Journal of Experimental Botany, Vol. 52, No. 362, pp. 1901-1911,
September 1, 2001
© 2001 Oxford University Press
Original Papers |
Does nitrogen supply affect the response of wheat (Triticum aestivum cv. Hanno) to the combination of elevated CO2 and O3?
Air Pollution Laboratory, Department of Agricultural and Environmental Science, Ridley Building, The University of Newcastle, Newcastle Upon Tyne NE1 7RU, UK
Received 15 November 2000; Accepted 30 May 2001
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Abstract |
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Spring wheat (Triticum aestivum cv. Hanno) was grown at ambient (350 µmol mol-1) or elevated CO2 (700 µmol mol-1) in charcoal/Purafil®-filtered air (CFA <5 nmol mol-1) or ozone (CFA +75 nmol mol-1 7 h d-1) at three levels of N supply (1.5, 4 and 14 mM NO-3), to test the hypothesis that the combined impacts of elevated CO2 and O3 on plant growth and photosynthetic capacity are affected by nitrogen availability. Shifts in foliar N content reflected the level of N supplied, and the growth stimulation induced by elevated CO2 was dependent on the level of N supply. At 60 d after transfer (DAT), elevated CO2 was found to increase total biomass by 44%, 29%, 12% in plants supplied with 14, 4 and 1.5 mM NO-3, respectively, and there was no evidence of photosynthetic acclimation to elevated CO2 across N treatments; the maximum in vivo rate of Rubisco carboxylation (Vcmax) was similar in plants raised at elevated and ambient CO2. At 60 DAT, ozone exposure was found to suppress plant relative growth rate (RGR) and net photosynthesis (A) in plants supplied with 14 and 4 mM NO-3. However, O3 had no effect on the RGR of plants supplied with 1.5 mM NO-3 and this effect was accompanied by a reduced impact of the pollutant on A. Elevated CO2 counteracted the detrimental effects of O3 (i.e. the same ozone concentration that depressed RGR and A at ambient CO2 resulted in no significant effects when plants were raised at elevated CO2) at all levels of N supply and the effect was associated with a decline in O3 uptake at the leaf level.
Key words: Elevated CO2, tropospheric ozone, N, net photosynthesis, growth.
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Introduction |
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Ground-level concentrations of CO2 and O3 continue to rise as a direct consequence of anthropogenic activities (Watson et al., 1990
). Over the same time-scale that the tropospheric CO2 concentration has risen from 
280 µmol mol-1 to 356 µmol mol-1, average background levels of O3 have approximately doubled (from 10–15 to 25–30 nmol mol-1) and UN-ECE critical levels (exposure limits) for the protection of vegetation are now regularly exceeded during the summer months in many parts of the developed world (Watson et al., 1990; Stockwell et al., 1997). Even if CO2 emissions are successfully restricted to the extent that the present rate of rise is sustained, atmospheric CO2 concentrations will have risen by a further 80% by the end of the 21st century (Wigley et al., 1997). Since ground-level O3 concentrations are also expected to continue to rise for the forseeable future (Chameides et al., 1994), it is important to understand how these concomitant changes in atmospheric composition are likely to affect terrestrial vegetation in order to expedite the necessary revision of plant growth simulation models designed to assess the impacts of atmospheric change on crop growth and productivity. The potential significance of considering the way in which rising atmospheric CO2 concentrations will influence plant responses to O3, and vice versa, are emphasized by calculations by Hertstein et al. (based on data for spring wheat, Triticum aestivum L. cv. Turbo) that cereal yield increases arising from a business-as-usual scenario for CO2 could be 20% less than predicted if simultaneous increases in O3 are taken into account (Hertstein et al., 1995).
Individually, elevated CO2 and O3 have contrasting effects on plant growth and physiology. Rising atmospheric CO2 concentrations are expected to enhance the growth and productivity of C3 plants, largely through stimulated rates of net photosynthesis (Cure and Acock, 1986; Drake et al., 1997). By contrast, exposure to O3 reduces growth and reproductive development in sensitive taxa, causes shifts in root/shoot carbon partitioning, depresses rates of net photosynthesis, and accelerates leaf senescence (Reich and Amundson, 1985; Cooley and Manning, 1987; Runeckles and Chevone, 1992; Pell et al., 1999).
Recent studies indicate that the decline in growth and photosynthesis observed in plants fumigated with O3 is often reduced when plants are raised at elevated CO2 (Barnes and Wellburn, 1998; Turcsányi et al., 2000). However, the existing database is inconsistent and several studies have shown that elevated CO2 did not ameliorate the detrimental effects of O3 (Barnes et al., 1995; Karnosky et al., 1998; Shimizu et al., 1997). The response of vegetation to the combination of rising CO2 and O3 is likely to be affected by N status (Cowling and Koziol, 1982; Paul and Driscoll, 1997; Stitt and Krapp, 1999), but it is not known to what extent differences in N availability may explain the contradictory responses observed in the literature. Several authors report plant growth and yield to be less responsive to elevated CO2 under conditions of limited N availability (Evans, 1989; Mitchell et al., 1993; McKee and Woodward, 1994; Rogers et al., 1996; Demmers-Derks et al., 1998; Lutze and Gifford, 1998) and photosynthetic acclimation to elevated CO2 has been reported to be accentuated in N-limited plants (Curtis, 1996; Drake et al., 1997); declines in Rubisco content and activity having been found to be greater in N-deficient plants than in plants raised with an optimal N supply (Nie et al., 1993; Miglietta et al., 1996; Rogers et al., 1996; Harmens et al., 2000). Other studies have reported the impacts of elevated CO2 to be independent of N status (Wong, 1979; Hocking and Meyer, 1991).
There is scant information on the effects of N supply on plant responses to O3, as the majority of studies have focused on the impacts of overall nutrient limitation (Maurer et al., 1997; Whitfield et al., 1998). However, those few studies that have been conducted indicate that plants exhibiting low-N content may be less susceptible to O3. For example, Brewer et al. reported a reduction in O3-induced visible injury when spinach (Spinacia oleraceae L.) and mangels (Beta vulgaris L.) were grown under a low-N regime, in comparison to plants supplied with a high level of N (Brewer et al., 1961). Similarly, O3-induced reductions in biomass were observed in radish (Raphanus sativus L.) (Pell et al., 1990) and loblolly pine (Pinus taeda L.) (Tjoelker and Luxmoore, 1991) at high, but not low, N supply.
Although several findings show the impacts of elevated CO2 or O3 to be modulated by N availability, little attention has been paid to the way in which N nutrition influences the combined effects of these gases on plant growth and physiology. The primary aim of this study was, therefore, to determine whether N availability affects the degree to which elevated CO2 counteracts the detrimental effects of O3. Secondary aims included the examination of whether acclimation to elevated CO2 would be accentuated at low N supply and whether N availability would affect plant response to O3. The study employed spring wheat (Triticum aestivum L. cv. Hanno) as a convenient model to study the combined effects of elevated CO2 and/or O3 on plant growth and physiology.
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Materials and methods |
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Plant culture and fumigation
Seed of spring wheat (Triticum aestivum L. cv. Hanno) was sown in seed trays containing vermiculite and germinated in a controlled environment chamber supplied with charcoal/Purafil®-filtered air (CFA). Six days after germination, seedlings at the same developmental stage (first leaf) were transplanted to 2.28 dm3 pots containing acid-washed sand and transferred to eight controlled environment chambers which form part of a fumigation system described in detail elsewhere (Zheng et al., 1998
). Chambers were illuminated by metal-halide lamps (Siemens HRH fittings plus 400 W HQI-T lamps supplied by Osram, St Helens, Merseyside, UK) providing a photosynthetic photon flux density (PPFD) of 
250 µmol m-2 s-1 at plant canopy height with a 14 h photoperiod. Diurnal temperature fluctuations ranged sinusoidally (tracking external changes in air temperature) from a night-time minimum of 15±0.2 °C to a day-time maximum of 23±0.3 °C. Air temperature in each chamber was logged continually (Delta-T Devices Ltd, Cambridge, UK). Relative humidity was maintained at 65±5%. A factorial experimental design was employed to achieve four duplicated treatments: ambient CO2 (346±3 µmol mol-1 CO2+CFA <5 nmol mol-1 O3); O3 (ambient CO2+75±1 nmol mol-1 O3 7 h d-1 (10.00–17.00 h)); elevated CO2 (702±7 µmol mol-1 CO2+CFA) and elevated CO2+O3 (elevated CO2+78±1 nmol mol-1 O3 7 h d-1). Gas concentrations were monitored, logged and adjusted using a computer-controlled feedback system described elsewhere (Barnes et al., 1995). Gas concentrations and air flows in duplicate chambers were monitored daily to ensure uniformity between replicate chambers. Ozone was generated from particulate/charcoal/Purafil®-filtered air by electric-discharge (Wallace and Tiernan BA.023 Laboratory Ozonator, Tonbridge, Kent, UK), while compressed CO2 was supplied from cylinders (Distillers MG, Reigate, Surrey, UK). Both gases were filtered (sensu Brown and Roberts, 1988) to remove potential contaminants prior to injection into the air stream entering individual chambers (Barnes et al., 1995). Plants were watered daily with a nutrient solution comprising 2 mM K2HPO4, 0.8 mM MgSO4, 70 µM Fe-EDTA, 46 µM H3BO3, 9 µM MnSO4, 1.6 µM CuSO4, 1.5 µM ZnSO4, 0.1 µM (NH4)6 Mo7O24. Three nitrogen treatments were imposed using Ca(NO3)2 as the source of NO-3: 14 mM NO-3, 4 mM NO-3, 1.5 mM NO-3. The change in calcium concentration was corrected using CaCl2. Pots were regularly flushed with deionized water to prevent accumulation of excess nutrient.
Growth and dry matter partitioning
Twenty plants were harvested upon initial transfer to the respective treatments. Subsequent harvests comprised five independent plants per chamber. These were made 32 d and 60 d after transfer (DAT) to the treatments. At each harvest, tiller number was counted, then plants were separated into root and shoot and washed prior to drying to constant weight in an oven at 70 °C. Plant relative growth rate (RGR), the RGR of component plant parts (RGR shoot=RGRS, RGR root=RGRR) and the allometric root : shoot coefficient (K=RGRR/RGRS) were calculated as described previously (Hunt, 1990). At the final harvest, dead leaf material was separated from the rest of the shoot.
Carbon and nitrogen analysis
At 32 and 60 DAT dried shoot material was ground and analysed for carbon (C) and N content using a RoboPrep-TCD C and N Element Analyser (Model 7002, Europa Scientific Ltd, Crewe, UK).
Gas exchange measurements
Leaves 4 and 7 on the main shoot were labelled upon emergence (15 and 33 DAT, respectively) and in situ rates of CO2/H2O exchange measured over the life-span of both leaves using a standard Parkinson leaf cuvette (model PCL-B, PP Systems, Hitchin, UK) linked to portable infrared gas analyser (Ciras-II, PP Systems, Hitchin, UK). Regular measurements were made at the growth CO2 concentration under chamber conditions (PPFD=182±3 µmol m-2 s-1 at the position occupied by the leaf in the cuvette; leaf temperature=23.5±0.1 °C; saturated vapour pressure deficit (SVPD)=0.91±0.01 kPa). When leaves reached full expansion (12 d after leaf emergence (DALE)), the response of CO2 assimilation to changes in the intercellular CO2 concentration (ci) was determined at cuvette CO2 concentrations of 100, 150, 200, 250, 350, and 1000 µmol mol-1. Measurements were made using an automated Parkinson leaf cuvette (model auto-PLC-B, PP Systems) employing a leaf temperature of 22.8±0.1 °C under a PPFD (1000±1 µmol m-2 s-1) shown by prior experimentation to be high enough to achieve the light-saturated rate of CO2 assimilation. Stomatal conductance to water vapour (gH2O), the rate of CO2 assimilation under growth conditions (A), the light-saturated rate of CO2 assimilation measured at 350 µmol CO2 mol-1 (A350) and the light- and CO2-saturated rate of CO2 assimilation (Amax) were calculated (von Caemmerer and Farquhar, 1981). The maximum in vivo rate of Rubisco carboxylation (Vcmax) was calculated from the linear portion of the measured A/ci response as described previously (Harley et al., 1992). Values for Kc (Michaelis constant for Rubisco carboxylation), Ko (Michaelis constant for Rubisco oxygenation), and 
(Rubisco specificity factor) (measured by Jordan and Ogren, 1984) were employed in calculations, correcting for temperature dependencies (according to Harley et al., 1992).
Leaf boundary layer resistance under chamber conditions was measured using the wet-blotting paper method described earlier (Unsworth et al., 1984). Ozone uptake at the leaf-level was calculated by analogy with Fick's first law for gas diffusion from measurements of leaf boundary layer conductance and gH2O, multiplying by 0.612 (Nobel, 1983) to account for the slower rate of molecular diffusion of O3 in air compared to H2O. The intercellular O3 concentration was assumed to be zero (based on the measurements of Laisk et al., 1989).
Statistical analyses
Statistical analyses were performed using SPSS (SPSS Inc., Chicago, Illinois, USA). Data were first checked for normal distribution and homogeneity of variance, and log-transformed prior to analysis as required. Within each N level, treatment means were compared using Duncan's multiple range test calculated at the 5% level. Due to significant CO2xO3 interactions, the main effects of elevated CO2 and O3 were tested using an independent t-test (sensu Olszyk et al., 2000). Interactions between elevated CO2xO3xN were tested by MANOVA. There were no significant differences between data for replicate chambers, so analyses were performed on the basis that plants in replicate chambers were as likely to be as similar to, or as different from, plants within an individual chamber.
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Results |
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Plant growth and dry matter partitioning
N supply significantly (P
0.001) affected RGR (Table 1) and biomass accumulation (Fig. 1) at 32 and 60 DAT. The increase in plant biomass at higher levels of N supply was mainly due to an increase in surviving tiller numbers (Table 2). In addition, plants grown at 14 mM NO-3 exhibited a reduction in the partitioning of biomass to the root relative to the shoot (i.e. reduced K) in comparison to their counterparts supplied with 4 and 1.5 mM NO-3 (Table 1).
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, (346±3 µmol mol-1 CO2+CFA <5 nmol mol-1 O3); O3, 
, (ambient CO2+75±1 nmol mol-1 O3 7 h d-1 (10.00–17.00 h)); elevated CO2, 
, (702±7 µmol mol-1 CO2+CFA); elevated CO2+O3, •, (elevated CO2+78±1 nmol mol-1 O3 7 h d-1) and supplied with 1.5, 4 or 14 mM NO-3. Plants were harvested 32 and 60 d after transfer (DAT) to fumigation chambers. Values represent mean±SE (n=10).
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Plants raised at elevated CO2 exhibited significantly (P
0.05) higher initial RGRs (Table 1
) and this was reflected in greater biomass accumulation at 32 and 60 DAT in comparison with equivalent plants grown at ambient CO2 (Fig. 1). However, the increase in RGR induced by elevated CO2 was not sustained, so that between 32 and 60 DAT the RGR was actually lower (P0.05) than that of equivalent plants grown at ambient CO2 (Table 1). The impact of elevated CO2 was associated with an increase in surviving tiller numbers by 32 DAT (Table 2). However, by 60 DAT this effect was no longer significant. CO2-enrichment also increased K (Table 1), indicating a relatively greater investment in root growth relative to the shoot compared with plants raised at ambient CO2.
The extent of the increase in growth under elevated CO2 was significantly affected by the level of N supply after both 32 and 60 DAT (CO2xN, P0.001), with the increment in biomass by the final harvest (60 DAT) greatest in the CO2-enriched plants supplied with the highest level of N (44%) compared to those supplied with 4 (29%) or 1.5 mM NO-3 (12%) (Fig. 1). Elevated CO2 also accelerated leaf senescence, hence the increase in dead leaf tissue in plants supplied with 1.5 and 4 mM NO-3, but this effect was not observed at 14 mM NO-3 (Table 2).
Ozone exposure significantly reduced plant biomass irrespective of N supply at 32 DAT (Fig. 1). Between 32 and 60 DAT there was no significant effect of ozone on RGR (Table 1). By 60 DAT, ozone significantly (P0.05) reduced RGR and accumulated biomass in plants supplied with 14 and 4 mM NO3-, but not in those supplied with 1.5 mM NO3-. Ozone-exposed plants supplied with 1.5 and 4 mM NO3- exhibited a greater decline in root growth relative to that of the shoot (reduction in K at 32 DAT) (Table 1). However, between 32 and 60 DAT this trend was reversed, so that by 60 DAT no significant differences in K were observed in plants supplied with 1.5 and 4 mM NO3-, but a significant (P0.05) reduction occurred in plants supplied with 14 mM NO3-. Ozone exposure resulted in no change in tiller number, but increased the relative percentage of dead leaf tissue across N treatments by 60 DAT (Table 2).
In the combined treatment, elevated CO2 counteracted (CO2xO3, P0.01) the negative effects of O3 on plant growth and root : shoot partitioning, irrespective of N availability (Tables 1, 2; Fig. 1).
Shoot carbon and nitrogen content
There was no consistent effect of the different treatments and/or N supply on shoot C content. Shoot N content corresponded to the level of N supplied (Table 3) and declined with plant age across treatments. At both harvests, shoot N content tended to be lower for plants raised at elevated CO2 in comparison with their ambient CO2-grown counterparts. Shifts in the C/N ratio were mainly due to effects on N content.
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Stomatal conductance and O3 uptake
Figure 2 shows the effects of elevated CO2 and O3 on gH2O over the life-span of leaves 4 and 7 on the main shoot of plants grown at contrasting levels of N supply. Independently, elevated CO2 and O3 reduced gH2O in both leaves, with the magnitude of effect being independent of N supply. However, in the combined treatment, O3 exposure resulted in no greater decline in gH2O than that induced by elevated CO2 alone (i.e. effects of the gases on gH2O were less than additive).
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Figure 3
shows O3 uptake modelled over the life-span of leaves 4 and 7 from measurements of boundary layer conductance, gH2O and O3 concentrations. Data revealed that CO2-enrichment reduced cumulative O3 uptake (on average by 25%) over the leaf life-span in plants supplied with 4 and 14 mM NO-3. Effects were less marked in plants supplied with 1.5 mM NO-3.
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Net photosynthesis
Figure 4 shows the impacts of elevated CO2 and/or O3 on A over the leaf life-span for plants grown under contrasting N regimes. Plants raised and measured at elevated CO2 exhibited significantly (P0.05) higher, and sustained, rates of A than equivalent plants raised and measured at ambient CO2, irrespective of N supply. Impacts of O3 were more complex. In leaf 4, exposure of ambient CO2-grown plants resulted in a decline in A following attainment of full leaf expansion (12 DALE) in plants supplied with 14 and 4 mM NO-3, but not in those supplied with 1.5 mM NO-3 (O3xN, P0.01). In leaf 7, O3 reduced A across N treatments, but effects were restricted to the latter stages of the leaf life-span in plants receiving 4 and 1.5 mM NO-3. Under CO2-enriched conditions, O3 exposure resulted in no significant change in A and this was unaffected by N status (i.e. elevated CO2 counteracted the negative effects of O3 on A over the leaf life-span irrespective of N supply).
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Measurements of A350, Amax and estimates of Vcmax for leaves 4 and 7 at full expansion are shown in Fig. 5
. Overall, measured photosynthetic parameters were c. 20% lower in plants supplied with low (1.5 mM NO-3) compared with high N (14 mM NO-3). There was no evidence of photosynthetic acclimation at elevated CO2; A350, Amax and Vcmax were not significantly different between elevated and ambient CO2-grown plants. A/ci response measurements were performed at the attainment of full leaf expansion (12 DALE), when O3 effects on A were marginal (see Fig. 3). Consistent with these findings, O3 was found to have no significant impact on A350, Amax and Vcmax at this stage of leaf development.
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, (702±7 µmol mol-1 CO2+CFA); elevated CO2+O3, 
, (elevated CO2+78±1 nmol mol-1 O3 7 h d-1) and supplied with 1.5, 4 and 14 mM NO-3. Values represent mean±SE (n=6).
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Discussion |
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Doubling the atmospheric CO2 concentration resulted in a significant increase in plant biomass and, in agreement with several other studies (Poorter et al., 1988
; Wong, 1990; Rogers et al., 1996; Jitla et al., 1997), this effect was primarily due to the stimulation of plant RGR in the early stages of growth. Consistent with previous reports (Fangmeier et al., 1996; Rogers et al., 1996; Demmers-Derks et al., 1998; Lutze and Gifford, 1998), the relative impact of elevated CO2 on plant growth was dependent on N supply as the gain in biomass at elevated CO2 after 60 d ranged from 44% (14 mM NO-3) to 29% (4 mM NO-3) and 12% (1.5 mM NO-3). The finding that the extra tillering induced by elevated CO2 was reduced at lower levels of N supply is consistent with the reported effects of the constraint of ‘sink development’ on the growth stimulation induced by elevated CO2 (Rogers et al., 1996). Extra tillers in rice (Oryza sativa L.) and wheat have been shown to be important in maximizing the growth and yield stimulation obtained under CO2-enriched conditions (Conroy et al., 1994; Jitla et al., 1997).
No evidence of photosynthetic adjustment (i.e. down-regulation of photosynthesis) at elevated CO2 was found in the present study, irrespective of N supply. This finding is consistent with the lack of photosynthetic adjustment reported in plants exposed to elevated CO2 in the field or in pot-based studies under conditions of adequate nutrient supply (Habash et al., 1995; Mulholland et al., 1997; Garcia et al., 1998). The present findings contrast with those of previous authors who have reported decreases in Rubisco content and activity under sustained exposure to elevated CO2 in plants grown under N limitation—though differences in the timing of gas exchange measurements in relation to leaf development may be important (Nie et al., 1993; Miglietta et al., 1996; Rogers et al., 1996). In the present study, no reduction in leaf Rubisco activity or photosynthetic potential was detected at elevated CO2 in the two low N treatments (1.5 and 4 mM NO-3), despite a significant reduction in sink development in plants raised at elevated CO2. This finding suggests that plants raised at elevated CO2 may have readjusted their resources to support photosynthesis and growth in relation to N availability, in order to maintain an adequate balance between supply and demand. This is supported by the finding that between 32 and 60 DAT, the RGR of plants grown under CO2-enrichment was actually lower than that of their ambient CO2-grown counterparts. As a result, shoot N content was only marginally reduced at elevated CO2 (12%) in plants supplied with 1.5 mM NO-3. The importance of balancing supply and demand to circumvent the need for photosynthetic adjustment at elevated CO2 has been illustrated in hydroponically-grown wheat, supplied with N in proportion to plant growth (Farage et al., 1998). Moreover, findings are consistent with reports that the decline in Rubisco activity under CO2-enriched conditions is strongly correlated with reductions in leaf N content (Nakano et al., 1997; Harmens et al., 2000; Makino et al., 2000). Additional studies indicate that leaf N content may have to fall below a species-specific threshold in order to trigger photosynthetic adjustment under CO2-enriched conditions (Delgado et al., 1994; Petterson and McDonald, 1994; Riviere-Rolland et al., 1996).
Environmentally-relevant O3-exposure reduced growth and photosynthetic rate in plants supplied with 4 and 14 mM NO-3. However, O3 had no impact on these parameters in plants supplied with the lowest level of N (1.5 mM NO-3). This result is consistent with findings for radish (Raphanus sativus L.) and loblolly pine (Pinus taeda L.) where O3 reduced biomass at high, but not low, N (Pell et al., 1990; Tjoelker and Luxmoore, 1991). The absence of a depression in A at low N supply was consistent with the lower gH2O, and hence cumulative O3 uptake, observed over the life-span of leaf 4 in plants supplied with 1.5 mM NO-3. However, increases in the reallocation of nitrogen from old to young leaves due to accelerated leaf senescence, as observed in plants exposed to O3, may have made some contribution in alleviating N limitation in the ‘young leaves’ of polluted plants (Manderscheid et al., 1992). This is supported by this study's finding that O3-exposed plants supplied with 1.5 mM O3 exhibited an 18% increase in A on the attainment of full leaf expansion. Further work is needed to evaluate the importance of N reallocation from senescent to young leaves as a compensation mechanism in plants exposed to O3 under conditions of low N supply.
In the combined treatment, elevated CO2 counteracted the negative effects of O3 on growth and photosynthesis observed at ambient CO2. Similar results have been reported for other cultivars of wheat (McKee et al., 1995, 1997a, b; Mulholland et al., 1997) and other herbaceous species (see tables fomulated by Cardoso-Vilhena et al., 1998; Turcsányi et al., 2000; Olszyk et al., 2000). One factor common to all these species is the responsiveness of the stomata to an increase in the atmospheric CO2 concentration such that the flux of O3 to the leaf interior would be substantially reduced under CO2-enriched conditions. Consistent with the view (Barnes and Wellburn, 1998) that this is a vital consideration governing the way in which rising CO2 concentrations will influence plant responses to O3 (and other gaseous pollutants that are not absorbed in significant quantities through the leaf cuticle), it is notable that the protection against O3 afforded by elevated CO2 in the present study was associated with a marked reduction (c. 25%) in O3 uptake over the life-span of individual leaves, as a consequence of the decline in stomatal conductance induced by CO2-enrichment. Moreover, the degree of protection against O3 afforded by elevated CO2 was unaffected by N supply. This observation is consistent with previous findings for sideoats grama (Bouteloua curtipendula Michx.), aspen (Populus tremuloides Michx.) and cotton (Gossypium hirsutum L.) (Volin and Reich, 1996; Heagle et al., 1999). These data suggest that attempts to introduce cost-effective control strategies with the aim of minimizing the impacts of photochemical oxidant pollution on vegetation may need to take into account the impact of rising atmospheric CO2 concentrations in modulating the effects of O3 on plant growth and productivity. First, however, there is an urgent need to establish exposurexresponse relationships for the combined effects of elevated CO2 and O3 on the yield of various crops.
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Acknowledgments |
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The authors thank Professor AW Davison (Newcastle University, UK) and Dr I McKee (Essex University, UK) for advice during the course of this study, and Mr P Green, Mr A White and K Taylor for technical assistance. The work was supported by a grant from The Portuguese Ministry of Science and Technology—PRAXIS XXI Scheme (BD/9155/96) and the work was undertaken during JDB's tenure as a Royal Society Research Fellow.
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Notes |
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1 To whom correspondence should be addressed. Fax: +44 191 222 5229. E-mail: J.D.Barnes{at}ncl.ac.uk
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Abbreviations |
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, Rubisco specificity factor; A, rate of CO2 assimilation; A350, light-saturated rate of CO2 assimilation rate at an ambient CO2 concentration of 350 µmol mol-1; Amax, light- and CO2-saturated rate of CO2 assimilation; CFA, charcoal/Purafil®-filtered air; DALE, days after leaf emergence; DAT, days after transfer; gH2O, stomatal conductance to water vapour; K, root/shoot allometric coefficient; Kc and Ko, Michaelis constants for Rubisco carboxylation and oxygenation, respectively; PPFD, photosynthetically active photon flux density; RGR, plant relative growth rate; RGRR, RGR of root; RGRS, RGR of shoot; Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; Vcmax; maximum in vivo rate of Rubisco carboxylation. |
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