Effect of elevated CO2 concentration on acclimation of tobacco plantlets to ex vitro conditions

Nicotiana tabacum L, plants grown in vitro were transferred to ex vitro con ditions and grown for 28 d in a greenhouse under normal CO2 concentration ( C, 330 pmol mol(-1)) or elevated CO2 concentration (E, 1000 mu mol mol(-1)) . Stomatal conductances of abaxial and adaxial epidermes measured under opt imal conditions were not significantly affected by growth under E, but the stomatal regulation of gas exchange was better. Leaf photosynthetic rate (A ) of elevated CO2 plants was similar to that of control plants when both we re measured under normal CO2, but higher when both were measured under elev ated CO2. The A of elevated CO2 plants was much higher than the A of contro l plants when measured under their respective growth CO2 concentration, whi ch resulted in their higher growth rate. Chlorophyll a and b contents, and activities of whole electron transport chain and of photosystem (PS) II wer e not markedly affected by growth under E, and the maximum efficiency of PS II measured as the ratio of variable to maximum fluorescence was even sligh tly increased. Hence no down-regulation of photosynthesis occurred in trans planted plants grown for 4 weeks under E. The contents of p-carotene and of xanthophyll cycle pigments (violaxanthin + antheraxanthin+zeaxanthin) were lower in E plants. The degree of de-epoxidation of xanthophyll cycle pigme nts was not changed or was even lower after transfer to ex vitro conditions , which indicated that no photoinhibition occurred, Therefore, CO2 enrichme nt can improve acclimation of in vitro-grown plantlets to ex vitro conditio ns.