Spectroscopic methods may be used for compositional analysis of plant materials or for the evaluation of properties of plants such as internal structure, metabolic activity or degree of damage. The stale of plants very often is indicated by changes in the functioning of the photosynthetic apparatus (PSA) and, therefore, can be studied by measurements of the chlorophyll fluorescence yield (CFY). A wide field of application of CFY and other spectroscopic methods is the determination of stress response of different plant species and cultivars. These methods were applied to monitor the plant reactions on environmental stress factors like: air pollution (SO₂, O₃, NO_x, NH₃ and other nitrogen compounds), - heavy metals (Pb, Cd, ... ) and other soil and water pollution, frost and unfavourable low temperature, high temperature and drought, radiation of high quantum energy (UV, X, and gamma rays), strong visible light and, of course, the combination of these effects, since the changes in the environmental conditions due to global problems (pollution, 'greenhouse effect', 'ozone hole') influence the state of plants.