A plot of the rate of a reaction, e.g. the phototropic response or photosynthesis, at different wavelengths of light. The action spectrum for photosynthesis (see diagram) is obtained by measuring the photosyn-thetic yield for a given amount of light incident upon the plant over a range of wavelengths. For green plants the action spectrum shows that chlorophyll is the pigment responsible for photosynthesis, since peak photosynthetic activity occurs at the absorption peaks of chlorophylls a and b.
For some algae the action spectrum shows peak photosynthetic activity at the absorption peak of one of the accessory pigments. In this situation although chlorophyll is still used for transfer of light energy into the photosynthetic process, it is not the primary light-absorbing pigment.