Two different rates of chlorophyll synthesis were observed in the cotyledons when squash (Cucurbita moscata) seedlings were exposed to continuous red radiation: a slow rate which lasted for about three hours and an accelerated rate that proceeded thereafter. Determination of red, far-red reversible reaction indicated that pigment formation in seedlings exposed initially to far-red was promoted by red radiation, and this promotion was repeatedly reversed by subsequent far-red treatments. On the other hand, seedlings exposed first to red radiation followed by far-red, red light cycles failed to respond according to the typical red-accelerated and far-red-inhibited photomorphogenic responses. Variations in these photoresponses were observed when the exposure time at each alternating waveband was changed, or the energy balance between red and far-red bands was changed or even when the seedlings were exposed to white light either before or after the red, far-red light treatments. The results of these experiments are reported.
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