Effects of Light-Dark Cycles

Plants vary considerably in their responses to light-dark cycles. Floral initiation may be promoted by short days, long days, an appropriate sequence of long and short days, or low temperature (vernalization) followed by an appropriate photoperiod. Some plants remain vegetative indefinitely unless they are exposed to one favorable photoperiod, which permits flowering ever after; others flower more profusely if the photoperiod is appropriate, but the effect is quantitative rather than qualitative. The literature on these phenomena is extensive, complex, and often contradictory, and synthesis of the data to yield accurate generalizations is a formidable challenge. Vince-Prue has not only succeeded in this task but has also provided incisive, original interpretations of much of the material. Time measurement in plants is regulated by the interaction of light absorbed by the pigment phytochrome with an internal oscillator, also called the biological clock. Although the molecular mechanisms of clock and phytochrome action have not been elucidated, Vince-Prue brings the reader up to date with good