Spring wheat (Triticum aestivum cv. Hanno) was grown at ambient (350 micromol mol(-1)) or elevated CO(2) (700 micromol 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 CO(2) and O(3) 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 CO(2) was dependent on the level of N supply. At 60 d after transfer (DAT), elevated CO(2) 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 CO(2) across N treatments; the maximum in vivo rate of Rubisco carboxylation (V(cmax)) was similar in plants raised at elevated and ambient CO(2). 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, O(3) 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 CO(2) counteracted the detrimental effects of O(3) (i.e. the same ozone concentration that depressed RGR and A at ambient CO(2) resulted in no significant effects when plants were raised at elevated CO(2)) at all levels of N supply and the effect was associated with a decline in O(3) uptake at the leaf level.

There is growing evidence that rising atmospheric CO2 concentrations will reduce or prevent reductions in the growth and productivity of C3 crops attributable to ozone (O3) pollution. In this study, the role of pollutant exclusion in mediating this response was investigated through growth chamber-based investigations on leaves 4 and 7 of spring wheat (Triticum aestivum cv. Hanno). In the core experiments, plants were raised at two atmospheric CO2 concentrations (ambient [350 micro l l(-1)] or elevated CO2 [700 micro l l(-1)] under two O3 regimes (charcoal/Purafil-filtered air [<5 nl l(-1) O3] or ozone-enriched air [75 nl l(-1) 7 h d(-1)]). A subsequent experiment used an additional O3 treatment where the goal was to achieve equivalent daily O3 uptake over the life-span of leaves 4 and 7 under ambient and CO2-enriched conditions, through daily adjustment of exposures based on measured shifts in stomatal conductance. Plant growth and net CO2 assimilation were stimulated by CO2-enrichment and reduced by exposure to O3. However, the impacts of O3 decreased with plant age (i.e. leaf 7 was more resistant to O3 injury than leaf 4); a finding consistent with ontogenic shifts in the tolerance of plant tissue and/or acclimation to O3-induced oxidative stress. In the combined treatment, elevated CO2 protected against the adverse effects of O3 and reduced cumulative O3 uptake (calculated from measurements of stomatal conductance) by c. 10% and 35% over the life-span of leaves 4 and 7, respectively. Analysis of the relationship between O3 uptake and the decline in the maximum in vivo rate of Rubisco carboxylation (Vcmax) revealed the protection afforded by CO2-enrichment to be due, to a large extent, to the exclusion of the pollutant from the leaf interior (as a consequence of the decline in stomatal conductance triggered by CO2-enrichment), but there was evidence (especially from flux-response relationships constructed for leaf 4) that CO2-enrichment resulted in additional effects that alleviated the impacts of ozone-induced oxidative stress on photosynthesis.

The impact of incubation in saline solutions of different concentrations on the uptake and cellular location of essential elements (Na + , K + , Mg 2+ and Ca 2+ ), and its effects on membrane integrity and on the photosynthetic apparatus, were investigated in the lichen Ramalina canariensis Steiner. Saline incubation resulted in a rapid uptake of Na + and Mg 2+ in the cell wall fraction, whereas in the intracellular fraction the accumulation of Na + was slower. No changes were observed for intracellular Mg 2+ , suggesting that no generalized membrane damage occurred. Concomitantly with the increase in intracellular Na + , there was a specific loss of K + from the cell interior, indicating that membrane permeability may have been compromised. Incubation in a 100% artificial sea water solution reduced the maximum photochemical efficiency of Photosystem II (F v /F m ) by 17% after 5 min, and this inhibition increased with incubation time. In samples incubated in 100% artificial sea water solution for 2 h followed by 2 h incubation in deionized water, ion distribution and F v /F m did not recover to control values. The present findings show the importance of determining the cellular location of elements when assessing their physiological impact. Results indicate that saline stress may irreversibly impair photosynthesis, thus compromising lichen vitality.

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