Flower

Many environmental cues signal a plant to produce flowers, and light is one of the most important of these. In many species, flowering is a photoperiodic response, in that it is controlled by the length of the light and dark periods to which the plant is exposed.

Some plants, such as Maryland mammoth tobacco, soybean, and hemp, are short-day plants, in that they flower in the spring and autumn when the days are shorter. Other plants, for example, spinach, mouse ear cress, and fuchsia, are long-day plants, in that they flower in the summer when the days are longer. Some plants, such as cucumber, corn, and garden peas, are day-neutral plants, in that they flower regardless of the daylength. Often, different varieties of the same species have different light requirements for flowering.

The dark period is as crucial as the light period for induction of flowering. In particular, when a short-day plant is exposed to short days, but given a pulse of light during the dark period, flowering is inhibited. When a long-day plant is exposed to short days but given a pulse of light during the dark period, flowering is promoted. Phytochromes are the photoreceptive plant pigments which detect these light pulses. Phytochromes also control other stages of plant growth and development, and phytochrome genes have been cloned and sequenced in many plant species.

Regrettably, plant physiologists have made little additional progress in understanding the mechanism of flower induction in recent years. Little is known about the biochemical reactions that follow from activation of phytochrome or how plants measure photoperiod. This is an area in which future botanists may make great progress.