Lecture 2 from Pat Heslop-Harrison for BS1003 - Cell and Developmental Biology. The transition to flowering. How do plants decide to flower? How do they respond to daylength (photoperiod) and temperature? For information from light, phytochrome is the photoreceptor, but not the clock/time measuring process. Pfr (phytochrome far red) is always the active form of phytochrome, but the function is different in long day plants and short day plants. Pfr promotes flowering in LDPs but inhibits flowering in SDPs.