Photoperiod response

Photoperiod describes the duration of daylight in the daily cycle, and the term photoperiodism describes an organism’s ability to detect changes in daylength. Photoperiodism was first described in detail by Garner and Allard, (1920) in studies on the differences in the control of flowering in tobacco varieties. A plants ability to measure photoperiod enables it to monitor seasonal changes, and to respond to them. Apart from the control of flowering, many other aspects of plant development are controlled by photoperiodism; these include asexual reproduction, onset of dormancy and the formation of storage organs (Thomas and Vince-Prue, 1997).

Photoperiods - short day and long day flowering responses

Plants can be categorised by their flowering response to different photoperiods. Short-day plants, which include rice (Oryza sativa) and Pharbitis, flower earlier when the photoperiod is shorter than a critical daylength. In contrast, long-day plants, which include Arabidopsis and wheat (Triticum aestivum), flower earlier when the photoperiod is longer than a critical daylength. Although described as short-day plants they are actually long-night plants, as night break experiments have demonstrated that these plants measure the duration of darkness as opposed to the duration of light (Hamner, 1940; Samach and Coupland, 2000; Taiz and Zeiger, 2002) . For long-day plants it is not so clear whether daylength or night length is measured (Thomas and Vince-Prue, 1997).

Non photoperiod responsive plants

Day-neutral plants represent a further class of photoperiod response type. Unlike short and long-day plants adapted to life at high latitudes, day-neutral plants evolved close to the equator where there is little change in daylength throughout the year. They therefore rely upon autonomous signals for floral initiation, and include species such as kidney bean (Phaseolus vulgaris) and desert plants such as desert sand verbana (Arbronia villosa), which respond quickly to rainfall (Taiz and Zeiger, 2002) .

In the photoperiod response of Arabidopsis, long days (16 hours light) promote flowering whereas short days (8-10 hours light) repress it (Martinez-Zapater, 1994) . An Arabidopsis plant will eventually flower regardless of photoperiod, and it is therefore described as a facultative long-day plant.

The two accessions of Arabidopsis mainly used in the research laboratory are Landsberg erecta (Ler) and Columbia ( Col ). Ler typically flowers after three weeks growth under long-days after forming 6-7 rosette leaves. When grown under short days flowering occurs much later, typically after 6 weeks growth and the formation of more than 30 rosette leaves. In addition to an increase in the number of rosette leaves, other developmental growth stages are prolonged when plants are grown under short-days, which leads to an increased number of cauline leaves and flowers (Schultz and Haughn, 1993) . The total number of leaves on a plant acts as a measure of flowering time independent of the growth rate (Koornneef et al., 1991) . The transfer of plants grown in short days into long days results in a rapid promotion of flowering, with exposure to a single long day being sufficient to promote flowering in the Columbia ecotype (Corbesier et al., 1996; Bradley et al., 1997; Onouchi et al., 2000).

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