Natural Scots pines have been exposed to filtered air, ambient air and air with elevated 0₃ or/and CO₂ in open top chambers. The trees showed no differences in their optical responses prior to the fumigations . After the fumigation period of three months the plants were in good healrh. The position of the maximum derivative of the green light reflectance in carbon dioxide fumigated pines was shifted from the control pines inflection point, by approximately 4 nm towards shorter wavelengths. The position of the red edge derivative maximum showed no significant changes. By fluorescence techniques (as OJIP - fast fluorescence transients) nearly no change was found in the quantum yield for electron transport (φ_o or excitation energy trapping φ_Po. However, the estimated activities as absorption, trapping or electron transport per cross-section increased considerably for all samples with elevated O₃ or CO₂, This increased activityseems to be due to an increased antenna size in O₃ treated samples. At elevated CO₂ the antenna size is decreased whereas the density of reaction centers per cross-section increased. This means that two different stress-adaptation mechanisms can lead to a similar macroscopic phenomenon like e.g. an increased metabolic activity.