INTRODUCTION

For flowering plants to cross pollinate, they must produce flowers at the same time of year. How do plants that may be different in size or age synchronize their flowering? In the 1930s, James Bonner and Karl Hamner discovered that the key flowering trigger in some plants is the length of night, which explains why a species will flower during a particular season year after year.

For historical reasons, plants are described as short-day or long-day plants, rather than the more appropriate long-night and short-night plants. Day-neutral plants, in which the flowering trigger does not depend on a specific length of darkness, are perhaps more common than short- or long-day plants. In this animation, we will examine flowering experiments performed on cocklebur, a short-day plant.

ANIMATION SCRIPT

In experiments by Karl Hamner and James Bonner, cocklebur plants were grown continuously on a 16-hour light/ 8-hour dark cycle. Under these conditions, the plants remained in a vegetative, nonflowering state indefinitely.

Reducing the photoperiod by just one hour produced plants that flowered abundantly.

From these results, it is not possible to determine the critical factor in flowering, because the length of darkness and length of light vary together in this experiment.

Hamner and Bonner also experimented with photoperiods that were not on a 24-hour cycle, but extended to 48-hour cycles. In the new treatment, plants received light during the first 16 hours, followed by a 32-hour dark period. Plants flowered under these conditions.

Other plants received treatments on 12-hour cycles, with 4 hours of light followed by 8 hours of darkness. Plants did not flower under these conditions.

From these experiments, it appears that nine hours or more of darkness after a light period triggers flowering.

Hamner and Bonner's experiments pointed to the length of darkness as important in flowering in cocklebur, with 9 hours as the critical length. To test whether a continuous 9 hours is important, they interrupted the dark period with 1 minute of illumination. This treatment resulted in no flowering.

The investigators determined that short-day and long-day plants had a timing mechanism that measures the length of a continuous dark period. Short-day plants require a long night of uninterrupted darkness in order to flower. If the night is short, or if a long night is interrupted by periods of illumination, the plant will not flower. Long-day plants flower when nights are short or when long nights are interrupted by periods of illumination. They do not flower when nights are long.

Both types of plants measure the length of the dark period. The nature of this timing mechanism has been partially revealed and involves a pigment, called phytochrome. Phytochrome exists in two forms—a P_r form and a P_fr form, which refer to red-light absorbing and far-red-light absorbing forms.

Red light converts P_r, which is inactive, into P_fr, the biologically active form. In turn, far-red light converts P_fr back into P_r. P_fr also steadily reverts back to the P_r form at night. Some of the P_fr molecules also decay.

Sunlight contains more red than far-red light. As night begins, phytochrome is almost all in the P_fr form. Over a period of several hours in the dark, these molecules begin to convert back to P_r or to break down. Depending on the length of darkness, only some of the P_fr remains.

If a flash of red light interrupts the long night, the phytochrome molecules reset themselves, converting from P_r back into P_fr.

Night interruptions have opposite effects in short-day and long-day plants. In short-day plants, P_fr inhibits flowering. In long-day plants, it promotes flowering. If the night is interrupted by light, with a final flash of red light, the red light essentially resets the night, creating a large pool of P_fr forms. This form of phytochrome inhibits flowering in short-day plants, but it promotes flowering in long-day plants. A final flash of far-red light essentially eliminates the pool of P_fr. Because P_fr inhibits flowering in short-day plants, short-day plants grown under these conditions flower. Because P_fr promotes flowering in long-day plants, long-day plants grown under these conditions do not flower.

CONCLUSION

How do plants measure the length of a dark night? The photoreceptor molecule, phytochrome, appears to play a key role. The active form of phytochrome—P_fr—promotes flowering in long-day plants, but inhibits flowering in short-day plants.

It was once hypothesized that the timing mechanism might simply be the slow conversion of a phytochrome during the night from the P_fr form—produced during the light hours—to the P_r 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, this suggestion is inconsistent with many experimental observations, such as the fact that when a plant is subjected to a dark period several days in duration, the plant's sensitivity to a light flash during the long night varies on a roughly 24-hour cycle. Such data suggest instead that the phytochrome is only a photoreceptor, and that the timekeeping role is played by a biological clock that is linked to the phytochrome (which sets the clock) and also to the production of flowers.