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a natural complex of plant hormones that induce flowering. The term “florigen” was introduced in 1936 by the Soviet plant physiologist M. Kh. Chailakhian.
The existence of the hormone was discovered in experiments on the photoperiodic regulation of flowering. Under favorable conditions in terms of the optimal length of the day, a physiologically active substance that stimulates flowering forms in the leaves of adult plants. Entering the stem buds from the leaves through living tissues, the substance induces the formation of flowers. Experiments with grafts of plants have established that florigen moves from a flowering-plant donor to a vegetative-plant receptor, fostering the flowering of the latter. In the same experiments it was made clear that florigen is not species-specific: it induces flowering in various species and photoperiodic groups of plants.
However, the mechanism of florigen formation and the hormone’s properties have been insufficiently studied, and the complete composition of florigen has not been established. Most widely accepted is Chailakhian’s hypothesis (1958) that florigen consists of two components: gibberellins and the hypothetical anthesins (from the Greek word for flowering— anthesis). A plant flowers only when both components are present. The gibberellins induce the formation of flower stems, and the anthesins induce the formation of the flowers. The flowering of short-day plants is retarded on long days because there is only weak synthesis of anthesins under those conditions. The flowering of long-day plants is retarded on a short day owing to an insufficiency of gibberellins. During a favorable photoperiod, both kinds of plants synthesize the lacking component parts of florigen.
The artificial treatment of plants with gibberellins stimulates flowering in long-day plants, which lack that hormone, whereas it does not influence the flowering of short-day ones, whose development is limited by insufficient anthesins. Hence, the hypothesis of the bicómponent composition of florigen is further supported. In a number of cases the flowering of two vegetative plants with different requirements as to length of day has been achieved by uniting the component parts of florigen contained in them through grafting.
REFERENCESChailakhian, M. Kh. Gormonal’naia teoriia razvitiia rastenii. Moscow-Leningrad, 1937.
Chailakhian, M. Kh. “Gormonal’nye faktory razvitiia rastenii.” Fiziologiia rastenii, 1958, vol. 5, fasc. 6.
Aksenova, N. P., T. V. Bavrina, and T. N. Konstantinova. Tsvetenie i ego fotoperiodicheskaia reguliatsiia. Moscow, 1973.
The Induction of Flowering: Some Case Histories. Edited by L. T. Evans. Ithaca, 1969.
Uince-Prue, D. Photoperiodism in Plants. New York, 1975.
N. P. AKSENOVA