Håkan Pleijel

Yield and grain quality of spring wheat (Triticum aestivum L., cv. Drabant) exposed to different concentrations of ozone in open-top chambers

Spring wheat (Triticum aestivum L., cv. Drabant) was exposed to different concentrations of ozone in open-top chambers for two growing seasons, 1987 and 1988, at a site located in south-west Sweden. The chambers were placed in a field of commercially grown spring wheat. The treatments were charcoal-filtered air (CF), non-filtered air (NF) and non-filtered air plus extra ozone (NF(+)). In 1988, one additional ozone concentration (NF(++)) was used. Grain yield was affected by the ozone concentration of the air. Air filtration resulted in an increase in grain yield of about 7% in both years, compared to NF. The addition of ozone (NF(+), NF(++)) reduced grain yield and increased the content of crude protein of the grain in both years. Filtration of the air had no significant effect on the content of crude protein, compared to NF. The results showed a strong positive chamber effect on grain yield in the cold and wet summer of 1987. In 1988, there was no net chamber effect on grain yield. The relative differences between the CF, NF and NF(+) treatments with respect to grain yield were of the same magnitude in the two years, despite the very different weather conditions.

Grain protein accumulation in relation to grain yield of spring wheat (Triticum aestivum L.) grown in open-top chambers with different concentrations of ozone, carbon dioxide and water availability

The present investigation was undertaken in order to study the influence of ozone, carbon dioxide and water availability on the relationship between grain protein and grain yield in wheat (Triticum aestivum L.). Results were combined from spring wheat, field grown in 16 different open-top chamber experiments, from four different countries. Protein concentration of the grain was negatively (linear) associated with grain yield. This relationship was symmetrical for yield reductions and yield stimulations, despite the fact that the major cause for increases in yield (elevated carbon dioxide concentrations) was different from that causing crop loss (elevated ozone concentrations). The relationship between off-take (the amount of protein taken away from the farmland per unit area) of grain protein and grain yield was clear and highly consistent, but not linear. Yield loss in relation to the reference used (open-top chamber with non-filtered air) was associated with a larger negative change in protein off-take than the positive change in protein off-take corresponding to a yield increase of the same size. The water treatments used in some of the experiments influenced yield and protein content to a very limited extent. It is concluded from the present study that the change of the grain protein from factors such as ozone and carbon dioxide can be explained largely by a simple relationship between grain protein and grain yield at a certain level of nitrogen availability to the plants.

An ozone flux-response relationship for wheat

Six open-top chamber experiments with field-grown wheat Triticum aestivum L. (five with spring wheat and one with winter wheat) were combined to test which of the two ozone exposure indices, AOT40 and CFO(3), that provided the most consistent relationship between relative yield loss and ozone exposure. AOT40 is the accumulated exposure over a threshold ozone concentration of 40 nl l(-1), while CFO(3) is the cumulative flux of ozone (uptake) to the flag leaves. The ozone uptake of the flag leaves was estimated using a stomatal conductance model, sensitive to phenology, light, vapour pressure deficit (VPD) and temperature in combination with measurements of the boundary layer conductance in the open-top chambers. Both indices were calculated for the grain-filling period, defined as the time from anthesis until 2 weeks before harvest. The duration of the grain-filling period was shown to be closely related to the rate of accumulation of thermal time above a base temperature of 0 degrees C. The CFO(3) index provided a much more consistent pattern in terms of ozone effects compared to the AOT40 index. This was especially the case for spring wheat, for which a linear regression between relative yield and CFO(3) using all five data sets is presented. According to the stomatal conductance model, VPD limited daytime stomatal conductance in warm and dry years, while temperature was the most important limiting factor during daytime in cool and humid years. The effect of light was mainly to delimit the time period of the day during which substantial uptake of ozone took place. It is concluded that, compared to the AOT40 index, the more mechanistically relevant flux-based index CFO(3) will estimate larger yield loss in the relatively humid parts of western and northern Europe, while smaller yield loss will be estimated for the dry summer climates in south and central Europe. The use of an ozone flux threshold, similar to the cut-off concentration 40 nl l(-1) in AOT40, did not improve the performance of the CFO(3) index.

Constraints to nitrogen acquisition of terrestrial plants under elevated CO2.

A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO2 (eCO2 ), and whether or not this constraint will become stronger over time. We explored the ecosystem-scale relationship between responses of plant productivity and N acquisition to eCO2 in free-air CO2 enrichment (FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) in all three ecosystem types, this relationship was positive, linear and strong (r(2) = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO2 caused neutral or modest changes in productivity. As the ecosystems were markedly N limited, plants with minimal productivity responses to eCO2 likely acquired less N than ambient CO2 -grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO2 , and this decrease was independent of the presence or magnitude of eCO2 -induced productivity enhancement, refuting the long-held hypothesis that this effect results from growth dilution. (iii) Effects of eCO2 on productivity and N acquisition did not diminish over time, while the typical eCO2 -induced decrease in plant N concentration did. Our results suggest that, at the decennial timescale covered by FACE studies, N limitation of eCO2 -induced terrestrial productivity enhancement is associated with negative effects of eCO2 on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation.

A stomatal ozone flux-response relationship to assess ozone-induced yield loss of winter wheat in subtropical China.

Stomatal ozone flux and flux-response relationships were derived for winter wheat (Triticum aestivum L.) grown under fully open-air ozone fumigation. A stomatal conductance (g(sto)) model developed for wheat in Europe was re-parameterized. Compared to European model parameterizations, the main changes were that the VPD and radiation response functions were made less and more restrictive, respectively, and that the temperature function was omitted. The re-parameterized g(sto) model performed well with an r(2) value of 0.76. The slope and intercept of the regression between observed and predicted g(sto) were not significantly different from 1 to 0, respectively. An ozone uptake threshold of 12 nmol m(-2) s(-1) was judged most reasonable for the wheat flux-response relationship in subtropical China. Judging from both flux- and concentration-based relationships, the cultivars investigated seem to be more sensitive to ozone than European cultivars. The new flux-response relationship can be applied to ozone risk assessment in subtropical regions.

Phenological weighting of ozone exposures in the calculation of critical levels for wheat, bean and plantain.

This paper presents phenological weighting factors to be applied to AOT40 (accumulated ozone exposure above a threshold of 40 nl l(-1)) ozone exposure-response relationships for crops at different growth stages. The quantification of such factors represents a step-forward in the derivation of Level II critical levels for ozone. The weighting factors presented are derived from published literature on the sensitivity of wheat (Triticum aestivum), bean (Phaseolus vulgaris) and plantain (Plantago major) to ozone at different growth stages. Weighting functions were calculated using either multiple linear regression or the reciprocal residual mean square (RMS(-1)). The resulting weights were transformed into multiplication factors to be applied to the monthly AOT40 during the 3-month assessment period of critical level exceedance. Interspecific differences were too large to allow for the development of a unified weighting function for the three species considered. For wheat grain yield, the derived multiplication factors varied by almost four-fold (0.40, 1.06, 1.54), while those for bean pod yield varied by only about 25% (0.85, 1.01, 1.14). The available data for plantain were restricted to short-term studies conducted under controlled conditions. These data were not suitable for the derivation of weighting factors comparable to those derived for bean and wheat. Based on known differences in wheat development and phenology across Europe, the need for a geographic differentiation of the time period for the calculation of the critical level exceedances is also discussed and examples provided of the adoption of the derived weightings in the mapping of critical level exceedances. Differences between critical level exceedance maps using weighted and unweighted AOT40 calculations are discussed.

Surface wetness enhances ozone deposition to a pasture canopy

A mass balance approach was used to estimate the deposition of ozone to a grass-clover canopy enclosed in open-top chambers. Three different concentrations of ozone were used. Deposition measurements were made before and after spraying the canopy with water. It was found that the wet canopy absorbed more ozone than the dry canopy. After spraying, the vapour pressure deficit (VPD) decreased in the chambers. Thus, an increase in stomatal conductance was expected, which would lead to an increase in ozone uptake by the plants. The change in VPD was, however, of a magnitude which is unlikely to account for more than part of the change in deposition velocity for ozone. It is concluded that the surface wetness as such accounted for a substantial part of the increase in ozone deposition after spraying.

Effects of ozone on biomass, non-structural carbohydrates and nitrogen in spring wheat with artificially manipulated source/sink ratio

Abstract Field grown spring wheat ( Triticum aestivum ) cv. Dragon, with artificially altered source/sink ratios, was exposed to ozone in open-top chambers in order to test whether the ozone sensitivity was affected by a shift in the source/sink relationship. The source and sink were manipulated by removing the flag leaf or the upper part of the ear, respectively. In 1995, three different chamber treatments were used: non-filtered air without extra ozone (NF), with ozone added before anthesis (NF+pre) or with ozone added during and after anthesis (NF+post). The ozone exposure during the treatment periods was 2417 and 2508 nmol mol −1 hours (AOT40) in NF+pre and NF+post, respectively. In 1997, the plants were exposed to filtered air (F) or to non-filtered air without extra ozone (NF) or with three different levels of ozone added (NF1+, NF2+ and NF3+). The ozone exposure expressed as AOT40 was 0, 13, 1924, 5881 and 10 375 nmol mol −1 hours in F, NF, NF1+, NF2+ and NF3+, respectively. The dry weight per grain and the amount of total non-structural carbohydrates and nitrogen per grain were reduced by ozone exposure in 1997. In 1995, the dry weight per grain in the source-manipulated shoots was reduced by ozone, especially when the exposure was conducted during and after anthesis. The ozone effect in 1997 was smaller in sink-manipulated shoots than in source-manipulated and non-manipulated shoots. It is concluded that the ozone sensitivity of wheat is higher when the grain growth is source-limited. It is also concluded that the ozone sensitivity of wheat is higher during and after anthesis than before anthesis, although source-limited wheat plants may be sensitive also to early ozone exposure.

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.

Ozone risk for vegetation in the future climate of Europe based on stomatal ozone uptake calculations

The negative impacts of surface ozone (O3) on vegetation are determined by external exposure, leaf gas exchange and plant antioxidant defence capacity, all dependent on climate and CO2 concentrations. In this study the influence of climate change on simulated stomatal O3 uptake of a generic crop and a generic deciduous tree at ten European sites was investigated, using the LRTAP Mapping Manual stomatal flux model. O3 concentrations are calculated by a chemistry transport model (MATCH) for three 30-yr time-windows (1961–1990, 2021–2050, 2071–2100), with constant precursor emissions and meteorology from a regional climate model (RCA3). Despite substantially increased modelled future O3 concentrations in central and southern Europe, the flux-based risk for O3 damage to vegetation is predicted to remain unchanged or decrease at most sites, mainly as a result of projected reductions in stomatal conductance under rising CO2 concentrations. Drier conditions in southern Europe are also important for this result. At northern latitudes, the current parameterisation of the stomatal conductance model suggest O3 uptake to be mainly limited by temperature. This study demonstrates the importance of accounting for the influences by climate and CO2 on stomatal O3 uptake, and of developing their representation in models, for risk assessment involving climate change.