A method is reported which allows an early diagnosis of stress on leaves and needles. The method is based on the kinetic measurement of fluorescence simultaneously at two different wavelenghts. Early diagnosis means a diagnosis whereby visible or spectroscopic modifications of the sample can be observed. An adaptation index A_p is defined and it can be calculated with the measured data for each sample. This index A_p is a measure for the ability of the sample to adapt to the variations of light conditions occurring always in nature and can be compared with the vitality index R_fd known from the literature. Examples are shown where trees have been treated with ozone of concentrations as they occur in nature e.g. 100 - 300 μg O₃/m³. The influence of this treatment can be detected on leaves or needles by the adaptation index A_p already at a time when no information at all is supplied by the vitality index R_fd.