Abstract
In etiolated zucchini seedlings the spectral properties of the far-red form of phytochrome (P fr) as measured in vivo were found not to be time invarient but rather to exhibit a circadian rhythm with respect to peak position. The peak was found to vary between 728 and 737 nm with a period of 22–24 h. In correlation with these changes it was found that the responsiveness of hook opening to a single red irradiation also exhibited a circadian period. Maximum responsiveness was found to occur during that phase when P fr exhibited its longwavelength form, and minimal responsiveness during that period when P fr exhibited its shortwavelength form. These results, it is suggested, support the conjecture that the longwavelength form of P fr corresponds to a complex of P fr with its native receptor and that it is the change in the level of this complex which governs circadian responsiveness.
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