Ambient ozone (O3) and adverse crop response: a unified view of cause and effect

This paper presents a cohesive view of the dynamics of ambient O(3) exposure and adverse crop response relationships, coupling the properties of photochemical O(3) production, flux of O(3) from the atmosphere into crop canopies and the crop response per se. The results from two independent approaches ((a) statistical and (b) micrometeorological) were analyzed for understanding cause-effect relationships of the foliar injury responses of tobacco cv Bel-W3 to the exposure dynamics of ambient O(3) concentrations. Similarly, other results from two independent approaches were analyzed in: (1) establishing a micrometeorological relationship between hourly ambient O(3) concentrations and their vertical flux from the air into a natural grassland canopy; and (2) establishing a statistical relationship between hourly ambient O(3) concentrations in long-term, chronic exposures and crop yield reductions. Independent of the approach used, atmospheric conditions appeared to be most conducive and the crop response appeared to be best explained statistically by the cumulative frequency of hourly ambient O(3) concentrations between 50 ppb and 90 ppb (100 and 180 microg m(-3)). In general, this concentration range represents intermediate or moderately enhanced hourly O(3) values in a polluted environment. Further, the diurnal occurrence of this concentration range (often approximately between 0900 and 1600 h in a polluted, agricultural environment) coincided with the optimal CO(2) flux from the atmosphere into the crop canopy, thus high uptake. The frequency of occurrence of hourly O(3) concentrations > 90 ppb (180 microg m(-3)) appeared to be of little importance and such concentrations in general appeared to occur during atmospheric conditions which did not facilitate optimal vertical flux into the crop canopy, thus low uptake. Alternatively, when > 90 ppb (180 microg m(-3)) O(3) concentrations occurred during the 0900-1600 h window, their frequency of occurrence was low in comparison to the 50-90 ppb (100-180 microg m(-3)) range. Based on the overall results, we conclude that if the cumulative frequency of hourly ambient O(3) concentrations between 50-62 ppb (100-124 microg m(-3)) occurred during 53% of the growing season and the corresponding cumulative frequency of hourly O(3) concentrations between 50-74 ppb (100-148 microg m(-3)) occurred during 71% of the growing season, then yield reductions in sensitive crops could be expected, if other factors supporting growth, such as adequate soil moisture are not limiting.     

Sensitivity of modelled sulphate and nitrate aerosol to cloud,pH and ammonia emissions

A Lagrangian dispersion model has been used to predict daily sulphate aerosol in 2006 at five UK rural measurement sites and hourly nitrate aerosol in April 2003 at Harwell (UK). The sensitivity of aqueous phase sulphate production to the meteorological input has been investigated. Large differences were found between cloud fraction and cloud liquid water output from the regional and mesoscale Met Office Unified Model. The impact on the sulphate aerosol was relatively small, with the mesoscale meteorology giving better results.Sulphate aerosol production in the aqueous phase was found to be very sensitive to modelled cloud pH. As the cloud becomes acidic sulphate production is greatly limited, conversely if the cloud is basic large amounts of sulphate aerosol are produced. A fixed model pH of 5.8 was found to produce better results than allowing the model to calculate pH which resulted in large over-predictions of measured sulphate aerosol in some episodes.Nitrate aerosol was not sensitive to cloud variables or pH, but showed a slight increase with 30% more ammonia emissions, and a slight decrease with 30% less ammonia.Sulphate production in model runs with fixed pH was not sensitive to changing ammonia emissions, however the sulphate production with modelled pH was very sensitive to plus or minus 30% ammonia. This work suggests that good modelling of ammonia is essential to correct estimation of aqueous phase sulphate aerosol if cloud pH is modelled. It is concluded that modelling to estimate the effect of reduced ammonia emission scenarios on future ambient aerosol levels should also take into account the neutralising effect of ammonia in cloud and hence the effect on aqueous phase production of sulphate.     

Acquired changes in stomatal characteristics in response to ozone during plant growth and leaf development of bush beans (Phaseolus vulgaris L.) indicate phenotypic plasticity

Bush bean (Phaseolus vulgaris L.) lines 'S156' (O3-sensitive)/'R123' (O3-tolerant) and cultivars 'BBL 290' (O3-sensitive)/'BBL 274' (O3-tolerant) were used to study the effects of O3 on stomatal conductance (gs), density, and aperture size on leaf and pod surfaces with the objective of establishing links between the degree of plant sensitivity to O3 and plasticity of stomatal properties in response to O3. Studies in open-top chambers (OTCs) and in continuously stirred tank reactors (CSTRs) established a clear relationship between plant developmental stages, degrees of O3 sensitivity and gs: while 'S156' had higher gs rates than 'R123' earlier in development, similar differences between 'BBL 290' and 'BBL 274' were observed at later stages. Gs rates on the abaxial leaf surfaces of 'S156' and 'BBL 290', accompanied by low leaf temperatures, were significantly higher than their O3-tolerant counterparts. Exposure to O3 in CSTRs had greater and more consistent impacts on both stomatal densities and aperture sizes of O3-sensitive cultivars. Stomatal densities were highest on the abaxial leaf surfaces of 'S156' and 'BBL 290' at higher O3 concentrations (60 ppb), but the largest aperture sizes were recorded on the adaxial leaf surfaces at moderate O3 concentrations (30 ppb). Exposure to O3 eliminated aperture size differences on the adaxial leaf surfaces between sensitive and tolerant cultivars. Regardless of sensitivity to O3 and treatment regimes, the smallest aperture sizes and highest stomatal densities were found on the abaxial leaf surface. Our studies showed that O3 has the potential to affect stomatal plasticity and confirmed the presence of different control mechanisms for stomatal development on each leaf surface. This appeared to be more evident in O3-sensitive cultivars.     

Evidence for potential impacts of ozone on Pinus cembra L. at mountain sites in Europe: an overview

We summarize what is known about the impact of ozone (O(3)) on Pinus cembra in the timberline ecotone of the central European Alps and the Carpathian Mountains. In the central European Alps exposure to ambient and two-fold ambient O(3) throughout one growing season did neither cause any visible injury nor affect the photosynthetic machinery and biochemical parameters in current to 1-year-old needles. By contrast, in the southern French Alps and in the Carpathians 1-year-old needles of Pinus cembra trees showed visual symptoms similar to those observed in O(3) stressed pine stands in southern California. For the southern French Alps the observed symptoms could clearly be attributed O(3) and differences in O(3) uptake seems to be the likely key factor for explaining the observed decline. For the Carpathians however, other reasons such as drought may not be excluded in eliciting the observed symptoms. Thus, the action of O(3) has always to be evaluated in concert with other environmental impacts, determining the tree's sensitivity to stress.     

Dynamics of biomass partitioning in field-grown radish varieties, treated with ethylenediurea

During the growing season of 1990, five staggered crops of radish (Raphanus sativus L.) were grown in the field, using the cultivars 'Cherry Belle', 'Red Prince', and 'Red Devil B'. Half of the plants received a soil drench (100 ml plant(-1); 100 mg litre(-1) of ethylenediurea (EDU) once, early in plant development. Destructive harvests were carried out at 2-day intervals during vegetative development. Non-linear growth kinetics, derived from Richards' function, were fitted to the dry weight data of the total plant, main organs (shoot and hypocotyl) and to the dry weight ratio between below-ground and above-ground organs. Estimating the parameters of these non-linear functions and testing their differences between EDU-treated and untreated plants unveiled biologically meaningful information on the impact of different levels of ambient ozone (O(3)) during the growth periods. The modified function which was applied to the data of biomass partitioning between the major plant parts was more powerful in detecting transient alterations in assimilate allocation compared to the growth dynamics of individual plant organs. At low levels of O(3), biomass partitioning towards the below-ground sink organs was slightly delayed and finally restricted in EDU-treated plants. When ambient O(3) reached moderate levels, which did not cause visible foliar injury, assimilate partitioning between organs was only insignificantly altered during early growth when EDU-treatments were compared. As growth progressed, however, less assimilates were allocated towards the hypocotyl and roots in the plants not protected by EDU. This pattern was similar in all cultivars tested, but was smallest in 'Cherry Belle', which is known to be sensitive to O(3) with respect to foliar injury. During the 15- to 19-day periods of rapid growth, the O(3)-exposure >80 nl litre(-1) ranged from 0.015 to 0.209 microl litre(-1) O(3) h, which corresponds to 7 h d(-1) mean values between 40 and 50 nl litre(-1) O(3), confirming that ambient ozone did not exceed a moderate level in this study.     

Radish (Raphanus sativus L.): a model for studying plant responses to air pollutants and other environmental stresses

The use of Raphanus sativus L. as a model crop for studies on plant response to environmental stresses is reviewed with emphasis on the effects of different atmospheric pollutants (O(3), SO(2), NO(2), acidic precipitation) and their combinations. Responses to temperature, light supply, water stress, and atmospheric CO(2) are also studied and discussed. In addition, the references reviewed are evaluated in terms of their experimental protocols on growth conditions and recommendations for optimal ranges of environmental and cultural variables, i.e. light, temperature, nutrient supply are given. Its distinct pattern of biomass partitioning, the small dimensions along with short and easy culture make radish an excellent experimental plant. The fleshy below-ground storage organ, formed by the hypocotyl and upper radicle, acts as the major sink during vegetative development. Abundant assimilate supply due to elevated levels of CO(2) along with high irradiation frequently promote hypocotyl growth more than shoot growth, whereas under conditions of stress shoot growth is maintained at the expense of the hypocotyl. This makes the hypocotyl: shoot ratio of radish a very sensitive and suitable indicator for various environmental stresses. Potential weaknesses and shortcomings of radish in its role as a model crop, particularly the high variability of injury and growth responses, are discussed along with possible solutions. Future research needs are derived from the summarized results presented and from some disparities among findings within the literature reviewed.     

Seasonal and diurnal gas exchange differences in ozone-sensitive common milkweed (Asclepias syriaca L.) in relation to ozone uptake

Stomatal conductance and net photosynthesis of common milkweed (Asclepias syriaca L.) plants in two different soil moisture regimes were directly quantified and subsequently modeled over an entire growing season. Direct measurements captured the dynamic response of stomatal conductance to changing environmental conditions throughout the day, as well as declining gas exchange and carbon assimilation throughout the growth period beyond an early summer maximum. This phenomenon was observed in plants grown both with and without supplemental soil moisture, the latter of which should theoretically mitigate against harmful physiological effects caused by exposure to ozone. Seasonally declining rates of stomatal conductance were found to be substantial and incorporated into models, making them less susceptible to the overestimations of effective exposure that are an inherent source of error in ozone exposure indices. The species-specific evidence presented here supports the integration of dynamic physiological processes into flux-based modeling approaches for the prediction of ozone injury in vegetation.