As measured by in vivo spectrophotemetry the phytoehrome content in etiolated turnip seedlings was higher in cotyledons than in hypocotyls; in the latter, it is confined to the apical part. During early growth in darkness the amount increased in both tissues to a maximum, reached about 40 hours after sowing; the levels then gradually declined. Separation of seedlings into hypoeotyl and cotyledons increased the rate of phytochrome loss in the former, but not in the latter.
Following 5 minutes of red light, P/r decayed very rapidly in darkness; after 1.5 hours all of the phytochrome was present as Pr, which was presumably not converted initially. In continuous red light the total phytochrome was reduced to below the detection level within 3 hours. Seedling age markedly affected the loss of phytochrome following red light; more was destroyed in older than in younger hypocotyls and apparent new synthesis occurred only in young seedlings. The capacity to synthesise phytochrome differed in cotyledons and hypocotyl. In cotyledons, synthesis occurred following shots of red light varying from 10 seconds, to 6 x 1 minute, but the amount of newly formed phytochrome was not related to the amount destroyed: after 5 hours of continuous red light no new synthesis occurred. In hypocotyls, the amount of phytochrome synthesised was related to the amount previously destroyed, and the phytoehrome content after 24 hours of darkness was similar following all red light treatments of I minute or longer: new synthesis occurred following 5 hours of continuous red light.
In far-red light phytochrome decayed very slowly, approaching the limit of detection after 48 hours. In cotyledons some loss was already observed after 5 hours of far-red and, in hypocotyls, after about 10 hours.
These results are discussed in relation to the possible role of phytochrome as the pigment mediating anthocyanin synthesis in prolonged far-red light.