Branch position in the moss Physcomitrella patens is regulated by blue light. In this study, fluence rate dependency of branch position determination was investigated by partial cell irradiation with a microbeam. With a 30 Wm–2 or lower fluence rate, branches formed at the microbeam area, but formed outside the microbeam when the fluence rate was raised to ≥200 Wm–2. Thus, both weak and strong light responses influence the determination of branch position. Further, light sensitivity of both responses was reduced in phototropin knock-out lines, revealing an involvement of phototropin as the blue light receptor.