Narrow band light was obtained by placing a narrow band interference filter between lamp 2 and an arm of the fibre optics. The light from the two lamps was projected fully mixed on the leaf surface. At each wavelength, photosynthesis was measured at the background light plus 0, 30 or 60molm−2 s−1 narrow band light, after a 3-min exposure to 100molm−2 s−1 of broad-band light. This low light intensity is usually adopted in research on spectral dependence of photosynthesis in order to count on a reliable light response,since, at this level, the relationship between photosynthesis and light radiation is supposed to be linear.