Under benign environmental conditions, plant growth is generally stimulated by elevated atmospheric CO2 concentrations. When environmental conditions become sub- or supra-optimal for growth, changes in the biomass enhancement ratio (BER; total plant biomass at elevated CO2 divided by plant biomass at the current CO2 level) may occur. We analysed literature sources that studied CO2×environment interactions on the growth of herbaceous species and tree seedlings during the vegetative phase. For each experiment we calculated the difference in BER for plants that were grown under 'optimal' and 'non-optimal' conditions. Assuming that interactions would be most apparent if the environmental stress strongly diminished growth, we scaled the difference in the BER values by the growth reduction due to the stress factor. In our compilation we found a large variability in CO2×environment interactions between experiments. To test the impact of experimental design, we simulated a range of analyses with a plant-to-plant variation in size common in experimental plant populations, in combination with a number of replicates generally used in CO2×environment studies. A similar variation in results was found as in the compilation of real experiments, showing the strong impact of stochasticity. We therefore caution against strong inferences derived from single experiments and suggest rather a reliance on average interactions across a range of experiments. Averaged over the literature data available, low soil nutrient supply or sub-optimal temperatures were found to reduce the proportional growth stimulation of elevated CO2. In contrast, BER increased when plants were grown at low water supply, albeit relatively modestly. Reduced irradiance or high salinity caused BER to increase in some cases and decrease in others, resulting in an average interaction with elevated CO2 that was not significant. Under high ozone concentrations, the relative growth enhancement by elevated CO2 was strongly increased, to the extent that high CO2 even compensated in an absolute way for the harmful effect of ozone on growth. No systematic difference in response was found between herbaceous and woody species for any of the environmental variables considered.

The growth response of plants to elevated CO2 under non-optimal environmental conditions

This paper presents a mathematical model which enables the semi-quantification of ozone (O3) detoxification, based upon the direct reaction of the pollutant with ascorbate (ASC) located in the aqueous matrix associated with the cell wall (i.e. the apoplast). The model describes the uptake of ozone into the leaf and its direct reaction with ASC, taking into consideration the regeneration of dehydroascorbic acid in the cytosol, the rate of replenishment of cell wall ASC and the distribution of ASC between sub-cellular compartments – based upon the permeability of biomembranes to the neutral species, ascorbic acid and the pH of various sub-cellular compartments. The importance of various physico-chemical characteristics (e.g. stomatal conductance, mesophyll cell wall thickness and tortuosity, chloroplast volume, apoplast pH, ASC:O3 reaction stoichiometry) in mediating the flux of ozone to the plasmalemma is analysed. Model simulations, supported by experimental observations, suggest that the ASC concentration in the leaf apoplast is high enough to scavenge a significant proportion of the O3 taken up into the leaf interior, under environmentally relevant conditions. However, there is considerable variation between taxa in the potential degree of protection afforded by apoplastic ASC, emphasizing the need for an improved understanding of the reaction chemistry of O3 in the cell wall.

https://www.jstor.org/stable/info/23385890

Simulating ozone detoxification in the leaf apoplast through the direct reaction with ascorbate.

Ground-level concentrations of ozone (O3) have risen rapidly over the past century (≈1.6% per annum in the Northern Hemisphere; Marenco et al. 1994), primarily due to increased emissions of nitrogen oxides and volatile hydrocarbons from all manner of domestic and industrial sources (Hough and Derwent 1990). Concentrations are expected to continue to rise for the foreseeable future; forecasts suggest that ground-level O3 concentrations will continue to climb at a rate of between 0.3% and 1.0% per annum over the next 50 years (Chameides et al. 1994). As a consequence, present-day concentrations of O3 commonly exceed the accepted threshold of 40 parts per billion (ppb; billion = 109) for damage to the most sensitive elements of natural and managed ecosystems (Fuhrer et al. 1997; Fuhrer and Achermann 1999) and the gas is now recognized to be the most prevalent and damaging air pollutant to which vegetation is exposed in many parts of Central/Southern Europe, North/South America and Asia (Yunus and Iqbal 1996). In many of these regions, there is incontrovertible evidence that O3 levels are high enough to reduce crop yields (Heck et al. 1983; Runeckles and Chevone 1992; Turcsanyi et al. 2000a), to cause shifts in the genetic composition of natural and seminatural vegetation (Davison and Barnes 1998; Barnes et al. 1999a) and predispose forest trees to damage by secondary stress factors (Sandermann et al. 1997; Chappelka and Samuelson 1998; Skarby et al. 1998; Matyssek and Innes 1999; Akimoto and Sakugawa 2000).

Plant Resistance to Ozone: the Role of Ascorbate

Tropospheric ozone (O 3 ) causes significant disruption of growth and yield in upland and Pima cottons Pima cotton (Gossypium barbadense L) was exposed to brief pulses (075 h) of a range of O 3 concentrations (nominally 00, 02, 05, and 08 μl l -1 ) to investigate effects on phloem translocation of 14 C-labelled recent photo-assimilate The initial phase of rapid efflux from source leaves was monitored with a Geiger-Muller Tube as activity remaining in the leaf as a function of time Visual inspection of unprocessed efflux curves revealed disruption of efflux by O 3  Single exponential decay functions were fitted to these efflux curves to extract first order rate constants for phloem loading and longitudinal transport of labelled carbohydrates A single compartment model was applied, with and without an asymptote of non-transported carbohydrate, to calculate leaf sugar contents The effect of O 3 in retarding efflux of label, decreasing the rate constant, and increasing calculated soluble sugar pools, was consistent regardless of the method of analysis Following incorporation of the asymptote, calculated rate constants and sugar pools were similar to values from the literature and to preliminary measurements of sugar contents in O 3 -treated cotton leaves Total carbohydrate transported from source leaves was reduced both by O 3 effects on assimilation (up to 20%) and by O 3 effects on efflux (up to 70%), but was clearly dominated by the impact on phloem translocation These rapid efflux kinetics likely reflect oxidant damage at the plasmalemma or plasmodesmata of mesophyll or phloem companion cells Evaluation of effects of O 3 on tonoplast function and consequences for carbohydrate translocation await a more complete compartmental efflux analysis

Acute exposure to ozone inhibits rapid carbon translocation from source leaves of Pima cotton

Spring wheat (Triticum aestivum cv. Hanno) was grown at ambient (350 mmol mol � 1 ) or elevated CO2 (700 mmol mol � 1 ) in charcoaluPurafil 1 -filtered air (CFA - 5n mol mol � 1 ) or ozone (CFA q75 nmol mol � 1 7h d � 1 ) at three levels of N supply (1.5, 4 and 14 mM NO � ), 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% ,2 9% ,1 2% in plants supplied with 14, 4 and 1.5 mM NO � , 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 )i n plants supplied with 14 and 4 mM NO � . However, O3 had no effect on the RGR of plants supplied with

Does nitrogen supply affect the response of wheat (Triticum aestivum cv. Hanno) to the combination of elevated CO2 and O3

Anthropogenic activities have significantly changed the composition of the global atmosphere — a situation that has raised serious International concerns. Much attention has been focused on the ‘warming’ that is expected as a consequence of the rising concentration of a number of radiatively active trace gases in the lower atmosphere (the troposphere) and also the enhanced levels of potentially harmful ultraviolet radiations that are reaching the earth’s surface (Houghton et al., 1990). Rather less attention has been paid to the direct effects of these environmental changes on natural and managed ecosystems, despite the fact that impacts are likely before statistically-discernible changes in climate are evident (Barnes, 1993). In this chapter, we concentrate on the past, present and likely future impacts of one of these gases — ozone (O3). Not only does this radiatively absorptive gas contribute directly to the “greenhouse effect”, but it is also a powerful oxidant and is recognised to be one of the most widespread toxic agents to which vegetation is exposed. It is also important to recognize that changes in climate and global atmosphere may not act in isolation - increases in temperature, atmospheric carbon dioxide (CO2) concentration and the enhanced penetration of ultraviolet-B (UV-B) radiation may, in turn, influence the tropospheric concentration of O3, and its effects on vegetation (Ashmore and Bell, 1991).

C. Gillespie

Papers

Impacts of Tropospheric Ozone: Past, Present and Likely Future