Abstract: | This paper shows that in Porphyridium cruentum and in Chlorella pyrenoidosa (but apparently not in Anacystis nidulans) “extreme red” light (> 720 mμ) can inhibit photosynthesis produced by “far red” light (up to 720 mμ). From the action spectrum of this phenomenon, it appears that an unknown pigment with an absorption band around 745 mμ must be responsible for it. |