Effects of ozone on the yield of spring wheat (Triticum aestivum L., cv. Albis) grown in open-top field chambers

Spring wheat (Triticum aestivum L., cv. Albis) was grown in the field at a site located in central Switzerland, and exposed to chronic doses of ozone (O(3)) in open-top chambers to study impacts on yield. The experiment was carried out in 1986, 1987 and 1988. The treatments used included charcoal-filtered air (CF), non-filtered air (NF) and non-filtered air to which constant amounts of O(3) (two levels, O(3)-1 and O(3)-2) were added daily from 09.00 until 17.00 local time. Mean solar radiation-weighted O(3) concentrations during the fumigation period were in the range 0.016-0.022 microl litre(-1) (CF), 0.036-0.039 microl litre(-1) (NF), 0.057-0.058 microl litre(-1) (O(3)-1, used in 1987 and 1988 only) and 0.078-0.090 microl litre(-1) (O(3)-2). Fumigation was maintained from the three-leaf stage until harvest. Ambient plots were used as a reference. Plant characteristics examined included straw yield, grain yield, number of grains per head, number of heads per surface area, weight of individual grains and harvest index (ratio of grain weight to total dry weight). Pollutant concentrations and other environmental parameters were monitored continuously inside and outside the chambers. In 1986 and 1987, enclosure mostly increased the values of different parameters, while in 1988, they were decreased. The negative enclosure effect was due to extremely turbulent winds, which caused lodging inside the chambers. In all 3 years, increasing O(3) concentrations negatively affected the parameters studied, except for the number of heads per surface area, which showed no treatment response. Grain yield showed a very sensitive response to O(3). The effect of O(3) on grain yield was due to an effect primarily on grain size and secondarily on grain number. The relative response of grain yield to O(3) was similar in all 3 years, despite year-to-year differences in climatic conditions and enclosure effects. The analysis of the data for combined years revealed an increase of about 10% in grain yield due to air filtration. The corresponding increase in straw yield was only about 3.5%. Exposure-response models were developed for individual years and combined years. It is concluded that, in the study area, ambient O(3) may affect grain yield in spring wheat.     

Effects of elevated CO2 nitrogen supply and tropospheric ozone on spring wheat. I. Growth and yield

Spring wheat (Triticum aestivum L. cv. Minaret) was exposed to three CO(2) levels, in combination with two nitrogen fertilizer levels and two levels of tropospheric ozone, from sowing to ripening in open-top chambers. Three additional nitrogen fertilizer treatments were carried out at the lowest and the highest CO(2) level, respectively. Plants were harvested at growth stages 31, 65 and 93 and separated into up to eight fractions to gain information about biomass partitioning. CO(2) enrichment (263 microl litre(-1) above ambient levels) drastically increased biomass of organs serving as long-term carbohydrate pools. Peduncle weight increased by 92%, stem weight by 73% and flag leaf sheath weight by 59% at growth stage 65. Average increase in shoot biomass due to CO(2) enrichment amounted to 51% at growth stage 65 and 36% at final harvest. Average yield increase was 34%. Elevated nitrogen application was most effective on biomass of green tissues. Yield was increased by 30% when nitrogen application was increased from 150 to 270 kg N ha(-1). Significant interactions were observed between CO(2) enrichment and nitrogen application. Yield increase due to CO(2) ranged from 23% at 120 kg N to 47% at 330 kg N. Triticum aestivum cv. Minaret was not very responsive to ozone at 1.5 times ambient levels. 1000 grain weight was slightly decreased, which was compensated by an increased number of grains.     

Crop loss assessment for California: modeling losses with different ozone standard scenarios

Crop yield losses were estimated for ambient O3 concentrations and for a series of potential O3 air quality standards for California, including the current statewide 1-h oxidant (O3) standard of 0.10 ppm (196 microg m(-3)), 12-h growing season averages, and other models. A model for statewide losses was developed using hourly O3 data for all sites in the State, county crop productivity data, and available O3 concentration-yield loss equations to determine potential yield losses for each crop in each county in California for 1984. Losses were based on comparison to an estimated background filtered air concentration of 0.025 or 0.027 ppm, for 12 or 7 h, respectively. Potential losses due to ambient air in 1984 were estimated at 19% to 25% for dry beans, cotton, grapes, lemons, onions, and oranges. Losses of 5% to 9% were estimated for alfalfa and sweet corn. Losses of 4% or less were estimated for barley, field corn, lettuce, grain sorghum, rice, corn silage, spinach, strawberries, sugar beets, fresh tomatoes, processing tomatoes, and wheat. Implementation of either a modified rollback to meet the current 1 h California O3 standard (0.10 ppm) or a three-month, 12-h growing season average of 0.045 ppm was necessary to produce large reductions in potential crop losses.     

Influence of ozone stress on growth processes, yields and grain quality characteristics among soybean cultivars

Field studies were conducted at USDA Beltsville Agricultural Research Center, Beltsville, Maryland, in 1984 and 1985 using open-top chambers to acquire information on the responses of 12 soybean (Glycine max L. Merr.) cultivars to O3 stress and to examine the interactions between maturity groups and O3 stress. Cultivars representing Groups III, IV, and V were exposed for approximately 3 months to charcoal-filtered air (CF) and nonfiltered air plus 40 nl litre(-1) O3 (NF + O3). Ozone was added 6 h d(-1), 5 d week(-1) for 13 weeks. The CF effectively reduced the accumulative oxidant exposure (AOX) to less than 1.0 microl litre(-1) h and the NF + O3 treatment approximately doubled the ambient AOX (16.7 microl litre(-1) h) to about 30 microl litre(-1) h. The AOX estimates the total O3 exposure above 30 nl litre(-1) during an entire growing season. Plant growth rates and relative growth rates were reduced by 17.0 and 14.4%, respectively, when averaged over cultivars. Based on growth rates, the Group III cultivars were the most affected by O3 stress. Averaged over cultivars, leaf expansion rates, leaf conductance, and transpiration rates were lower in the NF + O3 treatment compared to the CF control; however, wide variation was found with the stomatal results from field observations. Combined over years and cultivars, grain yield was reduced by an average of 12.5% by O3 stress with 3 of 12 cultivars showing significant reductions. Grain protein content was increased by 0.7% by O3 stress, but cultivar differences were equal to the differences caused by the O3 treatments. Grain oil content was unchanged by the O3 treatments. Group IV cultivars showed the greatest decrease in grain yield due to O3 stress. Multiple regression analyses were calculated using the difference between the CF and NF + O3 treatment as a measure of O3 stress. Significant positive relationships were found among net assimilation rates, plant growth rates, relative growth rates, and leaf expansion rates, which suggest that growth analysis characteristics would be useful in addition to yield in air pollution tolerance improvement studies with soybeans.     

Effects of pre-exposure to ultraviolet-B radiation on responses of tomato (Lycopersicon esculentum cv. New Yorker) to ozone in ambient and elevated carbon dioxide

Patterns of environmental change in the biosphere include concurrent and sequential combinations of increasing ultraviolet (UV-B) and ozone (O(3)) at increasing carbon dioxide (CO(2)) levels; long-term changes are resulting mainly from stratospheric O(3) depletion, greater tropospheric O(3) photochemical synthesis, and increasing CO(2) emissions. Effects of selected combinations were evaluated in tomato (Lycopersicon esculentum cv. New Yorker) seedlings using sequential exposures to enhanced UV-B radiation and O(3) in differential CO(2) concentrations. Ambient (7.2 kJ m(-2 )day(-1)) or enhanced (13.1 kJ m(-2) day(-1)) UV-B fluences and ambient (380 microl l(-1)) or elevated (600 microl l(-1)) CO(2) were imposed for 19 days before exposure to 3-day simulated O(3) episodes with peak concentrations of 0.00, 0.08, 0.16 or 0.24 microl l(-1) O(3) in ambient or elevated CO(2). CO(2) enrichment increased dry mass, leaf area, specific leaf weight, chlorophyll concentration and UV-absorbing compounds per unit leaf area. Exposure to enhanced UV-B increased leaf chlorophyll and UV-absorbing compounds but decreased leaf area and root/shoot ratio. O(3) exposure generally inhibited growth and leaf photosynthesis and did not affect UV-absorbing compounds. The highest dose of O(3) eliminated the stimulating effect of CO(2) enrichment after ambient UV-B pre-exposure on leaf photosynthesis. Pre-exposure to enhanced UV-B mitigated O(3) damage to leaf photosynthesis at elevated CO(2).     

Impacts of ozone on the growth and yield of field-grown winter wheat

Seed of winter wheat (Triticum aestivum L. cv Riband) was sown on 29 August 1992 in eight field plots. Four plots were exposed to elevated ozone (O(3)) concentrations on 16 days between 29 August and 2 October 1992, for 6 h day(-1), and on 27 days between 29 March and 24 August 1993, for 7 h day(-1). Mean daily O(3) concentrations were approximately 30 and 80 nmol mol(-1) in ambient and fumigated plots, respectively. Plants were sampled on 5 November (1992), 14 January, 16 February, 1 April, 25 May, 23 June and 24 August (1993). No visible symptoms of O(3) damage or premature senescence were observed at any time over the course of the experiment. Exposure to elevated O(3) decreased the above ground biomass by reducing plant density and individual plant relative growth rate. However, there was no significant influence of the pollutant on the growth of the root relative to the shoot. Assessment of yield characteristics at the final harvest revealed an O(3)-induced decrease in the number of grains per ear, as a result of fewer grains per spikelet and an increase in the number of infertile florets per spikelet. No significant effects of the pollutant on the number of ears per plant, spikelets per ear, or 1000 grain weight were found. As a result of the combined effects on the number of grains per ear and the decrease in plant density and growth rate, O(3) exposure reduced grain and straw yields (tonnes ha(-1)) by 13 and 8%, respectively. However, no significant change in the partitioning of dry matter between the grain and the straw was observed in fumigated plots. The findings are discussed within the context of United Nation Economic Commission for Europe critical level guidelines for the protection of crop yields, in relation to their application to winter-sown crops.     

Effects of nitrogen dioxide on growth and yield of black turtle bean (Phaseolus vulgaris L.) cv. 'Domino'

Twenty-six-day-old black turtle bean cv. 'Domino' plants were exposed to nitrogen dioxide (0.0, 0.025, 0.05 and 0.10 microl liter(-1)), 7 h per day for 5 days per week for 3 weeks, under controlled environment. Data were collected on net photosynthesis rate (PN), stomatal resistance (SR), and dark respiration rate (DR), immediately after exposure, 24 h after the termination of exposure and at maturity (when the leaves had just started turning yellow), using a LICOR 6000 Portable Photosynthesis System. Chlorophyll-a (Ch-a), chlorophyll-b (Ch-b), total chlorophyll (tot-Ch) and leaf nitrogen were measured immediately after exposure and at maturity. Growth characteristics-relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR) and root: shoot ratio (RSR)-were computed for treated plants. Net photosynthesis rate increased by 53% in 0.10 microl liter(-1) NO2 treated plants immediately after exposure compared to control plants. Dark respiration rates were also higher in treated plants. Ch-a, Ch-b and tot-Ch showed significant increases with 0.1 microl liter(-1) NO2 treatment immediately after exposure. Foliar nitrogen content showed an increase in treated plants both immediately after exposure and at maturity. Increases were also seen in RGR and NAR. Plant yield increased by 86% (number of pods), 29% (number of seeds) and 46% (weight of seeds), respectively. Nitrogen dioxide stimulated the overall plant growth and crop yield.     

Effects of ozone on managed pasture: II. Yield, species composition, canopy structure, and forage quality

Managed pasture composed of grasses, clover and weeds was exposed in open-top chambers to different levels of ozone (O(3)) during two consecutive seasons to study changes in yield, species composition, canopy structure, and forage quality. The pasture was established in 1990 and exposed in 1991 and 1992. Ozone treatments included charcoal-filtered air (CF), non-filtered air (NF), and two treatments with O(3) added to NF air during periods with global radiation >/= 400 W m(-2) (NF(+), NF(++)). The ratio between the 2-year cumulative, radiation-weighted O(3) concentration in ambient air (= 365 microl litre(-1) h) and in the different treatments was 0.50 (CF), 0.85 (NF), 1.11 (NF(+)), and 1.64 (NF(++)). Plots were harvested four times in 1991, and five times in 1992. The total forage yield for both seasons was modified little by O(3). The yield reduction in NF(++) was only 10% as compared to the CF treatment. Also, only marginal changes were observed in forage quality (Ca, crude protein, crude fibre), and in leaf area index and fractional light penetration. Ozone strongly reduced the yield of clover (Trifolium repens L. and Trifolium pratense L.). The O(3)-effect on clover growth was small after the first harvest and increased with each growth period. In NF, the 2-year cumulative clover yield was reduced by 24% relative to CF. In NF(++), clover growth almost ceased near the end of the second season. The reduction in clover yield with increasing O(3) was associated with a slight increase in the yield of grasses (mainly Dacytlis glomerata L). The increase in the proportion of invading species (weeds or herbs) (Taraxacum officinale L.) during the experiment was not significantly affected by O(3). A second order polynomial function was fitted to the data to establish an exposure-response model for the cumulative clover yield and the cumulative, radiation-weighted O(3) dose, and linear models were developed for total forage mass, grass yield and yield of weeds. Reducing O(3) from elevated levels (NF(+) and NF(++)) during the first season to near-ambient levels (NF) during the second season resulted in a significant recovery of clover yield after two re-growth periods. It is concluded that continuous exposure to ambient levels of O(3) negatively affects the yield of clover in frequently cut, managed pasture, but because of the relatively small proportion of clover, the shift in species composition only marginally affects total forage yield and forage quality. It is emphasised, however, that limitations of the experimental system must be taken into account before extrapolations to real field situations can be made.     

Interactive effects of elevated ozone and carbon dioxide on growth and yield of leaf rust-infected versus non-infected wheat

Spring wheat (Triticum aestivum L. cv. Turbo) was grown from seedling emergence to maturity (129 days) in chambers simulating the physical climate and ozone pollution of a field site in Northern Germany from 1 April to 31 July with a mean 1-h daily maximum of 61.5-62.4 nl l(-1) ozone compared to a constant low level of 21.5-22.8 nl l(-1) ozone. The two ozone levels were combined with either a current (374.1-380.2 microl l(-1)) or enriched (610.6-615.0 microl l(-1)) CO(2) atmosphere. Additionally, a leaf rust epidemic (Puccinia recondita f. sp. tritici) was induced at tillering stage by repeated re-inoculations with the inoculum formed on the plants. Leaf rust disease was strongly inhibited by ozone, but largely unaffected by elevated CO(2). Ozone damage on leaves was strongly affected by CO(2) and infection. On infected plants, ozone lesions appeared 2-4 weeks earlier and were up to fourfold more severe compared to non-infected plants. Elevated CO(2) did not delay the onset of ozone lesions but it significantly reduced the severity of leaf damage. It also enhanced the photosynthetic rate of flag leaves and increased the water use efficiency, biomass formation and grain yield. The relative increases in growth and yield induced by CO(2) were much larger on ozone-stressed than on non-stressed plants. Both ozone and fungal infection reduced biomass formation, number of grains per plant, thousand grain weight and grain yield; however, adverse effects of leaf rust infection were more severe. Elevated CO(2) largely equalized the negative effects of ozone on the photosynthetic rate, growth and yield parameters, but was not capable of compensating for the detrimental effects of fungal infection. The data imply that the impact of ozone in the field cannot be estimated without considering the predisposing effects deriving from fungal infections and the compensating effects deriving from elevated CO(2).     

Effects of nitrogen dioxide on biochemical and physiological characteristics of soybean

One month old soybean (Glycine max (L.) Merrill) cv. 'Williams' plants were exposed to nitrogen dioxide (NO2 at 0.1, 0.2, 0.3, and 0.5 microl liter(-1) and carbon filtered air (control), 7 h per day for five days, under controlled environment. Data were collected on net photosynthetic rate (PN), stomatal resistance (SR), and dark respiration rate (DR), immediately following the fifth day of exposure and 24 h after termination of exposure. Chlorophyll a (Ch a), chlorophyll b (Ch b), total chlorophyll (tot Ch) and foliar nitrogen (N) were measured before and after exposures. Growth characteristics--relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR), and root shoot ratio (RSR) -- were computed for treated plants using standard growth equations. Increases of 18% and 23% in PN were observed immediately following exposure to 0.2 microl liter(-1) NO2 and after 24 h recovery period, respectively. With 0.5 microl liter(-1) NO2 treatment, reductions in PN of 23% and 50% were observed, immediately after exposure and following 24 h recovery, respectively. DR rates with 0.2 l liter(-1) treatment were higher than the control. Chlorophyll a and tot Ch showed significant reduction with 0.5 microl liter(-1) NO2 treatment. The percent reduction in Ch a and tot Ch with 0.5 microl liter(-1) NO2 were 45% and 47%, respectively. Increases in foliar nitrogen content after 0.2 and 0.3 microl liter(-1) NO2 treatments were 46% and 69%, respectively. Nitrogen dioxide at 0.5 microl liter(-1) reduced RGR and NAR by 47% and 51%, respectively. Leaf area ratio was 42% higher in 0.5 microl liter(-)1 NO2 treated plants, compared with the control; this increase was insufficient to compensate for the decrease in NAR resulting in a net decline in RGR. Nitrogen dioxide up to 0.2 microl liter(-1) increased PN and foliar-N content of soybean. With 0.5 microl liter(-1) NO2, significant decreases were observed in PN, leaf chlorophyll, foliar-N, NAR and RGR. Nitrogen dioxide up to 0.2 microl liter(-)1 has a favorable influence on overall growth characteristics of soybean; however, inhibitory effects were seen with NO2 treatment at 0.5 microl liter(-1).     

Effects of elevated CO2, nitrogen supply and tropospheric ozone on spring wheat-II. Nutrients (N, P, K, S, Ca, Mg, Fe, Mn, Zn)

CO(2) enrichment is expected to alter leaf demand for nitrogen and phosphorus in plant species with C(3) carbon dioxide fixation pathway, thus possibly causing nutrient imbalances in the tissues and disturbance of distribution and redistribution patterns within the plants. To test the influence of CO(2) enrichment and elevated tropospheric ozone in combination with different nitrogen supply, spring wheat (Tritium aestivum L. cv. Minaret) was exposed to three levels of CO(2) (361, 523, and 639 microl litre(-1), 24 h mean from sowing to final harvest), two levels of ozone (28.4 and 51.3 nl litre(-1)) and two levels of nitrogen supply (150 and 270 kg ha(-1)) in a full-factorial design in open-top field chambers. Additional fertilization experiments (120, 210, and 330 kg N ha(-1)) were carried out at low and high CO(2) levels. Macronutrients (N, P, K, S, Ca, Mg) and three micronutrients (Mn, Fe, Zn) were analysed in samples obtained at three different developmental stages: beginning of shoot elongation, anthesis, and ripening. At each harvest, plant samples were separated into different organs (green and senescent leaves, stem sections, ears, grains). According to analyses of tissue concentrations at the beginning of shoot elongation, the plants were sufficiently equipped with nutrients. Elevated ozone levels neither affected tissue concentrations nor shoot uptake of the nutrients. CO(2) and nitrogen treatments affected nutrient uptake, distribution and redistribution in a complex manner. CO(2) enrichment increased nitrogen-use efficiency and caused a lower demand for nitrogen in green tissues which was reflected in a decrease of critical nitrogen concentrations, lower leaf nitrogen concentrations and lower nitrogen pools in the leaves. Since grain nitrogen uptake during grain filling depended completely on redistribution from vegetative pools in green tissues, grain nitrogen concentrations fell considerably with severe implications for grain quality. Ca, S, Mg and Zn in green tissues were influenced by CO(2) enrichment in a similar manner to nitrogen. Phosphorus concentrations in green tissues, on the other hand, were not, or only slightly, affected by elevated CO(2). In stems, 'dilution' of all nutrients except manganese was observed, caused by the huge accumulation of water soluble carbohydrates, mainly fructans, in these tissues under CO(2) enrichment. Whole shoot uptake was either remarkably increased (K, Mn, P, Mg), nearly unaffected (N, S, Fe, Zn) or decreased (Ca) under CO(2) enrichment. Thus, nutrient cycling in plant-soil systems is expected to be altered under CO(2) enrichment.     

Growth and yield responses of spring wheat (Triticum aestivum L. cv. Minaret) to elevated CO2 and water limitation

Spring wheat (Triticum aestivum L. cv. Minaret) was grown at two different CO2 concentrations (367 and 650 micromol mol(-1)) in open-top-chambers from sowing until final harvest. Furthermore two different watering treatments (well watered and water stressed) and two soil types of different fertility were used. At final harvest, which took place at growth stage 92, plants were separated into different fractions. Elevated atmospheric CO2 caused an accelerated chlorophyll-a breakdown and increased growth and yield. Total shoot biomass was enhanced by 43%, grain yield by 46% and main stem yield by 19%. Water stress also accelerated chlorophyll-a breakdown but reduced total shoot biomass by 40%, grain yield by 45%, main stem yield by 30% and thousand grain weight by 6%. On average, soil fertility altered shoot biomass by 30%, grain yield by 39% and main stem yield by 25%.     

Effect of carbon dioxide concentration on biomass production and partitioning in Betula pubescens Ehrh. seedlings at different ozone and temperature regimes

Seedlings of Betula pubescens were grown at two CO(2) concentrations, in combination with either two O(3) concentrations or two air temperatures, during 34-35 days at 24 h day(-1) photoperiod in growth chambers placed in a greenhouse. Increasing the CO(2) concentration from 350 to 560 micromol mol(-1) at 17 degrees C air temperature increased the dry weight of the main leaves, main stem, branches and root. The mean relative growth rate (RGR) was increased 10% by CO(2) enrichment, while increasing the O(3) concentration from 7 to 62 nmol mol(-1) decreased the RGR by 9%. The relative biomass distribution between the different plant components was not significantly affected by the CO(2) concentration irrespective of the O(3) concentration. No significant interactions between CO(2) and O(3) concentration were found except on leaf size, which was stimulated more by elevated CO(2) concentration at high, compared to low, O(3) levels. In another experiment, elevated CO(2) (700 micromol mol(-1)) significantly increased the dry weight of the different plant components, and more at 20 degrees C than at 15 degrees C. Raising the CO(2) concentration increased the RGR by 5 and 10% at 15 and 20 degrees C, respectively. CO(2) enrichment increased the branch dry weight relatively more than the dry weight of the other plant parts. Increasing the CO(2) concentration or temperature increased the plant height and stem diameter, however, no interactions between CO(2) and temperature were found.     

Growth responses of Populus tremuloides clones to interacting elevated carbon dioxide and tropospheric ozone.

The Intergovernmental Panel of Climate Change (IPCC) has concluded that the greenhouse gases carbon dioxide (CO2) and tropospheric ozone (O3) are increasing concomitantly globally. Little is known about the effect of these interacting gases on growth, survival, and productivity of forest ecosystems. In this study we assess the effects of three successive years of exposure to combinations of elevated CO2 and O3 on growth responses in a five trembling aspen (Populus tremuloides) clonal mixture in a regenerating stand. The experiment is located in Rhinelander, Wisconsin, USA (45 degrees N 89 degrees W) and employs free air carbon dioxide and ozone enrichment (FACE) technology. The aspen stand was exposed to a factorial combination of four treatments consisting of elevated CO2 (560 ppm), elevated O3 (episodic exposure-90 microl l(-1) hour(-1)), a combination of elevated CO2 and O3, and ambient control in 30 m treatment rings with three replications. Our overall results showed that our three growth parameters including height, diameter and volume were increased by elevated CO2, decreased by elevated O3, and were not significantly different from the ambient control under elevated CO2 + O3. However, there were significant clonal differences in the responses; all five clones exhibited increased growth with elevated CO2, one clone showed an increase with elevated O3, and two clones showed an increase over the control with elevated CO2 + O3, two clones showed a decrease, and one was not significantly different from the control. Notably. there was a significant increase in current terminal shoot dieback with elevated CO2 during the 1999-2000 dormant season. Dieback was especially prominent in two of the five clones, and was attributed to those clones growing longer into the autumnal season where they were subject to frost. Our results show that elevated O3 negates expected positive growth effects of elevated CO2 in Populus tremuloides in the field, and suggest that future climate model predictions should take into account the offsetting effects of elevated O3 on CO2 enrichment when estimating future growth of trembling aspen stands.     

Effects of elevated carbon dioxide and ozone on the growth and yield of potatoes (Solanum tuberosum) grown in open-top chambers

Potato (Solanum tuberosum cv. Bintje) was grown in open-top chambers under three carbon dioxide (ambient and seasonal mean concentrations of 550 and 680 mumol mol-1 CO2) and two ozone concentrations (ambient and an 8 h day-1 seasonal mean of 50 nmol mol-1 O3) between emergence and final harvest. Periodic non-destructive measurements were made and destructive harvests were carried out at three key developmental stages (24, 49 and 101 days after emergence) to establish effects on growth and tuber yield. Season-long exposure to elevated O3 reduced above-ground dry weight at final harvest by 8.4% (P < 0.05), but did not affect tuber yields. There was no significant interaction between CO2 and O3 for any of the growth and yield variables examined. Non-destructive analyses revealed no significant effect of elevated CO2 on plant height, leaf number or green leaf area ratio. However, destructive harvests at tuber initiation and 500 degrees Cd after emergence showed that above-ground dry weight (8 and 7% respectively) and tuber yield (88 and 44%) were significantly increased (P < 0.05) in the 550 mumol mol-1 CO2 treatment. Responses to 550 and 680 mumol mol-1 CO2 were not significantly different for most parameters examined, suggesting the existence of an upper limit to the beneficial influence of CO2 enrichment. Significant effects on above-ground dry weight and tuber yield were no longer apparent at final harvest, although tuber numbers were increased (P < 0.05) under elevated CO2, particularly in the smaller size categories. The results show that the O3 treatment imposed was insufficient to reduce tuber yields and that, although elevated CO2 enhanced crop growth during the early stages of the season, this beneficial effect was not sustained to maturity.     

Modeling the impact of ozone x drought interactions on regional crop yields

The influence of soil moisture stress on crop sensitivity to O3 was evaluated for corn (Zea mays L.), cotton (Gossypium hirsutum L.), soybean (Glycine max L. Merr.), and wheat (Triticum aestivum L.) grown in the United States. This assessment was accomplished by using yield forecasting models to estimate the influence of soil moisture deficits on regional yield and a previously developed model to predict moisture stress x O3 interactions. Reduced crop sensitivity to O3 was predicted for those regions and years for which soil moisture stress reduced yield. The models predicted a drought-induced reduction in crop sensitivity to O3 of approximately 20% for the 1979 to 1983 period; i.e. a hypothetical O3-induced yield reduction of 5% for adequately watered crops would have been reduced to a 4% effect by the 1979 to 1983 distribution of soil moisture deficits. However, predicted drought effects varied between crops, regions, and years. Uncertainties in the model predictions are also discussed.     

Interactive effects of ozone and elevated carbon dioxide on the growth and physiology of black cherry, green ash, and yellow-poplar seedlings

Potted seedlings of black cherry (Prunus serotina Ehrh.) (BC), green ash (Fraxinus pennsylvanica Marsh.) (GA), and yellow-poplar (Liriodendron tulipifera L.) (YP) were exposed to one of the four treatments: (1) charcoal-filtered air (CF) at ambient CO(2) (control); (2) twice ambient O(3) (2 x O(3)); (3) twice ambient CO(2) (650 microl l(-1)) plus CF air (2 x CO(2)); or (4) twice ambient CO(2) (650 microl l(-1)) plus twice ambient O(3) (2 x CO(2) + 2 x O(3)). The treatments were duplicated in eight continuously stirred tank reactors for 10 weeks. Gas exchange was measured during the last 3 weeks of treatment and all seedlings were destructively harvested after 10 weeks. Significant interactive effects of O(3) and CO(2) on the gas exchange of all three species were limited. The effects of elevated CO(2) and O(3), singly and combined, on light-saturated net photosynthesis (A(max)) and stomatal conductance (g(s)) were inconsistent across species. In all three species, elevated O(3) had no effect on g(s). Elevated CO(2) significantly increased A(max) in GA and YP foliage, and decreased g(s) in YP foliage. Maximum carbon exchange rates and quantum efficiencies derived from light-response curves increased, while compensation irradiance and dark respiration decreased in all three species when exposed to 2 x CO(2). Elevated O(3) affected few of these parameters but any change that was observed was opposite to that from exposure to 2 x CO(2)-air. Interactive effects of CO(2) and O(3) on light-response parameters were limited. Carboxylation efficiencies, derived from CO(2)-response curves (A/C(i) curves) decreased only in YP foliage exposed to 2 x CO(2)-air. In general, growth was significantly stimulated by 2 x CO(2) in all three species; though there were few significant growth responses following exposure to 2 x O(3) or the combination of 2 x CO(2) plus 2 x O(3). Results indicate that responses to interacting stressors such as O(3) and CO(2) are species specific.     

Growth and yield responses of spring wheat (Triticum aestivum L. CV. Turbo) grown in open-top chambers to ozone and water stress

Spring wheat (Triticum aestivum L.) cv. Turbo was exposed to different levels of ozone and water supply in open-top chambers in 1991. The plants were grown either in charcoal filtered air (CF), not filtered air (NF), in charcoal filtered air with proportional addition of ambient ozone (CF1), or in charcoal filtered air with twice proportional addition of ambient ozone (CF2). The mean seasonal ozone concentrations (24 h mean) were 2.3, 20.6, 17.3, and 24.5 nl litre(-1) for CF, NF, CF1, and CF2 treatments, respectively. Ozone enhanced senescence and reduced growth and yield of the wheat plants. At final harvest, dry weight reductions were mainly due to reductions in ear weight. Grain yield loss by ozone mainly resulted from depressions of 1000 grain weight, whereas numbers of ears per plant and of grains per ear remained unchanged. Pollutants other than ozone did not alter the response to ozone, as was obvious from comparisons between CF1 and NF responses. Water stress alone did not enhance senescence, but also reduced growth and yield. However, yield loss mainly resulted from reductions in the number of ears per plant; 1000 grain weight was not influenced by water stress. No water supply by ozone treatment interactions were detected for any of the estimated parameters.     

Photosynthetic responses to temperature, light flux-density, CO2 concentration and vapour pressure deficit in Eucalyptus tetrodonta grown under CO2 enrichment

Seeds of Eucalyptus tetrodonta were sown under ambient or CO(2) enriched (700 microl litre(-1)) conditions in tropical Australia. Four sets of measurements were made, the first two after 12 months, on trees growing either in pots or planted in the ground. The third and fourth set were made after 18 and 30 months exposure to CO(2) enrichment, on trees growing in the ground. After 12 months exposure to CO(2) enrichment, the rate of light-saturated assimilation (Amax) of plants growing in the ground was determined. Responses of CO(2) assimilation to variations in leaf temperature, leaf-to-air vapour pressure deficit (LAVPD), light flux density and CO(2) concentration were also measured in the laboratory using plants growing in large pots. There was no significant difference in Amax between pot and ground located plants. Assimilation of E. tetrodonta was relatively insensitive to changes in LAVPD for both ambient and CO(2) enriched plants but the temperature optimum of assimilation was increased in plants grown and measured under CO(2) enrichment. Plants grown with CO(2) enrichment had an increased rate of light-saturated assimilation and apparent quantum yield was significantly increased by CO(2) enrichment. In contrast, carboxylation efficiency was decreased significantly by CO(2) enrichment. After 18 months growth with CO(2) enrichment, there was no sign of a decline in assimilation rate compared to measurements undertaken after 12 months. At low LAVPD values, assimilation rate was not influenced by CO(2) treatment but at moderate to high LAVPD, plants grown under CO(2) enrichment exhibited a larger assimilation rate than control plants. Specific leaf area and chlorophyll contents decreased in response to CO(2) enrichment, whilst foliar soluble protein contents and chlorophyll a/b ratios were unaffected by CO(2) treatment. Changes in soluble protein and chlorophyll contents in response to CO(2) enrichment did not account for changes in assimilation between treatments. After 30 months exposure to CO(2) enrichment, the rate of light-saturated assimilation was approximately 50% larger than controls and this enhancement was larger than that observed after 18 months exposure to CO(2) enrichment.     

Effects of ozone on 14C translocation velocity and growth of spring wheat (Triticum aestivum L.) exposed in open-top chambers

Growth and yield were reduced but (14)C translocation velocity was not affected by increasing levels of ozone in spring wheat exposed in open top chambers to the following treatments: charcoal filtered air (CF), non-filtered ambient air (NF), or NF with addition of 30 microl litre(-1) ozone, 8 h daily (NFO). Destructive harvests were performed at anthesis and at maturity. Parts of the flag leaf or the second leaf were exposed to (14)CO(2) in small cuvettes for 5 min before, during and after anthesis. The translocation velocity was followed by autoradiography and scintillation counting of the plants frozen and lyophilized at different times after labelling. The label was transported at the same velocity in all the treatments. Ozone induced changes in carbon allocation or partitioning should probably be explained as amounts of carbon transported (mg s(-1)), rather than as transportation velocity (mm s(-1)). The amount translocated may be governed by source conditions under O(3) stress: reduced healthy green biomass and photosynthesis, but perhaps also by impairment of phloem loading because of membrane damage.