In Chapter 2, two actions of light were distinguished in the photoperiodic control of flowering in the SDP Pharbitis nil. Light acts to control the phase of the photoperiodic rhythm. Light also interacts with a specific phase of that rhythm, (pj, to inhibit flowering. Although these two actions of light have not clearly been differentiated in the majority of SDP, there is much circumstantial evidence for their existence and they are considered as two separate reactions in the following discussion. Additionally, it is well documented that removal of the Pfr form of phytochrome early in the night may inhibit flowering in many SDP. The photoreceptor thus has multiple actions in the photoperiodic control of flowering (Vince-Prue and Takimoto, 1987). These are shown schematically in Fig. 4.1.
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