Along the equator, the lengths of day and night remain constant because the sun rises and sets at the same time throughout the year.
The lengths of the day and night are also equal (each is six months long) at the exact North and South Poles due to the fact that the sun remains below the horizon for six months each year and above the horizon for the other six months. Any place else in the world, the days become longer in the summer and shorter in the winter.
The reproductive cycles of many organisms, both plant and animal, are regulated by the length of the light and dark period, called the photoperiod. In flowering plants (angiosperms), flowers are organs for sexual reproduction, and photoperiodism refers to the process by which these plants flower in response to the relative lengths of day and night.
The synchronization of reproduction with seasonal time is a very important aspect of plant physiology. Reproduction in many angiosperms is dependent on cross-pollination, the process of pollen being transferred from one flower to another. Hence, it is important for all of the plants of the same species in a given region to flower at the same time.
Even in nonflowering plants such as mosses, ferns, and some algae, it is usually beneficial for reproductive structures to be formed in a given season. The ability to detect the length of the day or night or both makes it possible to synchronize the reproductive event to a particular time of year.
While there have been hundreds of studies which show that many plants respond to changes in the photoperiod, there have been no broad sweeping generalities to provide a better understanding of this phenomenon. Each species, and often each cultivar or variety within a species, appears to have its own photoperiodic response.
The photoperiodic classification of plants is usually made on the basis of flowering, but other aspects of their development may also be affected by day length. Based on their flowering response, plants are classified as short-day plants (SDPs), long-day plants (LDPs), intermediate-day plants, ambiphotoperiodic, or day-neutral plants.
Short-day plants flower when the days are relatively short (generally nine hours or less), such as in the late fall or early winter.
In some SDP species flowering is qualitative, meaning that short days are absolutely required, while in other SDP species flowering is quantitative, which means flowering is accelerated under short days, but short days are not an absolute requirement. Some examples of SDPs include rice, cocklebur, and soybean.
Long-day plants flower when the days are relatively long (generally fifteen hours or greater), as would occur in late spring and early summer. As with SDPs, there are qualitative and quantitative species of LDPs.
Intermediate-day plants require quite narrow day lengths (between twelve and fourteen hours) in order to flower, and flowering is inhibited by either short or long days. Sugarcane is an example of an intermediate-day plant.
Ambiphotoperiodic plants are a specialized group of plants that will flower in either short days or long days, but flowering is inhibited by intermediate day lengths.
In day-neutral plants, flowering is not regulated by day lengths. In other words, day-neutral plants flower regardless of the day length. There are also many interesting interactions between photoperiod and temperature.
A plant may respond to a certain day length at one temperature but exhibit a different response at another temperature. For example, both the poinsettia and morning glory are absolute SDPs at high temperature; however, they are absolute LDPs at low temperature and day-neutral at intermediate temperatures.
Flowering is regulated by chemicals produced in the plant, and a variety of plant hormones, including auxins, ethylene, gibberellins, cytokinins, and abscisic acid, have been shown to influence flowering in different species. The critical aspect of photoperiodism, however, is the measurement of seasonal time by detecting the lengths of day and night.
The discovery of the night break phenomenon, which showed that interruption of the night period with light inhibited flowering in SDPs, established that the length of the dark period is the most critical for initiating a photoperiodic response.
The chemical phytochrome is responsible for measuring the dark period. Phytochrome, found in the leaves of plants, exists in two forms, Pr and Pfr. Pr absorbs red light during the day and is converted to Pfr. Pfr absorbs far-red light during the night and is converted to Pr.
The prevailing hypothesis is that Pfr inhibits flowering, and the length of the dark period has to be sufficient for the Pfr to fall below some critical level. When the Pfr falls below this level, chemical messages are sent to the floral regions, and flowering is initiated.
While phytochrome definitely has been shown to trigger the flowering response, it is not the only chemical involved. It has been shown that a bluelight photoreceptor may also play a role in photoperiodism.
In addition, phytochrome is not translocated in the plant. It remains in the leaves. Hence, other chemicals which have not been positively identified are responsible for signaling the photoperiodic response.