Photoperiodism

Photoperiodism is an organism's response to the relative lengths of day and night (i.e., the photoperiod). We have always known that plants are tied to the seasons: each kind of plant forms flowers at about the same time each year; for example, some in spring, some in summer, some in autumn. Botanists knew that plants responded in various ways to temperature and other changes in the environment, but it was not until World War I (1914-18) that anyone tested plant responses to photoperiod. At that time Wightman W. Garner and Henry A. Allard at the U.S. Department of Agriculture in Maryland began to control various parts of the environment in their greenhouses to see if they could make a new hybrid tobacco bloom in summer rather than only in winter. Nothing worked until they put plants into dark cabinets for various times overnight in midsummer. Long nights caused their tobacco plants to flower, and they soon tested other species. They published their results in 1920.

Long-Day, Short-Day, and Day-Neutral Plants

Garner and Allard (and others) discovered that tobacco, soybeans, chrysanthemums, and several other species flowered only when the days were shorter than some maximum length and the nights were longer than some minimum length, with the exact times depending on the species. They called plants with this response "short-day plants." Such plants flower in either spring or fall. Radishes, spinach, a different species than their experimental tobacco, and other species had an opposite response: they flowered only when days were longer than some minimum length and nights were shorter than some maximum length. These are called long-day plants. These plants flower primarily in the summer. Tomato, sunflower, yet another species of tobacco, and several other species formed flowers almost independently of daylength. These are called day-neutral plants.

Later work by other investigators found a very few species, called intermediate-day plants, that flowered only when the days were neither too short nor too long. The opposite is also known: some plants flower on long or short but not on intermediate days. A few species have an absolute photoperiod requirement while others are promoted by some photoperiod but eventually flower without it. Although light intensity sometimes influences the response, typically plants respond not to the amount of light but only to the durations of light and dark. A short-day cocklebur plant (Xanthium strumarium), for example, blooms only when nights are longer than about 8.3 hours, while long-day henbane (Hyoscyamus niger) flowers only when the nights are shorter than about 12 hours.

Although the effective durations of light and dark are typically almost independent of temperature, temperature often influences the type of response. Some species, for example, may require cool temperatures followed by long, warmer days (e.g., sugar beet). Some species may be day-neutral at one temperature and have a photoperiod requirement at another temperature.

Many plant responses in addition to flowering are controlled by photoperiodism. (Animal breeding times, migration times, fur color, and many other phenomena are also influenced by photoperiod.) Photoperiod influences stem lengths, dormancy and leaf fall in autumn, germination of some seeds, tuber and bulb formation, and many other plant manifestations. In flowering, it is the leaf that senses the photoperiod, so some signal must be sent from the leaf to the buds where flowers form. Although numerous attempts have failed to isolate a chemical signal for flower formation—a hormone—most researchers still feel confident that such a so-called florigen must exist.

Measuring Time

The essence of photoperiodism is the measurement of time, the durations of day and night. Early experiments showed that the night was especially important for many species. Interrupting the night with even a brief period of light (seconds to an hour or two, depending on species and light intensity) stops the short-day response or promotes the long-day response. If the total of light plus dark adds up to more or less than twenty-four hours, it is the dark period that seems to be important. More recent experiments, however, show that photoperiod-sensitive plants measure the durations of both day and night. Time measurement in photoperiodism is clearly related to circadian leaf movements and other manifestations of the biological clock.

How do the plants know when it is light or dark? The pigment phytochrome, so important in many plant responses, couples the light environment to the mysterious biological clock. Phytochrome exists in two forms, both of which absorb certain wavelengths (colors) of light. One form, called P_r, absorbs red light, which converts it to the other form, P_fr. P_fr absorbs longer wavelengths of light, called far red, which convert it back to P_r. During the day, red light predominates so most of the pigment is in the P_fr form, signaling to the clock that it is light; the clock measures how long it is light. As it begins to get dark, the P_fr begins to break down, and some of it is spontaneously converted to P_r. This drop in P_fr level signals the clock that it is getting dark, and the clock begins to measure the length of the dark period. When the lengths of both day and night are right for the particular species, the next steps in the response to photoperiod are initiated; for example, florigen may begin to be synthesized.

Much study has gone into understanding these phenomena, and recent work has emphasized the role of specific genes in the flowering process.

Photoperiodism and the Distribution of Plants

Photoperiodism influences the distribution of plants on Earth's surface. As expected, species that require long days for flowering (in spring or summer) occur far from the equator. Short-day species occur in the same regions but flower in late summer. Tropical short-day species also occur, growing only 5° to 20° from the equator. These species detect very small changes in daylength (e.g., one minute per day in March and September at 20° north or south of the equator).

With respect to photoperiod, there can be many ecotypes within a species. For example, the northern ecotypes of short-day cocklebur or lambs-quarters (Chenopodium rubrum) or the long-day alpine sorrel (Oxyria digyna) require longer days and shorter nights to flower than their more southern counterparts. In these examples, the different photoperiod ecotypes within a species are virtually identical in appearance but have different clock settings.

Advantages of Photoperiodism to a Species

The ecotype differences are often clearly of advantage to the species. For example, frost comes much earlier in the year in more northern climates, and the various ecotypes of cocklebur all flower about six to eight weeks before the first killing frost in autumn, allowing time for seed ripening.

Because of photoperiodism, flowering and other responses within an ecotype population of plants are synchronized in time. This is certainly an advantage if the plants require cross pollination; it is essential that all bloom at the same time. Garner and Allard noticed that soybean plants, despite being planted at various times from early spring to early summer, all came into bloom at the same time in late summer. Photoperiodism had made the small plants, which were planted late, flower at almost the same time as the large plants, planted much earlier.

There is much to learn about the ecological importance of photoperiodism. So far, responses of only a few hundred of the approximately three hundred thousand species of flowering plants have been studied.

Frank B. Salisbury

Bibliography

Bernier, George, Jean-Marie Kinet, and Roy M. Sachs. The Physiology of Flowering. Boca Raton, FL: CRC Press, 1981.

Dole, J. M., and W. F. Wilkins. Floriculture, Principles and Species. Upper Saddle River, NJ: Prentice-Hall, 1999.

Garner, W. W., and H. A. Allard. "Effect of the Relative Length of Day and Night and Other Factors of the Environment on Growth and Reproduction in Plants." Journal of Agricultural Research 18 (1920): 871-920.

Halevy, Abraham H., ed. Handbook of Flowering. Boca Raton, FL: CRC Press, 1985.

Salisbury, Frank B., and Cleon Ross. Plant Physiology, 4th ed. Belmont, CA:Wadsworth Publishing Co., 1992.

Thomas, Brian, and Daphne Vince-Prue. Photoperiodism in Plants, 2nd ed. San Diego, CA: Academic Press, 1997.