Jürgen Bender

Growth responses and foliar sensitivities of native herbaceous species to ozone exposures

This paper reports on open-top chamber studies investigating the effects of different O3 exposures on native herbaceous plant species. Plants were grown up from seeds, potted into natural soils and exposed to near-ambient O3 levels during one growing season. A wide range of visible symptoms was apparent during the exposures. Species such as Rumex obtusifolius, Senecio vulgaris or Sonchus asper showed leaf colorations (e.g. reddish pigmentation) that probably indicate a non-specific stress response. In other species especially of the genera Malva and Cirsium the symptoms produced by O3 appeared to be similar to those characteristic for O3-specific foliar injury (stippling, flecking). In almost all species tested, O3 caused premature leaf senescence, which was sometimes associated with premature leaf abscission. However, earlier senescence did not necessarily result in changes in plant growth. Of all species tested, Malva sylvestris was found to be the most sensitive in terms of growth reduction and lower seed production.

Response of cellular antioxidants to ozone in wheat flag leaves at different stages of plant development

The effect of ozone (O3) on growth, yield and foliar antioxidants of spring wheat (Triticum aestivum L. cv. Turbo) was investigated in 1990 and 1991 in Braunschweig, Germany. Plants were grown full-season in pots in open-top chambers ventilated with charcoal-filtered (CF) air to which one or two levels of O3 were added. Mean 8 h day(-1) (10.00-18.00 h) O3 concentrations in the CF and CF + O3 treatments were 5.9, 61.2 and 92.5 nl litre(-1) in 1990, and 4.7 and 86.4 nl litre(-1) in 1991. Plants that received the high O3 level showed symptoms of premature senescence of the oldest leaves and yield reductions in both growing seasons. The contents of ascorbate and glutathione and the enzyme activities of ascorbate peroxidase and glutathione reductase were measured in symptomless flag leaves in weekly intervals before and after the beginning of anthesis. Leaf age had a significant effect on all antioxidants investigated. The O3 exposures of about 90 nl litre(-1) increased the activity of ascorbate peroxidase and the concentration of glutathione, but there were no pollutant effects on ascorbate content and glutathione reductase activity. Measurements of the antioxidant levels throughout one day showed no clear indications of diurnal changes in the antioxidative capacity in wheat flag leaves. The results are discussed in relation to the role of antioxidants in O3 detoxification.

Updated stomatal flux and flux-effect models for wheat for quantifying effects of ozone on grain yield, grain mass and protein yield

Field measurements and open-top chamber experiments using nine current European winter wheat cultivars provided a data set that was used to revise and improve the parameterisation of a stomatal conductance model for wheat, including a revised value for maximum stomatal conductance and new functions for phenology and soil moisture. For the calculation of stomatal conductance for ozone a diffusivity ratio between O(3) and H(2)O in air of 0.663 was applied, based on a critical review of the literature. By applying the improved parameterisation for stomatal conductance, new flux-effect relationships for grain yield, grain mass and protein yield were developed for use in ozone risk assessments including effects on food security. An example of application of the flux model at the local scale in Germany shows that negative effects of ozone on wheat grain yield were likely each year and on protein yield in most years since the mid 1980s.

Physiological and Biochemical Responses of Bush Bean (Phaseolus vulgaris) to Ozone and Drought Stress

Summary Bush bean ( Phaseolus vulgaris L.) plants were exposed to ozone (O 3 ) episodes in open-top field chambers in early and late season studies at Corvallis, Oregon. Plants were grown in cultural systems that controlled plant water status. The 7-h seasonal mean O 3 concentrations were 0.067 and 0.054 ppm for the early and late season experiments, respectively. At anthesis, part of the plants were subjected to a drought-stress treatment that was maintained for 14 days, after which it was relieved. Both O 3 and drought-stress treatments affected physiological and biochemical processes in beans during their reproductive stages of development. Measurements of water potential, osmotic potential and relative water content revealed a mild leaf drought stress in plants grown under water deficit conditions. Drought stress increased the foliar concentrations of several nutrients (Mg, K, Fe, S, P), while O 3 exposure had very little effect. At final harvest when plants had reached pod maturation, the concentrations of total soluble sugars were reduced by O 3 . Both O 3 and drought stress significantly affected the sum of free amino acids. Of all amino acids quantified, proline showed the most remarkable increase (up to three-fold) to drought stress. Ozone exposure impaired osmoregulation to drought stress.

Ozone exposure of field-grown winter wheat affects soil mesofauna in the rhizosphere.

A 2-year open-top chamber experiment with field-grown winter wheat (Triticum aestivum L. cv. Astron) was conducted to examine the effects of ozone on plant growth and selected groups of soil mesofauna in the rhizosphere. From May through June in each year, plants were exposed to two levels of O(3): non-filtered (NF) ambient air or NF+ 40 ppb O(3) (NF+). During O(3) exposure, soil sampling was performed at two dates according to different plant growth stages. O(3) exposure reduced above- and below-ground plant biomass in the first year, but had little effect in the second year. The individual density of enchytraeids, collembolans and soil mites decreased significantly in the rhizosphere of plants exposed to NF+ in both years. Differences were highest around anthesis, i.e. when plants are physiologically most active. The results suggest that elevated O(3) concentrations may influence the dynamic of decomposition processes and the turnover of nutrients.

Current and future ozone risks to global terrestrial biodiversity and ecosystem processes

Abstract Risks associated with exposure of individual plant species to ozone (O3) are well documented, but implications for terrestrial biodiversity and ecosystem processes have received insufficient attention. This is an important gap because feedbacks to the atmosphere may change as future O3 levels increase or decrease, depending on air quality and climate policies. Global simulation of O3 using the Community Earth System Model (CESM) revealed that in 2000, about 40% of the Global 200 terrestrial ecoregions (ER) were exposed to O3 above thresholds for ecological risks, with highest exposures in North America and Southern Europe, where there is field evidence of adverse effects of O3, and in central Asia. Experimental studies show that O3 can adversely affect the growth and flowering of plants and alter species composition and richness, although some communities can be resilient. Additional effects include changes in water flux regulation, pollination efficiency, and plant pathogen development. Recent research is unraveling a range of effects belowground, including changes in soil invertebrates, plant litter quantity and quality, decomposition, and nutrient cycling and carbon pools. Changes are likely slow and may take decades to become detectable. CESM simulations for 2050 show that O3 exposure under emission scenario RCP8.5 increases in all major biomes and that policies represented in scenario RCP4.5 do not lead to a general reduction in O3 risks; rather, 50% of ERs still show an increase in exposure. Although a conceptual model is lacking to extrapolate documented effects to ERs with limited or no local information, and there is uncertainty about interactions with nitrogen input and climate change, the analysis suggests that in many ERs, O3 risks will persist for biodiversity at different trophic levels, and for a range of ecosystem processes and feedbacks, which deserves more attention when assessing ecological implications of future atmospheric pollution and climate change.

Analyses of enzyme activities and other metabolic criteria after five years of fumigation

Enzymatic activity (peroxidase, glutamate dehydrogenase, glutamine synthetase), foliage buffering capacity, soluble protein and nitrogen content were measured in current and previous year needles from young spruce (Picea abies) and fir (Abies alba). The trees were exposed to low levels of SO(2) and/or O(3) and simulated acidic precipitation (pH 4.0) in open-top chambers from 1983 through 1988. Needle samples were taken during March 1988 at the end of the five-year fumigation period. Exposure to SO(2) substantially increased sulphur content in both needle age classes of spruce and fir, and concomitantly reduced the foliage buffering capacity index (BCI), whereas the combined fumigation with SO(2) and O(3) had no effect on BCI. Peroxidase activity was markedly higher in year-old needles compared to current-year needles. However, trees from the SO(2) and SO(2) + O(3) treatments exhibited statistically significant stimulated peroxidase activities. Similarly, changes in the activities of the nitrogen-metabolizing enzymes indicated an altered cellular function of the trees after the long-term pollution stress. Levels of activity of both glutamate dehydrogenase and glutamine synthetase were increased by exposure to SO(2), especially in spruce. Although glutamate dehydrogenase in spruce was affected by all treatments, such changes in activity were found in fir only with the SO(2) treatment. The highest activity of glutamine synthetase, however, occurred in the older needles of trees exposed to SO(2) + O(3). Total nitrogen concentration was either unaffected by the pollutant treatments or decreased in spruce compared to the controls. No statistically significant changes due to the fumigation were found in soluble protein concentrations. Results indicated that chronic exposure to air pollutants lead to alterations in metabolic processes in conifer needles, detectable either by changes in typical stress indicating values or by increases in ammonium assimilation capacity.

Low doses of ozone affect nitrogen metabolism in bean (Phaseolus vulgaris L.) leaves

Summary Bush bean ( Phaseolus vulgaris L. cv. Rintintin) plants were exposed 34 days for 8 h day -1 to three levels of ozone (7, 62 and 111 ppb) in open-top chambers. During anthesis concentrations of amino acids, polyamines and soluble protein as well as the activity of the enzyme glutamine synthetase were measured in leaves in order to investigate the effect of chronic ozone stress on characteristics of nitrogen metabolism. Protein content and enzyme activity of glutamine synthetase were both shown to decrease with increasing ozone concentrations. Spermidine was elevated at the highest ozone level. Analysis of amino acids yielded similar results showing a significant increase of total amino acids at the highest treatment level. This was associated with an increase of the concentrations of the individual amino acids glutamate, glutamine, alanine, threonine and, especially, asparagine. At the intermediate ozone level only the concentration of glutamate was found to be higher.

Mineral content in the soil and tree foliage.

Seedlings of fir (Abies alba Mill.) and spruce (Picea abies L. Karst.) were fumigated with SO(2), O(3) and SO(2) + O(3) in open-top chambers (OTCs) for almost 5 vegetation periods. As background stress, simulated rain of pH 4.0 was applied. Nutrient content of soil, soil solutions, and trees was investigated and balanced. In the upper partition of the soil high concentrations of exchangeable Ca(2+) were found in all chambers. The SO(2) and SO(2) + O(3) treatments led to increased Ca(2+), Mg(2+) and Mn(2+) concentrations in soil solution and the pool of exchangeable protons increased. This response was most evident in the SO(2) and SO(2) + O(3) chambers and less clear in the filtered pH 5.0 control chamber. In the SO(2) treatment increased Mn and S levels were found in the needles. Ca content in the needles showed a decreasing trend. O(3) alone had no consistent effect on needle nutrient content.

Plant-Mediated Ecosystem Effects of Tropospheric Ozone

Tropospheric ozone (O3) is considered as the most significant phytotoxic pollutant in the atmosphere and is already responsible for widespread effects on crops, trees and native plant species. Globally, there is evidence that the background O3 concentrations are further increasing. Most research has been conducted on plant and tree species of commercial value, but very little is known about the impacts of O3 on the scale of forest-, agro- or grassland ecosystems. Exposure to elevated O3 causes oxidative stress, which results in reduced photosynthesis, visible injury, decreased growth and productivity. We present examples showing that impacts of O3 on vegetation may lead to long-term effects on ecosystem structure and function. Recent experiments have shown that O3 can cause a shift in plant species composition and can indirectly affect soil processes. Ozone has also been shown to affect water cycling through its effect on stomata and can alter overall ecosystem productivity.