Time measurement and the control of flowering in plants

Many plants are adapted to flower at particular times of year, to ensure op timal pollination and seed maturation. In these plants flowering is control led by environmental signals that reflect the changing seasons, particularl y daylength and temperature. The response to daylength varies, so that plan ts isolated at higher latitudes tend to flower in response to long daylengt hs of spring and summer, while plants from lower latitudes avoid the extrem e heat of summer by responding to short days. Such responses require a mech anism for measuring time, and the circadian clock that regulates daily rhyt hms in behaviour also acts as the timer in the measurement of daylength. Pl ants from high latitudes often also show an extreme response to temperature called vernalisation in which flowering is repressed until the plant is ex posed to winter temperatures for an extended time. Genetic approaches in Ar abidopsis have identified a number of genes that control vernalisation and daylength responses. These genes are described and models presented for how daylength might regulate flowering by controlling their expression by the circadian clock. (C) 2000 John Wiley & Sons, Inc.