Plant timing responses

• Like animals, plants have both exogenous and endogenous factors that control rhythms.

• Circadian rhythms shown by plants include:a) opening and closing of stomatab) sleep movements, for example,

leaves of beans droop at night

• Circaanual rhythms include: seed germination, flowering and leaf fall. These rhythms are controlled by daylength and temperature.

Flowering and day length

• Flowering plants can broadly be grouped into:

1) short day plants – flowering is initiated when the day length is short, night is long.

2) long day plants – flowering is initiated when the day length is long and the night is short.

3) day neutral – flower occurs within a short life cycle e.g. dandelions

How do plants measure time in response to changing day length?

• Photoperiodism:

• Plants detecting change in day length and responding with according rhythms.

How do plants measure day length?

• The phytochrome system

The ability of plants to respond to the length of day or night is triggered by a light sensitive pigment called phytochrome. This is found in the leaves.

Phytochrome exists in 2 forms corresponding to 2 wavelengths of red (665nm) and far red (725nm) light.

Two forms= Pr (P665) and Pfr (P725)• Pr is the inactive form of phytochrome• Pfr is the active form.

• Plants measure day length by the amount of phytochrome in each form.

FLOWERINGFlowering (as well as a number of other responses) is controlled by the phytochrome system.

LeafDetects light

Flower budBegins

developmentMessage sent with

hormone called Florigen (theoretically).

Phytochrome has 2 forms:

Pr665 is “phytochrome red”Pfr725 is “phytochrome far-red”

Pr665 Pfr725

Fast with red light (from the sun)

Slow overnight

Fast with far red light

• When Pfr accumulates the plant detects that the day is long and the night is short. Long day plants will flower, short day plants will not.

• When Pr accumulates the plant detects that the day is short and the night is long. Short day plants will flower, long day plants will not.

PHYTOCHROME SYSTEM

Pr665 Pfr725

Fast with red light (from the sun)

Slow overnight

Pfr → Pr can also be achieved quickly (for experimental purposes) by using far-red light (hence it’s name).

Pfr promotes flower growth in long-day (summer) plants.

Pfr inhibits flower growth in short-day (winter) plants.

Day neutral plants tend to flower all of the time.

After a short night (summer) there will still be

lots of Pfr left.

After a long night (winter) there will

be lots of Pr.

Fast with far red light

Confused?

DON’T PANIC!

What are critical light periods?

Short day plants and long day plants require a certain amount of sunlight hours to start flowering.

In SDP this is usually less than 12 hours

In LDP this is usually more than 12 hours

• If plants are exposed to natural light, Pr converts to Pfr quickly.

• If plants are exposed to red light, Pr converts to Pfr quickly.

• In the dark, Pfr is slowly converted to Pr.

• If plants are exposed to far-red light, Pfr is rapidly converted to Pr.

Make some predictions

16 hours light 8 hours dark

SDP LDP

What form of phytochrome will accumulate?

•Pfr

What type of plant will flower?

X

Make some predictions

8 hours light 16 hours dark

SDP LDP

What form of phytochrome will accumulate?

•Pr

What type of plant will flower?

X

Make some predictions

What form of phytochrome will accumulate?

•Pfr

What type of plant will flower?

X 8 hours light

SDP LDP

Make some predictions

What form of phytochrome will accumulate?

•P 725

What type of plant will flower?

X 8 hours light

SDP LDP

red

Make some predictions

What form of phytochrome will accumulate?

•P 665

What type of plant will flower?

X8 hours light

SDP LDP

redFar

red

Germination of Lettuce seeds kept in darkness

Germination of Lettuce seeds exposed to red light for 1 min

Germination of Lettuce seeds exposed to red light for 1 min and then far red light for 4 mins

Germination of Lettuce seeds exposed to red light for 1 min, far red light for 4 mins then red light for1 min

• Far red flashed last few seeds germinated

• Red flashed last seeds germinated

What conclusions can you draw from this data?

•That darkness inhibits lettuce seed germination

•That far red light inhibits lettuce seed germination

•That red light promotes lettuce seed germination

Further investigations on the flowering of plants have revealed that :

•Exposure to darkness, red and far red light can affect flowering of plants

•In some plants flowering is inhibited by red light and promoted by far red light

•In some plants flowering is inhibited by far red light and promoted by red light

Florigen – a hypothetical hormone

• The system that detects the length of day/night occurs in the leaves because phytochromes are present in the leaves. In order for flower buds to develop, a “messenger” needs to go to the site of future bud growth. This message is thought to be delivered by a hormone called “florigen”.

Manipulation of photoperiodism by growers

• In controlled environments, it is possible to manipulate flowering of short and long-day plants by flashing light at the appropriate time.

• Flowering plants can be made to flower out of season. In sugar cane, flowering can be delayed so more sugar accumulates before harvest.