Photoperiodism and Flowering Time

Many plants flower in response to day length, using light receptors and an internal clock to read the seasons and time reproduction, in one of the classic problems of plant physiology now understood at the molecular level.

Definition

Photoperiodism is the response of plants to the relative length of day and night, and flowering-time control is the integration of these and other cues into the decision to switch from vegetative growth to flowering.

Scope

This topic covers photoperiodism and the classification of plants as long-day, short-day, or day-neutral, the photoreceptors and circadian clock that measure day length, the florigen signal, and the role of vernalization in the genetic control of flowering time.

Core questions

• How do plants measure day length to time flowering?
• What is the nature of the mobile signal that induces flowering?
• How do photoperiod, vernalization, and internal cues combine to control flowering time?

Key theories

Coincidence model of photoperiodism

Day length is sensed when light coincides with a clock-regulated window of sensitivity, so that photoreceptors and the circadian clock together determine whether a plant perceives long or short days.

Florigen as a mobile flowering signal

Leaves perceiving the inductive photoperiod produce a mobile signal, identified as the FT protein, that travels to the shoot apex and triggers the transition to flowering.

Mechanisms

Photoreceptors — phytochromes and cryptochromes — feed light information into the circadian clock, which gates the expression of regulators such as CONSTANS so that flowering-promoting genes are activated only when light falls within a clock-defined window, distinguishing long-day from short-day conditions. Induced leaves produce FT protein, the long-sought florigen, which moves through the phloem to the shoot apex and, with partner proteins, activates floral identity genes. In many species a separate prolonged-cold pathway, vernalization, removes epigenetic repression of flowering, aligning reproduction with spring.

Clinical relevance

Control of flowering time is central to agriculture and horticulture, determining the geographic adaptation of crops, the scheduling of greenhouse production, and the breeding of varieties matched to particular day lengths and growing seasons.

History

Garner and Allard described photoperiodism in 1920, and Chailakhyan proposed the florigen concept in the 1930s; molecular genetics in model plants later identified the photoreceptors, clock components, and the FT protein that embodies the florigen signal.

Key figures

• Wightman Garner
• Harry Allard
• Mikhail Chailakhyan

Seminal works

• taiz2015
• buchanan2015

Frequently asked questions

What is a short-day plant?

A short-day plant flowers when the night length exceeds a critical value — that is, when days are short — even though it is actually the length of uninterrupted darkness that the plant measures; examples include chrysanthemums and many autumn-flowering species.

What is florigen?

Florigen is the mobile flowering signal produced in leaves under inductive day length; long sought, it was identified as the FT protein, which travels to the shoot tip to switch on flowering.