Effects of Different Carbon Dioxide and Ozone Concentrations on Shoot Growth of Phleum pratense L. and Betula pubescens Ehrh. as Influenced by Day Length and Irradiance
Seedlings of Phleum pratense L. (timothy) and Betula pubescens Ehrh. (mountain birch) were grown for 37 or 42 days at all combinations of two CO2 concentrations (350 and 700 mol mol-1), two O3 concentrations (13 and 59 nmol mol-1 in 8 h day-1), two day lengths (17 and 24 h DL) and two levels of supplementary lighting (150 and 210 mol m-2 s-1 photosynthetic photon flux, PPF) in 16 growth chambers placed in a greenhouse. Elevated CO2 concentration increased the mean shoot dry weight by 47% in timothy and by 39% in birch. No significant interactions were found between CO2 and O3, DL or PPF with respect to shoot dry weight in the two species. The number of shoots in timothy was generally enhanced by CO2 enrichment. The number of branches in birch was strongly enhanced by elevated CO2 at 17 but not at 24 h DL, and the ratio of the fresh weight of branches to main shoot was significantly increased irrespective of DL. Increasing the O3 concentration caused visible leaf injuries both in timothy (chlorosis/necrosis) and in birch (yellow stipples/brown spots), while the shoot weight was not significantly affected. The number of O3-induced injuries in timothy was decreased by increasing the CO2 concentration or the total irradiance (increasing DL and/or PPF). The number of injuries in birch was slightly decreased by increasing PPF; however, CO2 enrichment had no effect.
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