The photoperiodic stimulus in both LDPs and SDPs is perceived by the leaves. For example, treatment of a single leaf of the SDP Xanthium with short photoperiods is sufficient to cause the formation of flowers, even when the rest of the plant is exposed to long days. Thus, in response to pho-toperiod the leaf transmits a signal that regulates the transition to flowering at the shoot apex. The photoperiod-reg-ulated processes that occur in the leaves resulting in the transmission of a floral stimulus to the shoot apex are referred to collectively as photoperiodic induction.
Photoperiodic induction can take place in a leaf that has been separated from the plant. For example, in the SDP Perilla crispa, an excised leaf exposed to short days can cause flowering when subsequently grafted to a noninduced plant maintained in long days (Zeevaart and Boyer 1987). This result indicates that photoperiodic induction depends on events that take place exclusively in the leaf.
Grafting experiments, which have contributed greatly to our understanding of the floral stimulus, will be discussed in more detail later in the chapter.
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