ABSTRACT
Photoperiod, the daily cycle of day and night, is one of the most fundamental environmental signals for living organisms. Photoperiods regulate numerous biological events across all life forms, including cell division in yeast, diapause of insects, and coat color and sexual behavior of birds and animals (Classen et al. 1991, Danilevskii 1965, Eppley et al. 1967, Hoffmann 1973). In humans, disruptions of the photoperiod cycles result in metabolic and psychological disorders. Photoperiodism, a word derived from the Greek words for “light” and “duration of time,” refers to the developmental and physiological reactions of organisms to the length of day and night, enabling them to adapt to seasonal changes in their environment. Photoperiodism thus may imply the ability and the means of organisms not only to recognize but also to memorize and anticipate seasonal changes in the length of day and night. Such mechanisms are especially important for organisms in the temperate zones, where a “clock” mechanism controlling seasonal developmental and physiological alternatives apparently measures the length of day and night (Saunders 2002). The mechanisms of photoperiodism are studied particularly well in plants. Photoperiodism involves various aspects of plant
4.1 Introduction .......................................................................................................................... 121 4.2 Photoperiodic Flowering Research in the Premolecular Era ............................................... 122
4.2.1 Photoperiodism in Plants .......................................................................................... 122 4.2.2 Photoperiodic Time Measurement ............................................................................ 122
4.2.2.1 Plants Measure the Duration of Darkness ................................................. 122 4.2.2.2 External Coincidence Model ..................................................................... 123
4.2.3 Florigen ..................................................................................................................... 123 4.3 Circadian Clock in Plants .....................................................................................................124 4.4 Photoreceptors in Photoperiod ............................................................................................. 125
4.4.1 Phytochromes ........................................................................................................... 125 4.4.2 Cryptochromes ......................................................................................................... 126 4.4.3 Zeitlupe Family ......................................................................................................... 127
4.5 Molecular Mechanism of Photoperiodic Flowering in Plants .............................................. 128 4.5.1 Arabidopsis ............................................................................................................... 128 4.5.2 Rice ........................................................................................................................... 129 4.5.3 Morning Glory .......................................................................................................... 130 4.5.4 Soybean ..................................................................................................................... 131 4.5.5 Sugar Beet ................................................................................................................. 132 4.5.6 Poplar ........................................................................................................................ 132 4.5.7 Sunower .................................................................................................................. 133
4.6 Conclusions ........................................................................................................................... 133 References ...................................................................................................................................... 134
development, including seedling growth, dormancy, tuberization, leaf fall, and owering induction. This chapter introduces the pioneering work and the recent progress in photoperiodism studies in plants with a specic focus on owering, plants’ decision to switch to reproductive growth from vegetative growth in response to seasonal changes in photoperiods.
