Some herbaceous sun and shade plants were grown under two red/far-red ratios to investigate their adaptation in morphology, growth and net photosynthesis to the light quality component of shade light. All species reacted in the same way to a low red/far-red ratio, but the sun species reacted more, showing more stem elongation, a lower leaf weight ratio and a lower relative growth rate. It can be inferred that photosynthetic functions are not influenced by the red/far-red ratio, but dark respiration increases, and probably to a greater extent in sun species.