One major role of environmental signals is to control the timing of the production of florigen and antiflorigen. This link between environmental and internal signals has been most clearly established for photoperiod. The role of day length in the regulation of flowering had been recognized by 1913. The impact of photoperiod on flowering in numerous species soon became apparent.
In the 1930's W. W. Garner and H. A. Allard found an unusually large tobacco plant growing in a field. The plant stood out because it failed to flower, they named it the Maryland Mammoth. Maryland Mammoth cuttings flowered in a greenhouse that December, and subsequent experimentation demonstrated that flowering would occur only when days were short and nights long. The Maryland Mammoth is an example of a short-day plant. Short-day plants generally flower in the spring or fall, when day lengths are shorter. Other examples of short-day plants are poinsettias, cockleburs, Japanese morning glories, and chrysanthemums. Plants such as spinach, lettuce, and henbane will flower only if a critical day length is exceeded, they are categorized as long-day plantsand generally flower during long summer days.
Photoperiodic control mechanisms may be more complex, as in the case of ivy, a short-and-long-day plant, which requires at least a twelve-hour photo-period followed by a photoperiod of at least sixteen hours. Still other plants, including sunflowers and maize, are day-neutral. They flower independent of photoperiod. By the 1940's it was established that night length, not day length, is critical in the photo-periodic control of flowering. For example, flowering in the short-day Japanese morning glory can be prevented by a brief flash of light during the critical long night. In comparing short-day and long-day plants, the distinguishing factor is not the absolute length of night required, rather, the difference is whether that night length provides the minimum (short-day plants) or maximum (long-day plants) period of darkness required to permit flowering.
How a plant perceives night length and translates this into the appropriate response in terms of flowering is not fully understood. A pigment known as phytochrome, however, plays a critical role. Phytochrome exists in two forms (Pr and Pfr) that are interconvertible. Pr absorbs red light and is converted to Pfr which absorbs far-red light and is subsequently converted back to the Pr form of the pigment. Sunlight contains both red and far-red light, and thus equilibrium between the two forms is achieved. At noon, about 60 percent of the phytochrome is in the Pfr form. In the dark, some Pfr reverts to Pr, and some breaks down. Because of the absence of red light, no new Pfr is generated.
The relationship between phytochrome and photoperiodic control of flowering has been established using night-break experiments with red and far-red light. (In these experiments, darkness is interrupted by momentary exposure to light.) Flowering in the Japanese morning glory, a short-day plant, can be inhibited by a flash of red light (as well as light equivalent to sunlight) in the middle of a long night. Far-red light has no effect. A flash of far-red light following a flash of red negates the inhibitory effect of the red light. In long-day plants, flowering can be induced when the dark period exceeds the critical night length with a red-light night break. Farred light flashes do not result in flowering. The effect of the red flash can be negated by a subsequent far-red flash. In these experiments, the light flashes alter the relative amounts of Pr and Pfr.
See also: Circadian Rhythms, Temperature, Genetic Control of Flowering
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