Biomass allocation, needle structural characteristics and nutrient composition in Scots pine seedlings exposed to elevated CO2 and O3 concentrations

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were exposed to ambient or elevated ozone (O₃) (1.52ambient) and carbon dioxide (CO₂) (590 µmol mol^-1) concentrations during two growing seasons in open-top field chambers (OTCs). Five different treatments were applied in the chambers: filtered air, ambient air, elevated O₃, elevated CO₂, and elevated O₃ and CO₂ combined. Ambient plots outside the OTCs were also included, but the chamber ambient was used as a control in O₃ and CO₂ treatments due to a significant chamber effect. Increases in yellowing and chlorotic mottling of previous-year (C+1) needles and in the amount of cytoplasmic ribosomes and electron density of the chloroplast stroma in current-year (C) and C+1 needle mesophyll cells were observed in elevated O₃ at both CO₂ concentrations. Elevated O₃ alone caused a non-significant 10.9% decrease in plant total dry mass and a significant decrease in manganese (Mn) content of C needles. CO₂ enrichment caused a significant increase in needle cross-sectional width after the first year of exposure, and an accumulation of starch and slight curling and swelling of the chloroplast thylakoids in the mesophyll tissue of C needles after the second year of exposure. Calcium and Mn contents were increased and copper and nitrogen contents were decreased, significantly, in CO₂-exposed needles. A non-significant 19.1% increase in plant total dry mass was measured in elevated CO₂ alone, whereas a 14.8% reduction in total dry mass, together with a significant reduction in current-year main shoot length, was found in the combined treatment. Overall, in spite of decreases in O₃-induced visible injuries by CO₂, elevated CO₂ levels were not able to counteract the impact of O₃ in this experiment.