To study the possible differences in tropospheric ozone (O 3) tolerance of oat ( Avena sativa L.) and wheat ( Triticum aestivum L.), two oat and two wheat cultivars were exposed to 0, 50, or 100 nL.L -1 O 3 concentrations in growth chambers. Measurements on volatile organic counpound emission and physiological, biochemical, and leaf structural characteristics were conducted with 2- and 4-week-old seedlings. Neither of the studied species was particularly O 3 sensitive, but O 3 sensitivity should rather be defined on the basis of the characteristics of the cultivars within species. Visible leaf injuries increased with leaf age and with increasing O 3 concentration. Net photosynthesis ( Pn), stomatal conductance ( gs), and chlorophyll fluorescence ( Fv/ Fm) of 2-week-old seedlings were more detrimentally affected by O 3 compared with older seedlings. Wheat generally invested more in photosynthesis and related processes, such as gs, Fv/ Fm, concentrations of Rubisco, chlorophylls and carotenoids, and synthesis of starch compared with oat. O 3 increased Rubisco concentration in 2-week-old and carotenoid concentration in 4-week-old seedlings, especially in wheat. Lower extent of O 3-caused visible leaf injuries in the other oat cultivar can supposedly be explained by its low stomatal conductance and high monoterpene production.
