Photoperiodism is a term used to describe the plants various responses to day length, explained here in terms of flowering; other responses include bud dormancy and leaf fall.

Many plant species flower at about the same time each year, e.g. in the UK Magnolia stellata in April, Philadelphus delavayi in June and chrysanthemum in September. In many cases, flowering is in response to the changing day length, which is the most consistent changing environmental factor, in comparison with above-ground temperature which is more variable.

In a day length sensitive species, the flowering process is ' switched on ' by a specific period of daylight (or darkness) called its critical period. In the chrysanthemum the critical period is sixteen hours of daylight (or eight hours of darkness), which occurs in September in the UK. If repeated over several weeks, the internal structure of the buds begins to change from a vegetative meristem to a flowering meristem (see p00).

The Phytochrome'Switch'

Since 1920 much research has attempted to explain the photoperiodic flowering response, including using artificial lighting and investigating genetic and biochemical control. Recently, the following stages have been identified, namely switching on at the leaf, mobilizing leaf genes, moving the message from leaf to bud, and developing a flowering meristem. The first stage represents one of the best examples of the horticulturist manipulating the biology of the plant and an understanding of the science has enabled the grower to control the flowering process with the consequent valuable worldwide industry of year-round flower production. Phytochrome is the chemical produced by the plant to operate the switching mechanism.

Phytochrome is a large blue-coloured molecule (molecular weight about 125 000). It is made up of two relatively small colour-sensitive sub-molecules (chromophores) and two very long protein chains. It is thought that the chromophores change their shape in response to light, and that this vital 'day-length message' is passed through the proteins to the next stage in the flowering sequence described. Investigations of phytochrome suggest that, in addition to its involvement in the flowering stimulus, the chemical is used in as many as 24 other light-induced reactions ranging from opening a seed's plumule hook as it emerges from the soil (see p154) to increasing the respiration rate of cells.

A two-way chemical process is involved, requiring a different light colour for each direction. Phytochrome Pr660 is sensitive to red light of wavelength 660 nm, found in daylight from dawn to dusk. Pr660 is changed to a less stable form of phytochrome (Pfr730) after a days' exposure. Pfr730 refers to phytochrome as far red light, is found in shaded conditions and is the form which brings about the plant response.

Day length sensitive plants respond to changing seasons as either long day plants or short day plants. In species such as Hosta, sweet pea, Lobelia and radish, long days are essential for flowering, while the flowering of carnations and snapdragons, among others, is improved. In these species, the presence of Pfr730 in a concentration above a critical limit results in the promotion of flowering, because the summer nights are not long enough to allow sufficient Pfr730 to revert back to Pr660. A more accurate term here therefore would be 'short night plant ' .

  • Day neutral species are switched on to flowering by a range of situations involving plant size and development and temperature, e.g. Begonia elatior and tomato.
  • Short day plants, e.g. chrysanthemum, poinsettia and kalanchoe, respond differently in that the presence of Pfr730 above a critical limit inhibits flowering. In chrysanthemum the critical period of dark is eight hours and this condition over a period of several weeks will induce flowering. To enable all-year-round production as cut flowers and pot plants, the day length manipulation is sophisticated. Immature plants must initially be prevented from flowering, then flower buds must later be induced, often at a time of year when the natural day length would not be suitable.

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