Lecture 22 - UW-Parkside: Help for Personal Homepages

Lecture 22: Photoperiodisms & circadian rhythms
• Photoperiodisms
• Short day & long day flowering plants
• Internal “clocks” / circadian rhythms
• Sleep movements (nyctonastic)
• Cryptochrome involved in circadian rhythms
plants and animals
Reading: Chapter 19
Photoperiodism (daylength) Is Key to timing of
Development over Season(s)
• germinate in spring / early summer
• reproduction and flower production
• senesce leaves (result in fall tree “colors”)
• induction of bud dormancy in perennial plant
(needed to make it through a winter)
• produce tubers for over-winter storage
Most are Phytochrome (Red light) Responses
Day length at 50 o latitude (northern USA)
- general life-events related to daylength

Flowering: Plants generally fall into one of three
(see table 19.1 for more examples)
Long-day plants – Flower in summer when days are long
(LD) Examples, spring wheat & rye
henbane
Short-day plants – Flower in fall when days are short
(SD) Examples, soybean
poinsettia
cocklebur
Day-neutral plants – Flower independent of day length
(DNP) Examples, corn
bean
sunflower
“Critical Daylength” with respect to flowering
It is NOT based on whether plants flower at an
absolute length of day light
Instead, it is based on behavior relative to a critical
daylength
Example, plants that flower when days are shorter
than a critical daylength are “short-day plants”
A Short-day and a Long-day Flowering Plant
- both flower during a ~13 hr daylength
- Yet, these are different types
Fig. 19.3
Henbane
(Long Day)
Cocklebur
(Short Day)

Soybean flowering is controlled by daylength more so
than the number of “days-to-flowering”
- Explains why soybean planting dates are less critical to
farmers than corn planting dates (a DNP)
- Also explains why soybeans are so synchronized in
fall senescence harvesting
Short day
Long day
Morning glory (SD) Henbane (LD)
Fig. 19.1
A Short-day and a Long-day Flowering Plant
- Four plants are same age, exposed to different daylength
Range in Daylength Depends on Latitude
Similar to
Fig. 19.2
- Extreme latitudes have largest change in daylength
Daylength
in hours
Month
Fig. 19.20

Photoperiodism Effects Ecological Distribution
- helps define longitudinal zones
- plant species and variants within species sometimes use
different daylength clues depending on latitude
- common ragweed (Ambrosia artemisifolia) for example:
- an annual SD plant
- critical daylength is 14.5 hr (mid August)
- north of latitude 50 o N, it does not grow
Result: less ragweed pollen allergies in central
and northern Canada
Flower Induction
Fig. 19.20
Induction has occurred when a plant has irreversibly
started floral production (apical meristem starts to
pro-floral organs instead of stem/leaf organs)
Induced plants can be returned to “non floral” conditions
and they will continue to flower
Cocklebur is well studied for its flower induction because
it only takes one short-day exposure to induce flowers
(a good model system)
What is being measured in Photoperiodism?
The day length is NOT being measured
Instead, it is the length of night. Photoperiodisms is a
misnomer (skodoperiodisms may be better)
During long nights some compound is being reduced
below some threshold amount
That compound is the photoreceptor phytochrome!
(Red light response)

Classic Photoperiodism Experiment with Cocklebur (SD)?
hr day (hr night)
In this case, the
Cockleburs flowered
With nights >9 hr
A short interruption of
red light during “night”
“reverses” phenotype
What is happening at night?
Fig. 19.4
• During long nights Pfr (active form of phytochrome) is
gradually disappearing overnight
• This explains why a short pulse of light at night prevents
flowering.
• Red light reforms high concentration of Pfr (resets the level)
Leaves are the site of perception and flowering cues
Perilla & cocklebur are SD plants induced to flower by
treating leaf (youngest) with SD
Fig. 19.5

Flower stimuli are transported through out plants
Fig. 19.6 (grafted cocklebur plants)
Chemical nature of flower-inducing hormone/compound
(florigen?) remains unknown!
Biological Clocks, Circadian Rhythms
These day/night phenotypes are based on internal
clocks and so much direct light responses
These are examples of Nastic movements
Day position Night position
Responses Remain even after day-night light removed
These are endogenous signals that persist over several
cycles even when put in continuous light
Require blue light and cryptochrome, blue-light
photoreceptor. Common to plants and animals.

Examples of Circadian Rhythmic Processes in Plants
(Table 19.2)
• sleep movements (nyctinastic)
• stomatal opening
• Stem growth
• Gas uptake
• gene expression