A feature that underscores the importance of the dark period is that it can be made ineffective by interruption with a short exposure to light, called a night break (see Figure 24.19A). In contrast, interrupting a long day with a brief dark period does not cancel the effect of the long day (see Figure 24.19B). Night-break treatments of only a few minutes are effective in preventing flowering in many SDPs, including Xanthium and Pharbitis, but much longer exposures are often required to promote flowering in LDPs.
In addition, the effect of a night break varies greatly according to the time when it is given. For both LDPs and SDPs, a night break was found to be most effective when given near the middle of a dark period of 16 hours (Figure 24.20).
The discovery of the night-break effect, and its time dependence, had several important consequences. It established the central role of the dark period and provided a valuable probe for studying photoperiodic timekeeping. Because only small amounts of light are needed, it became possible to study the action and identity of the photore-ceptor without the interfering effects of photosynthesis and other nonphotoperiodic phenomena. This discovery has also led to the development of commercial methods for regulating the time of flowering in horticultural species, such as Kalanchoe, chrysanthemum, and poinsettia (Euphorbia pulcherrima).
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