Controlling when and how vigorously plants flower is a major discovery in horticultural science. Its use has spawned vast industries worldwide supplying flowers and potted plants out-of-season. The control mechanism was uncovered by two American physiologists in the 1920s. Temperate plants inhabit zones where seasonal daylength varies between extending light periods in spring and decreasing ones in autumn.
Those environmental changes result in plants which flower in long-days and those which flower in short-days. ‘Photoperiodism’ was coined as the term describing these events. Extensive subsequent research demonstrated that it is the period of darkness which is crucially important. Short-day plants flower when darkness exceeds a crucial minimum, usually about 12 hours which is typical of autumn. Long-day plants flower when the dark period is shorter than the crucial minimum.
Irises are long day flowers. Image: Geoffery R Dixon
A third group of plants usually coming from tropical zones are day-neutral; flowering is unaffected by day-length. Long-day plants include clover, hollyhock, iris, lettuce, spinach and radish. Gardeners will be familiar with the way lettuce and radish “bolt” in early summer. Short-day plants include: chrysanthemum, goldenrod, poinsettia, soybean and many annual weed species. Day-neutral types include peas, runner and green beans, sweet corn (maize) and sunflower.
Immense research efforts identified a plant pigment, phytochrome as the trigger molecule. This exists in two states, active and inactive and they are converted by receiving red or far-red wavelengths of light.
Sunflowers are day neutral flowers. Image: Geoffery R Dixon
In short-day plants, for example, the active form suppresses flowering but decays into the inactive form with increasing periods of darkness. But a brief flash of light restores the active form and stops flowering. That knowledge underpins businesses supplying cut-flowered chrysanthemums and potted-plants and supplies of poinsettias for Christmas markets. Identifying precise demands of individual cultivars of these crops means that growers can schedule production volumes gearing very precisely for peak markets.
Providing the appropriate photoperiods requires very substantial capital investment. Consequently, there has been a century-long quest for the ‘Holy Grail of Flowering’, a molecule which when sprayed onto crops initiates the flowering process.
Chrysanthemums are short day flowers. Image: Geoffery R Dixon
In 2006 the hormone, florigen, was finally identified and characterised. Biochemists and molecular biologists are now working furiously looking for pathways by which it can be used effectively and provide more efficient flower production in a wider range of species.