Photoperiodism is a biological response to a change in the proportions of light and dark in a 24‐hour daily cycle. Plants use it to measure the seasons and to coordinate seasonal events such as flowering.
Plants make such adjustments by utilizing the pigment phytochrome, which exists in two forms: P r, which absorbs red light, and P fr, which absorbs far‐red light. Each can convert to the other when they absorb light. During the day, the two forms convert back and forth (P r becomes P fr, and vice versa), until they reach an equilibrium of 60:40 P fr: P r in plant tissues. During the night, P fr slowly converts to P r or else disintegrates. P r is stable in the dark.
P fr is the biologically active form, acting as the switch that turns on such plant responses as flowering or seed germination. When the threshold concentration of P fr is attained, the response is stimulated. Thus, it is the length of the night period, not the day period, that determines the response. Short nights (meaning long days) favor activities that require large amounts of P fr; conversely, if the night is long (and the day short), more P fr is converted back to P r and responses triggered by small amounts of P fr are favored. P r, synthesized from amino acids, is the inactive form.
Photoperiodism was first studied in relation to flowering. Plants can be described in relation to their photoperiod responses as short‐day, long‐day, day‐neutral, and intermediate‐day plants. Plants that flower in late summer and fall are short‐day; they have a critical period of light exposure of less than about 16 hours. Long‐day plants are summer flowering and have a critical period of longer than 9 to 16 hours. Day‐neutral plants flower in photoperiods of any length, while intermediate‐day plants flower only in periods neither too long nor too short for the particular plant (that amount of time is different for each plant studied to date but not classifiable as either long‐day or short‐day).
Other photoperiodic responses involving the phytochrome system include seed germination and the early growth of seedlings.
Because hormones control so many metabolic activities in plants, flowering has long seemed likely to be under the control of one or more hormones. Early experiments sought to determine which part of a plant is sensitive to the light that initiates flowering. The results suggested the presence of a substance that moved from the leaves to the flower buds. Although the substance was not identified then—nor has it been isolated now—it was named florigen. Florigen is the hypothetical flowering hormone; it may or may not actually exist. Note that flowering most likely is not controlled by a single hormone, but is the result of a combination of internal and external signals and responses.