How do plants know when to flower?

The The Phytochrome Phytochrome SystemSystem

Flowering of plants is Flowering of plants is determined in part by determined in part by the length of the day.the length of the day.

long days & short nights

short days & long nights

Plants are of three general Plants are of three general types: types: ExampleExample

short-dayshort-day

long-daylong-day

day-neutralday-neutral

strawberry

sunflower

spinach

Short-day plants flower Short-day plants flower in spring or autumn in spring or autumn when the day lengths when the day lengths are shorter than the are shorter than the critical day length..critical day length.. For example, the common cocklebur (Xanthium strumarium) is induced to flower only when exposed to 16 hours or less of light.

Examples of short-day Examples of short-day plants :plants :

poinsettiaspoinsettias

strawberriestrawberriess

primrosesprimroses..

Long day plants flower Long day plants flower chiefly in the summer chiefly in the summer and will flower only when and will flower only when exposed to light for exposed to light for longer than a critical longer than a critical length.length.

Spinach has a Spinach has a critical critical daylength of 13 daylength of 13 hours.hours.

Examples of long-day Examples of long-day plantsplants

spinachspinach

lettucelettuce

henbanhenbanee

What determines if What determines if plants are long or short-plants are long or short-day is not so much the day is not so much the length of the exposure length of the exposure time but whether it is time but whether it is longer or shorter than longer or shorter than some critical interval. some critical interval.

Day-neutral plants flower Day-neutral plants flower without respect to without respect to daylength. daylength.

Examples of day-neutral Examples of day-neutral plantsplants

corncorn sunflowsunflowerer

cucumbcucumberer

cucumbecucumberr

Plants contain a Plants contain a photoreceptor called photoreceptor called

phytochrome (P)phytochrome (P) which comes in two forms which comes in two forms which are which are

interconvertible, interconvertible, PrPr and and

PfrPfr.. PrPr PfrPfr

Pr Pr absorbs red absorbs red light andlight and Pfr Pfr absorbs far-red absorbs far-red light.light.

Visible light lies between 400 and 700 nm.

660 nm

730nm

Red light

Far Red light

The Electromagnetic spectrum

Pr

Pfr

Dark reversion

Physiological response

Far-red light

Red light

Light induced synthesis

Slowly reverts during the night.

Straight away

Straight away