Ultrastructural and growth responses of young Scots pine seedlings (Pinus sylvestris) to increasing carbon dioxide and ozone concentrations

Three-year-old Scots pine seedlings were exposed to ambient or elevated (2 × ambient) O3 and CO2 levels, singly and in combination, during one growth period in open-top field chambers. Growth measurements showed increased shoot length and needle width in response to CO2 enrichment, whereas O3 exposure resulted in visible injury (chlorotic mottling and increased yellowing of previous year needles). At the ultrastructural level, O3 caused increased electron density of chloroplast stroma and increased number of cytoplasmic ribosomes at both CO2 levels. CO2 enrichment also resulted in an increase in the size of starch grains in chloroplasts. In general, simultaneous exposure to elevated O3 reduced the impact of elevated CO2.