Photoperiodism is the mechanism plants use to sense the seasons, which allows them to time important developmental events like flowering. This biological response is dictated by the relative lengths of the light and dark periods within a 24-hour cycle. Short-day plants (SDPs) are those that initiate flowering when the days become short, which corresponds naturally to the late summer and fall seasons.
The Critical Role of Night Length
The name “short-day plant” is actually misleading because the true trigger for flowering is not the short duration of light, but rather the continuous, uninterrupted duration of darkness. These plants must experience a night that exceeds a specific time frame, known as the critical night length. This required dark period can vary significantly, but it often needs to be 12 to 14 hours long or more, depending on the plant species.
If the long night is interrupted by even a brief flash of light, the timing mechanism is reset. An interruption as short as a few minutes of artificial light, such as from a streetlamp or a window, can prevent the plant from flowering completely. If the dark period is continuous and long enough to surpass the critical night length, the signal for flowering is produced. This requirement is why growers can manipulate the flowering of SDPs, such as chrysanthemums and poinsettias, by artificially extending the period of darkness.
The Biological Mechanism of Flowering
Plants measure the length of darkness using a light-sensing protein pigment system called phytochrome. This system acts as a biological switch, existing in two interconvertible forms: Pr and Pfr. The Pr form absorbs red light and is rapidly converted into the Pfr form during the day.
The Pfr form is the biologically active state of the pigment, and it is inhibitory to flowering in short-day plants. During the long, uninterrupted night, the Pfr form slowly and steadily converts back into the inactive Pr form. Only when the concentration of Pfr drops below a certain low threshold, indicating that the night was long enough, is the flowering process initiated.
Once the critical night length is met and the phytochrome switch is flipped, a mobile signal molecule is synthesized, primarily in the leaves. This signal, historically referred to as florigen, is a protein that travels from the leaves through the plant’s vascular system to the shoot apical meristem. Upon reaching the meristem, this protein induces the genetic and structural changes necessary for the formation of a flower bud.
Comparing Short-Day Plants to Other Types
Long-day plants (LDPs) are essentially the opposite of SDPs, as they require a light period longer than a critical threshold to flower. For LDPs, which typically bloom in late spring and summer, a short night is the necessary trigger for reproduction. Examples of LDPs include many common garden vegetables like spinach, lettuce, and radishes, which flower when the days are longest. These plants use the same phytochrome system, but the genetic response to the Pfr level is reversed. For LDPs, a high level of the active Pfr form of phytochrome, maintained by long daylight exposure, actually promotes flowering.
The third group is known as day-neutral plants (DNPs), which do not rely on the length of the light or dark period to initiate flowering. DNPs flower once they have reached a certain stage of maturity or when other environmental factors are met, such as adequate moisture or temperature. Corn, tomatoes, and cucumbers are common examples of day-neutral plants.
Short-day plants like rice and soybeans flower as days shorten, while LDPs like oats and clover flower when days lengthen. Day-neutral plants possess a photoperiodic mechanism that is essentially uncoupled from the flowering signal.