Abstract. A prolonged light treatment strongly increases responsivity to Pfr in many instances of phytochrome-controlled biogenesis of flavone or cyanidin glycosides. The present investigation deals with the question of whether light also leads to a corresponding increase of responsivity towards Pfr in such photoresponses which are not related to synthesis of flavonoid pigments in outer tissue layers of seedlings. Phytochrome-mediated accumulation of the chloroplast GPD (glyceraldehyde-3-phosphate dehydrogenase, EC 126.96.36.199) was chosen as a response and the milo shoot (Sorghum vulgare Pers. cv. Weider, hybrid) as an appropriate subject. It was found that responsivity towards Pfr is extremely weak in a dark-grown shoot while prolonged light pretreatments lead to a dramatic increase in responsivity. Blue and UV light are far more effective than red light in eliciting this effect within a few hours. High responsivity is only maintained in the light. When the seedlings are placed in darkness the level of responsivity drops rapidly with a half-life of the order of 2 h. The data allow more complete explanations for intriguing phenomena of plant life under natural light/dark conditions such as shade detection or sensing of light → dark transitions.