The participation of the red/far-red reversible reaction of phytochrome in the control of anthocyanin formation in turnip seedlings has been demonstrated. A brief exposure to red light following a preliminary irradiation period in blue, increased anthocyanin content compared with blue alone; this effect was reversed by a subsequent short exposure to far-red. The sensitivity to red light was largely restricted to 24 hours old seedlings when grown in water at 25°C. Sensitivity was restored in older seedlings when they were grown in phenylalanine and kept in high temperature (35°C) for several hours before light was given; under these conditions, the phytochrome effect was greater in 48 hours old than in 24 hours old seedlings. In the youngest seedlings the largest increase occurred when red followed a preliminary blue exposure of at least 12 hours; in older seedlings the maximum response to red was almost attained after only 4 hours of blue light. Hypocotyl elongation was shown to be hardly affected by the reversible reaction of phytochrome. Possible reasons for these changes in sensitivity to phytochrome are discusse
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