4. Plant Orientation in Time

Orientation in Space

Orientation in Time

Animal Orientation in Time

Species Interactions

Concept 14: Photoperiodism and Flowering

Success Criteria & Vocabulary

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Critical day length (CDL): The period of light required in a 24-hour cycle to induce flowering of long-day plants or to inhibit flowering of short-day plants.

Dormancy: A period of arrested growth and development. 

Long-day plant (LDP): A plant that flowers when daylength is more than a critical value (~12 hours).

Perennial: A plant that lives for several years.

Period: Length of time/duration

Photoperiod: The period of time each day during which an organism receives illumination; day length.

Photoperiodism: The physiological reaction of plants and animals to the presence and absence of light.

Phytochrome: A pigment in plants responsible for the photoperiodism effect. Regulates the timing of flowering with different effects in long day and short day plants.

Phytochrome far red (Pfr): The active form of phytochrome.

Phytochrome red (Pr): The inactive form of phytochrome. 

Short-day plant (SDP): A plant that flowers when daylength is less than a critical value (~12 hours).

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Concept 14: Support Notes

What is Photoperiodism?

All parts of the Earth experience seasonal changes in climate that have a direct effect on organisms (both plants and animals). Most organisms are able to anticipate such changes by preparing in advance for the. To do so, organisms must be able to 'know' what time of year it is. 

The most reliable seasonal cue is day length or PHOTOPERIOD (except in the tropical regions where it varies very little throughout the year). Most organisms can respond to photoperiod, an ability called photoperiodism.

A wide variety of adaptations to seasonal change involve PHOTOPERIODISM, for example:

Flowering time in many plants.

Stimulus to migrate in many birds.

Production of winter buds in many temperate tree species.

Laying down fat prior to hibernation in many mammals. 

Production of tubers in potato plants.

Production of winter coat in Arctic fox and other arctic mammals. 

From now on, we will be focusing on photoperiodism in plants, and the adaptation of flowering time. 

Types of flowering in plants

Flowering plants can be grouped into three categories, according to their flowering requirement:

1) Short-day plants (SDP)

SHORT-DAY PLANTS need a PHOTOPERIOD less than a critical value, the CRITICAL DAY LENGTH (CDL). In other words, SDP flower in autumn, when days are shortening. Examples are oat, ryegrass, clover, and soy bean.

It is worth nothing that many spring-flowering PERENNIALS actually produce flower buds the previous autumn, for example tulips, daffodil, and crocus. The buds remain DORMANT during the winter, and expand the following spring. 

2) Long-day plants (LDP)

LONG-DAY PLANTS need a photoperiod greater than a critical value. In other words, LDPs flower when the days are lengthening, in late spring and early summer. Examples are strawberry and tobacco. 

3) Day-neutral plants

In these plants, flowering is insensitive to photoperiod. Examples are cucumber and tomato.

What is the plant actually measuring?

Night length is actually the important factor (not day length)

Although we classify plants as SHORT-DAY or LONG-DAY, plants are actually measuring the length of the night. In other words, it is the length of the PERIOD of continuous darkness, not the length of the period of continuous light, that determines whether or not the plant flowers.

When a SDP is exposed to short days with the middle of the night interrupted by a few minutes of light, flowering is prevented. Similarly, when a long day plant is exposed to short days with the middle of the night interrupted by a few minutes of light, flowering is induced. 

So it is not actually the day length, but uninterrupted night length, that is important in inducing flowering. 

Photoperiodism is controlled by the Phytochrome System

PHOTOPERIODISM it is controlled by the protein pigment called PHYTOCHROME. Phytochrome exists in two photo-reversible two forms: Pr and Pfr. 

I highly recommend that you watch this video below.

Pr (phytochrome red) is the inactive form of phytochrome. The 'r' indicates that it absorbs red light very strongly. When Pr is exposed to red light during the day (sunlight has a lot of red light), it absorbs it and rapidly gets converted into Pfr. In fact, this occurs so rapidly that all of the phytochrome is converted into its active state regardless of day length. 

Pfr (phytochrome far-red) is the active form of phytochrome. The 'fr' indicates that it absorbs far-red light very strongly. When Pfr is exposed to far-red light, it absorbs it and gets converted back to Pr. However, there is more red than far-red light in sunlight, so ultimately, more Pfr (active) accumulates throughout the day than Pr (inactive).

In the darkness of the night, the active Pfr is very slowly converted back into inactive Pr. But because this conversion is so slow, inactive Pr can only accumulate if the plant experiences a long uninterrupted period of darkness.