were less in RW than in WW for total biomass per plant. A stomatal O3 flux-based dose-response relationship was parameterized considering water stress effects, which explained biomass losses much better than a concentration-based approach. The O3 critical level of Phytotoxic Ozone Dose over a threshold of 7 nmol O3.m−2.s−1 (POD7) for a 4% biomass loss in this poplar clone under different water regimes was 4.1 mmol m−2. Our results suggest that current O3 levels in most parts of China threaten poplar growth and that interaction with water availability is a key factor for O3 risk assessment.

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

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

The effects of ozone-exposed sugar maple seedlings on the biological performance and the feeding preference of the forest tent caterpillar (Malacosoma disstria Hbn.)

The effects of exposure of sugar maple (Acer saccharum Marsh.) to ozone on the entire larval stage of a native insect have not been previously investigated. This study reports the effects of sugar maple seedlings exposed to different ozone concentrations on the relative performance and the feeding preference of the forest tent caterpillar (Malacosoma disstria Hbn.). Three-year-old seedlings were set in nine open-top field chambers in the spring of 1992 and 1993. Three ozone concentrations were generated: charcoal-filtered ambient air (0×), ambient air (1×) and three times ambient air (3×). In 1992, female and male larval development time did not differ among ozone treatments. In 1993, female larvae reared on 3× developed faster than those on 0× and 1×, while male larvae were not affected. Ozone treatments did not influence pupal weights except for males in 1993 where pupae reared on 0× were heavier than 1× but did not differ from 3×. Larval and pupal survival rates were not affected by ozone in either year. Finally, 4th and 5th instar larvae showed a significant feeding preference for 3× foliage in 1993 but not in 1992. The response of the forest tent caterpillar to ozone exposed seedlings varied between years and could be more sensitive to annual climatic variations than ozone.

Critical levels of O3 for wood production of European beech (Fagus sylvatica L.)

During recent decades, ozone (O3) has gained much attention as a possible contributor to forest decline. Attempts have been made to set critical levels for O3 in order to protect vegetation. Damage to vegetation seems to depend on the pattern of exposure. Episodic peaks followed by periods with low concentrations are more phytotoxic than exposures with generally elevated concentrations but without peaks. The present experiment aims to examine whether O3 affects the wood production of beech seedlings. The seedlings were exposed to three different air pollution regimes: charcoal-filtered ambient air (CF), non-filtered ambient air (NF), and NF+30nl1−l−1 O3 8 hours day1 during summer periods (NFO). Basal stem diameter was measured regularly during three growing seasons. The relative diameter increase was significantly reduced with increased O3 concentration. AOT40 is calculated for all treatments and evaluated in relation to the relative diameter growth.

Effects of sulphur dioxide and nitrogen dioxide, singly and in mixture, on the macroscopic growth of three birch clones.

An investigation of some aspects of the effects of low concentrations of the gases, sulphur dioxide and nitrogen dioxide, singly and in mixture, was made on the growth of three birch clones, two of Betula pendula Roth. (silver birch) and one of Betula pubescens Ehr. (downy birch). Comparative measurements of the growth form and dry mass increment were made over one year in glasshouses supplied with charcoal-filtered ambient air, and SO(2) and NO(2), singly or in mixture, at mean concentrations of 62 ppb (nl litre (-1)) of one or both gases. The main effects were found in those plants that were fumigated with SO(2) singly, and SO(2) and NO(2) together. Both treatments induced premature leaf loss and reduction in mass, especially of roots, the effects increasing over time. The heights and initial leaf areas were maintained, apparently at the expense of other parameters. NO(2), if present singly, had little or no effect, but it tended to enhance the damaging effect of SO(2) when the two were applied together. The different clones showed different degrees of response to the pollutants, but these differences became less marked during the second season of fumigation. The effects found are discussed in relation to the annual growth of trees, particularly birch.