Responses of plants to their external environment

Plant Timing responses

Plant rhythms

Like animals, plants can exhibit rhythms based on the movement of earth.

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Recall what are the possible rhythms that we have studied so far.

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Daily - circadian

Annual - circannual

Tidal - circatidal

Monthly - circalunar

Half month - circasemilunar

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Plants show both daily rhythms and annual rhythms.

Daily rhythms

Daily rhythms include:

• Opening and closing of flowers
• Some flowers open during the day and close at night, while others open during the night but close during the day.
• Leaf movements (also known as sleep movements)
• Some plants will have their leaves raised/horizontal during the day but drop them to vertical during the night

You tube clips plant movements

Sleep movements in leaves:

http://www.youtube.com/watch?v=sErX8NlVy8I&safe=active

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Flower movements:

http://www.youtube.com/watch?v=g8mr0R3ibPU&safe=active

http://www.youtube.com/watch?v=vqI7mJinlw4&safe=active

Were the movements shown likely to be exogenous or endogenous?

Why? How would we test it?

What was the zeitgeber?

Biological plants- Plants

https://www.pathwayz.org/Tree/Plain/BIOLOGICAL+CLOCKS+-PLANTS

Annual rhythms

Annual rhythms include:

• The production of flowers
• The loss of leaves (in deciduous trees)
• Germination of seeds

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Many of these changes occur in response to a change in photoperiod

• Photoperiodism is the regulation of activity by the length of daylight (the photoperiod)

Advantages Biological Clock

• Enables plants to anticipate environmental changes such as sunrise and maximise photosynthesis
• Enables plants to prepare for periodic environmental extremes (winter, drought etc)
• Enable plants to flower at the same time / when pollinators are available, increasing the chance of pollination

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Photoperiodism in flowering

• Flowering (production of flowers) in plants is brought on by the length of photoperiod.
• The photoperiod is detected by special pigments in the leaves.
• Plants will fall into one of three categories based on the effect of photoperiod on their flowering
• Short-day plants (SDPs): Need a period of long nights before they can flower.
• Long-day plants (LDPs): Need a period of short nights before they can flower
• Day-neutral plants: Flower independently of the length of photoperiod. e.g. carnations and many weeds.

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Critical day length

• SDPs require a photoperiod of less than a certain critical day length (CDL) before they can flower
• e.g. if a species has a 10 hours CDL than it needs a night of 14 hours or more before it will flower.
• What length of night would be req for 11 hours CDL?
• What seasons are these likely to flower in?

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• LDPs require a photoperiod of more than a certain critical day length (CDL) before they can flower
• e.g. if a species has a 14 hours CDL than it needs a night of 10 hours or less before it will flower.
• What length of night would be req for 15 hours CDL?
• What seasons are these likely to flower in?

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Autumn (when days are shortening)

Late Spring and early Summer (when days are lengthening)

13 hours or more

9 hours or less

Phytochrome

The reason plants can respond to a photoperiod, is due to special light sensitive pigments in their leaves called phytochrome.

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Wavelengths of visible light absorbed by phytochrome

Phytochrome absorbs light of specific wavelengths depending on their type.

They are named for the size of wavelength they absorb!

• P665 (or P_red or P_r) absorbs red light and is the inactive form
• P725 (or P_far-red or P_fr) absorbs far-red light and is the biologically active form (it causes a response e.g. germination or control of flowering)

Sunlight contains more red than far-red light.

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Conversion of phytochrome

P_red and P_farred can change from one to another

• When P_red absorbs red light it turns into P_farred (active) which builds up in the plant
• This happens during the day
• P_farred (active) in the absence of red light turns slowly back into P_red
• This happens during the night t
• If P_farred (active) is exposed to far red light the conversion to P_red is more rapid

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Long day vs Short day

Long day plants (LDP):

• Require day to be long enough (photoperiod more than CDL) for P_farred to build up enough to cause flowering.
• P_red ------red light during day---------> P_farred (builds up in plant)

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Short day plants (SDP):

• Require night to be long enough (photoperiod less than CDL) for P_red to build up allowing it to flower (possibly due to absence of P_farred rather than presence P_red)
• P_farred -- absence red light during night --> P_red (builds up in plant slowly)

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Effect of far-red light

Short day plants require a long night to flower.

• They will not flower if their 'long night' is interrupted by red or white light/daylight.
• Because it converts P_red back to P_far-red
• However, if following such exposure they are then exposed to far-red light, this will reverse the effect as it causes rapid conversion of P_far-red to P_red

sciPAD

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