Photosynthetic and stomatal responses of potatoes grown under elevated CO2 and/or O3—results from the European CHIP-programme

The physiological effects of elevated CO2 and/or O3 on Solanum tuberosum cv. Bintje were examined in Open-Top Chambers during 1998 and 1999 at experimental sites across Europe as part of the EU 'Changing Climate and Potential Impacts on Potato Yield and Quality' programme (CHIP). At tuber initiation (≈20 days after emergence, DAE) elevated CO2 (680 μl l(-1)) induced a 40% increase in the light saturated photosynthetic rate (A(sat)) of fully expanded leaves in the upper canopy. This was 16% less than expected from short-term exposures of plants grown under ambient CO2 (360 μl l(-1)) to elevated CO2, indicating that photosynthetic acclimation began at an early stage of crop growth. This effect resulted from a combination of a 12% reduction in stomatal conductance (g(s)) and a decline in photosynthetic capacity, as indicated by the significant reductions in the maximum carboxylation rate of Rubisco (Vc(max)) and light-saturated rate of electron transport (J(max)) under elevated CO2. The seasonal decline in the promotion of photosynthesis by elevated CO2 reflected the concurrent decrease in gs. Vcmax and Jmax were both reduced in plants grown under elevated CO2 until shortly after maximum leaf area (MLA) was attained. Although non-photorespiratory mitochondrial respiration in the light (Rd) increased during the later stages of the season, net photosynthesis was consistently increased by elevated CO2 during the main part of the season. Photosynthetic rate declined more rapidly in response to elevated O3 under ambient CO2, and the detrimental impact of O3 was most obvious after MLA was attained (DAE 40-50). Several exposure indices were compared, with the objective of determining the critical ozone level required to induce physiological effects. The critical O3 exposure above which a 5% reduction in light saturated photosynthetic rate may be expected (expressed in terms of cumulative exposure above 0 nl l-1 O3 between emergence and specific dates during the season (AOT0-cum)) was 11 μl l-1 h; however this value should only be extrapolated beyond the CHIP dataset with caution. The interaction between O3 and stomatal behaviour was more complex, as it was influenced by both long-term and daily exposure levels. Elevated CO2 counteracted the adverse effect of O3 on photosynthesis, perhaps because the observed reduction in stomatal conductance decreased O3 fluxes into the leaves. The results are discussed in the context of nitrogen deficiency, carbohydrate accumulation and yield.