Abstract: Described and represented is a method for determining the content of at least one plant substance of at least one part of a plant. In order for the content of plant substances, in particular secondary plant substances, of at least one part of a plant to be determined and optimized more expediently, it is provided that the at least one part of the plant is irradiated successively with light of different wavelengths and/or wavelength ranges and that, in response to the irradiation of the at least one part of the plant with light of each wavelength and/or at each wavelength range, the chlorophyll fluorescence at least substantially the same wavelength and/or at least substantially the same wavelength range is measured in each case.