No flowering

Time (hours)

Only plants given 10 hours of dark flowered.

No flowering

Time (hours)

Conclusion: Short-day plants measure the length of the night and could more accurately be called long-night plants.

39.11 Night Length and Flowering The length of the dark period, not the length of the light period, determines flowering.

► For one group of plants, the light period was kept con-stant—either shorter or longer than the critical day length—and the dark period was varied.

► For another group of plants, the dark period was kept constant and the light period was varied.

The plants flowered under all treatments in which the dark period exceeded 9 hours, regardless of the length of the light period. Thus, Hamner and Bonner concluded that it is the length of the night that matters; for cocklebur, the critical night length is about 9 hours. Thus, it would be more accurate to call cocklebur a "long-night plant" than a short-day plant.

In cocklebur, a single long night is sufficient photoperiodic stimulus to trigger full flowering some days later, even if the intervening nights are short ones. Most plants are less sensitive than cocklebur and require from two to several nights of appropriate length to induce flowering. For some plants, a single shorter night in a series of long ones, even one day before flowering would have commenced, inhibits flowering.

By means of other experiments, Hamner and Bonner gained some insight into how plants measure night length. They grew SDPs and LDPs under a variety of light conditions. In some experiments, the dark period was interrupted by a brief exposure to light; in others, the light period was interrupted briefly by darkness. Interruptions of the light period by darkness had no effect on the flowering of either short-day or long-day plants. Even a brief interruption of the dark period by light, however, completely nullified the effect of a long night (Figure 39.12a). An SDP flowered only if the long nights were uninterrupted. An LDP experiencing long nights flowered if those nights were interrupted by exposure to light. Thus, the investigators concluded, these plants must have a timing mechanism that measures the length of a continuous dark period.

The nature of this timing mechanism has been partially revealed, beginning with the determination of the effective wavelengths of light and the identity of the photoreceptors. In the interrupted-night experiments, the most effective wavelengths of light were in the red range (Figure 39.12b), and the effect of a red-light interruption of the night could be fully reversed by a subsequent exposure to far-red light, indicating that a phytochrome is the photoreceptor. Phy-tochromes and blue-light receptors, which affect several aspects of plant development (see Chapter 38), also participate in the photoperiodic timing mechanism.

What might that mechanism consist of? It was once hypothesized that the timing mechanism might simply be the slow conversion of a phytochrome during the night from the Pfr form—produced during the light hours—to the Pr form. Such phytochrome conversion would function as an "hourglass," and the effect of a night would depend simply upon whether all the phytochrome had been converted. However,

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