Seasonal Behaviour in Plants

Photoperiodism in Plants: Flowering

• Photoperiodism: regulation of seasonal activity by day length (photoperiod)

• Garner & Allard’s (1920) hybrid tobacco plants:– Wouldn’t flower outdoors even if 2-3m in height (ie

mature enough to flower)– Young plants would flower well in winter if kept warm

enough– Experiments with light intensity, temperature showed

day length was important (the hybrid tobacco needed short days to cause flowering

Garner & Allard’s three flowering groups:• Long day plants (LDP)– Flower when photoperiod > a certain value (they called

this critical day length or CDL)– LDP flower when days lengthening, getting longer than

CDL (ie spring, early summer)– LDP mostly found in higher latitudes (where day length

varies a lot)– Examples: grasses, radish

• Short day plants (SDP)– Flower when photoperiod < CDL– SDP flower when days shortening (autumn)– Examples chrysanthemum, strawberry

LDP flower when day length increasing (above CDL value)SDP flower when day length reduces below CDL

• Day-neutral plants– Insensitive to day length– Examples: ephemeral plants (life cycle too short

for season changes to be important AND dandelion, tomato

BUT: photoperiodic requirements may be affected by plant age, temperature…

Night length is what counts!Normally:

But, if long night interrupted with a few minutes of light:

(SDP stopped from flowering, LDP induced to flowerIf long day interrupted with darkness:

(no effect)

So, night length is the real factor. A bit moot as in real world a long night will always = a short day (SDP will flower)

Non 24 hour cycles…Cocklebur ( a SDP) given:

4h light: 8h darkNo flowering. But 4 hours <<<CDL of 15.5hThen given:

16h light: 32h darkFlowers! Even though “day” is > CDL of 15.h

Conclude: night length is more importantShort day plants are really long night plants, Night length muster be longer than a certain period (and vice versa for LDP)

Even this is not the full story – flowering probably a complicated example of an endogenous rhythm.

Leaves detect the flowering stimulus

Flowering Stimulus Detection

• In experiments* Chrysanthemums (an SDP):– Flower when leaves only are given short days– Don’t flower when leaves are given long days– Stem apices (where flower buds grow) seem to be

insensitive to photoperiod.

* Diagram previous slide

What is the flowering signal?

Flowering signal is same in SDP & LDP

• Closely related SDP & LDP grafted*• Regardless of photoperiod both parts flower

(flower stimulus same for both plants)• Stimulus remains unknown but:– Probably a combination of chemicals– Is carried by living phloem tissue (temperature

and energy levels affect flowering)

* Diagram previous slide

Flowering involves phytochrome

Photoperiodic induction of flowering involves phytochrome

• Remember that a brief period of light in long night reverses photoperiodic responses…– SDP: flowering halted– LDP: induced to flower

• Obvious question “What wavelength of light is most effective?”

(Knowing this may tell us which pigment is absorbing the light, causing the flowering response)• Results*:

- Red light is most effective- AND far-red (infra red) is most effective at reversing effect of red.

* Diagram previous slide

• Therefore:– Pigment is blue – Phytochrome fits as a candidate as it is blue and also known that a

red/far-red reversal affects other processes involving phytochrome

• The two phytochrome states:– Pr absorbs red light of 660nm, converts to Pfr– Pfr absorbs far-red light of 725nm, converts to Pr

• In the real world:– Sunlight has red and far-red (forward and back reactions occur),– BUT sunlight has more red than far red, so red light effects dominate (Pfr

builds up)

Graph shows:Pr absorbs 660nm red light wellPfr absorbs 725nm far-red light well

Is phytochrome really involved?• If so, then red light effect should be reversed if

followed by far-red (it is!) and reversed again by red (it is!)

Furthermore: the best light to reverse effect of red is 725nm (characteristic of Pfr)

So, how is all this used to measure photoperiod…

…and by extrapolation night length (the real cue for flowering)?• Pfr reverts to Pr slowly at night:

• Could conversion of Pfr to Pr act as a clock?- and the effect of red light burst in night resets this?

NO!

Because:• Pfr Pr is too quick, only 2-3 hours (night could be 6-16

hours long)• Pfr Pr is temperature sensitive (and we know that CDL

isn’t)

Actual mechanism:• Unknown (sigh!) but probably involves an internal clock.

- Experiment: Soybean (SDP) grown in 8h:64h light dark regime with night interruption at varying intervals

- Result: effectiveness of night interruption (at stopping flowering) shows peaks at circadian intervals

Other Photoperiodism Responses in Plants

Bud DormancyProcess in temperate trees (eg sycamore) where bud formation is triggered by shortening days but buds remain small and undeveloped until this dormancy is broken (by a period of chilling ie winter) and further development continues.

Maintained by asbcisic acid (ABA). Chilling increases giberellin which stimulates growth (ABA levels drop too).

Without chilling plant may be late to resume growth in spring.

Q: What happens in a mild winter?Q: What happens in Northland?Q: What happens in the tropics?

• Leaf fall

Leaf Fall (Abscission)In temperate regions ground frozen in winter, can’t draw water into roots to meet demands of photosynthesis and transpiration at leaves.Solution: store starch in stem (for next spring), lose leaves.Leaf fall caused by leaf aging and short days – trees near street lamps lose leaves later as the light artificially extends the day.

Few NZ trees lose leaves. In those that do low temperature is the trigger.

Leaves can’t just drop off (the open tissue would get fungal infections). Abscission precedes leaf fall – a complex process where plant resorbs leaf nutrients (proteins, chlorophyll) and a scar of cork forms, sealing the wound.Tubers in potatoes (short days)

Other photoperiodic responses:Bulb formation in onions (long days)

Runners in strawberries (long days)

Other roles of phytochrome

• GerminationSmall seeds only germinate in light (they would run out of food if buried deeply). Red most effective at causing germination, Far red inhibits germination and cancels effect of red.

Sounds like our friend phytochrome!

Under a leaf canopy more far fed than red gets through. So, if shaded, seeds don’t germinate until canopy loses leaves or gap in canopy occurs.

• VernalisationPromotion of flowering by a period of chilling (normally winter). Causes plants to flower in spring.Biennials grow vegetatively in first season and flower (after chilling) in the next. In a mild winter they may not flower or flowering and fruiting may be late.

Don’t confuse germination and bud dormancy (rate of growth) with vernalisation (type of growth)