1. Plant Timing Responses

2. Plant Timing Responses

3. Biological Plant Rhythms
Plants show many rhythms, just as animals do.
These are controlled by exogenous or endogenous factors, such as a biological clock or timing mechanism.
Some plants show daily or circadian rhythms e.g. Opening and closing of stomata, sleep movements of the leaves of bean plants, and many flowers open during the day and close at night.
Bean leaf simulation
Endogenous rhythms are genetically based.

5. Annual Rhythms
Plants can show annual rhythms such as; seed germination, flowering, and leaf fall.
These may be controlled by factors such as temperature and day length.
A plant will only respond to changing day length if it has a mechanism to measure it.

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7. Photoperiodism
The regulation of plant processes by changing day length.

8. Flowering plants can be broadly grouped into
1. Short-day Plants: Length of daylight must be shorter than a critical length in order for the plant to flower.
2. Long-day Plants: Length of daylight must be longer than a critical length in order for the plant to flower.
3. Day-neutral: These flower regardless of day length as long as they have enough light for normal growth (often native to tropics).

9. Important points!
Plants actually measure the length of the dark period NOT the period of light.
Even a bright flash of light during the dark period will break it.
The leaves actually detect the period of darkness and then send a message to the flower bud via the hormone florigen.
In long day plants Pfr induces flowering.
In short day plants Pfr inhibits flowering.
Can you explain why??

10. Photoperiodism
The regulation of plant processes by changing day length.
a pigment call phytochrome measures the time that has passed.
Phytochromes exist in two forms
Pr absorbs red (R; 660 nm) light
Pfr absorbs far red (FR; 725 nm) light
Pr absorbs red light and converts into Pfr
Pfr absorbs far red light and converts into Pr.
In the dark, Pfr spontaneously converts back to Pr.
Sunlight is richer in red (660 nm) than far red (730 nm) light so at sundown, all the phytochrome is Pfr.
During the night, the Pfr converts back to Pr.

11. If this process is interrupted by a flash of 660-nm light, the Pr is immediately reconverted to Pfr and the night's work is undone .
A subsequent exposure to far red (730 nm) light converts the pigment back to Pr and the steps leading to the release of florigen can be completed.

12. Phytochrome has Active (Pfr) and Inactive (Pr) Forms
daylight
Pfr reverts slowly to Pr in the dark
There is little far-red light in sunlight
Pr is inactive, absorbs
PROMOTES OR
INHIBITS GROWTH
Absorbed (Pfr is active)
Photoperiodism simulation

14. poinsettias

15. A. B. 1.What type of flowering plants are these?
2. Estimate each examples critical day length.

16. Other Phytochrome Responses
Pigment formation.
Leaf expansion.
Sleep movements of leaves.
Some small seeds will only germinate in the light (Germination is promoted by the Pfr produced in sunlight or red light).
Why would this response be an advantage?
Shade avoidance response or etiolation occur in the dark or in absence of red light. The plant grows long and thin until they reach light.

17. Other environmental cues and plant responses.
Dormancy and its Causes.
Some Viable seeds do not germinate even when there is water, oxygen and a suitable temperature.
Dormancy can be regulated by the environment or by the seed.
Seed regulated dormancy comes in two forms: 1) dormancy caused by external or physical factors e.g. impervious seed coat
2) Dormancy caused by internal or chemical factors.

18. Scarification.
This is when the impervious seed coat is broken so that germination can occur.
Scarification may be done using
Acid or hot water
Abrasion
Passing through the digestive tract of an animal.
Decomposition of seed coat by soil organisms
Fire (in some cases).
Exposure to moist chilling (Stratification). The seed must spend time at or near freezing temperature. This ensures that the seed germinates in spring rather than autumn when it is produced.

19. Vernalisation Temperature can also be a cue for flowering.
Some plants will not flower unless they experience a cold spell e.g. Lilac, stone fruit.

20. Key Point Summary
Plants show many biological rhythms. Some are controlled exogenously, some endogenously.
Phytochromes are involved in many plant processes.
Phytochromes exist in two forms
Pr (absorbs red light) -- Pfr (absorbs far red light)
In many plants, Photoperiodism determines flowering (night length). Phytochrome is involved as an on/off switch.
Other environmental cues affect plants e.g. seed dormancy and stratification or scarification.
Some plants need a cold spell to flower (vernalisation)
Plants show many circadian rhythms such as sleep movements of flowers and leaves.