19.10: With photoperiodism and dormancy, plants detect and prepare for winter.

Flower production is an energetically expensive task, and because it is so costly, plants benefit by flowering only when it will most effectively enhance their reproductive success. The optimal flowering time differs from species to species, depending on the activity of pollinators, the time it takes for fruits to develop and be dispersed, and a variety of other factors.

Plants must respond not just to environmental changes over the course of a 24-hour cycle but also to seasonal changes over the course of a year. For some plants, the best time to flower is in the spring or summer. For others, it is best to wait until early autumn. In other words, plants need more than just a biological clock—they need a “biological calendar.” When it comes to producing flowers or scaling back metabolic activity and becoming dormant for winter, for example, a plant’s reproductive success or even survival can depend on choosing the right moment (FIGURE 19-23). Timing is everything.

How do plants know when it is spring or summer? What environmental change do they use as a cue to the beginning or end of a season? Temperature? Precipitation levels? Both do tend to change with the seasons—particularly as you get farther and farther from the equator—but they are too variable for a plant to rely on. A plant shouldn’t be fooled by a cloudy spell in summer or an unseasonably dry winter. There is, however, one consistent and unvarying cue to the time of year: the number of hours of daylight or darkness. Plants use one of these cues—the length of the dark period in a day—in a process known as photoperiodism, to regulate their flowering time and numerous other responses to seasonal changes.

All flowering plants fall into one of three categories when it comes to regulating flower production. There are (1) long-day plants, (2) short-day plants, and (3) day-neutral plants (FIGURE 19-24). The groups are so named because the amount of daylight seems to determine when they produce flowers. Long-day plants begin producing flowers only when the length of daylight exceeds a critical amount. Short-day plants are the opposite: they flower only when the length of daylight becomes less than some threshold amount. The critical amounts of daylight occur generally in spring for long-day plants and in late summer or fall for short-day plants, when conditions are best suited for reproduction for the particular species. Day length has no effect on day-neutral plants, which flower when they reach a sufficient state of growth and maturity.

Although short-day and long-day plants are named for the amount of daylight that, apparently, determines when they flower, experiments have shown that short-day and long-day plants are actually sensitive to the length of the night rather than the day. That is, flowering in short-day plants is triggered when the nights become longer than some threshold period, and flowering in long-day plants is triggered by the onset of short nights. This seemingly unimportant distinction had disappointing implications for the California Department of Transportation. The department wanted to line a highway with bright-red flowering poinsettias. Unfortunately, these short-day plants never flowered as expected, because the car headlights on the highway kept interrupting the plants’ measurement of the night length. As a consequence, the poinsettia plants—which require at least 14 uninterrupted hours of darkness if they are to flower—never detected an appropriately long night (and thus short day), and they remained green.

How do plants detect night length? Earlier in this chapter we saw that phototropism occurs as photoreceptors in plants respond to blue light. Photoperiodism, on the other hand, is regulated in large part by plant pigments in the leaves that are sensitive to red and far-red wavelengths of light. The ratio of activated red-sensitive pigments to activated far-red-sensitive pigments varies at different points during the day and night. These pigments also play important roles in regulating seed germination and plant elongation, as well as in several aspects of leaf growth and shape.

Flowering may be disrupted, or its timing altered, in plants cultivated indoors. It can be particularly difficult to trigger flowering in short-day plants, as people who keep poinsettias as houseplants discover. Plants growing under the artificial lights in houses don’t usually experience the long, uninterrupted period of darkness required for blooming. Long-day flowers, on the other hand, are easier to cultivate indoors, because flashes of light or continuous light during the night can trick the plants into behaving as if the night is short, leading to flowering. Day-neutral plants are not influenced by artificial lights.