photo periodicity and vernal is at ion

Photoperiodicity and Vernalisation

By : Asri IvoDewi Sahfitri Tanjung

Dwi RahmanarsihShofia LubisSiti Indriani

Concept of the biological clock1. Endogenous circadian rhythms2. photoperiodism

Biological clock

A roughly 24-hour cycle in the biochemical, physiological, or behavioral processes of living entities on Earth

Occur within the cell

Endogenous circadian rhythms

Many plant activities seem to occur on a daily schedule - some plants open their leaves at dawn and shut them at dusk - some plants may open and shut flowers as the day changes from dawn to dusk

Photosynthesis, auxin production, and the rate of cell division all have regular daily rhythms and these rhythms continue even when environmental conditions are kept constant

Controlled by independent time measuring system (biological clock)

Circadian Rhythms

Entrainment occurs when a periodic repetition of light and dark (or some other external cycle) causes a biological clock to remain synchronized with the same cycle as the entraining factor

Light-dark cycles, light intensity and temperature cycles are the principle sources of entrainment

The main advantage of a biological clock is that it allows the plant to respond to the changes in seasons by accurately measuring changing day length - thus changes in the environment trigger responses that result in adjustments of growth, reproduction, and other activities of the organism

More Circadian Rhythms

PHOTOPERIODISM The biological measurement of the relative lengths of day and night

Garner and Allard (1920’s)

The discovery of photoperiodism

Soybeans (Glycine max) planted over a three-month period all flowered about the same time

Photoperiodism the response by an organism to synchronise its body with changes in day length

At high latitudes this is important because the change in length of the day indicates the season

Days getting shorter indicate winter is approaching

Days getting longer indicate summer is approaching

Some plants regulate their flowering this way

Photoperiodism

Photoperiodism: Types of Plants

3 different types of plants:

◦ “Short Day” flower when days are short, nights are long (Ex. poinsettias, chrysanthemums)

◦ “Long Day” flower when days are long and nights are short (Ex. Spinach, Radish)

◦ “Day Neutral” flowering does not depend on length of day or night (Ex. tomato)

Response varies according to the age of the plant and varies in its intensity

Broadly they can be grouped into three categories

Response to day length

Response to day length

CucumberHollyMaize

Do not respond to day length

DAY-NEUTRAL PLANTS

Chrysanthemum < 15hTobacco < 14h

< 10-16 hSHORT-DAY PLANTS(“LONG-NIGHT”

PLANTS)

Red clover > 9hOats > 12hWinter wheat > 12h

>9-16 hLONG-DAY PLANTS

EXAMPLESDAY LENGTHRESPONSE

Response to day length

Some plants flower only after a CRITICAL DAY LENGTH

Some plants only flower after a CRITICAL NIGHT LENGTH

Chrysanthemum

Critical daylength

CriticalDaylength

(CD)

Xanthium: a short day plant, flowers when CD is LESS than 15.5 hours.

Hyoscyamus: a long day plant, flowers when CD is MORE than 11 hours.

PHOTOPERIODISM K. Hamner (U of Cal.) and J. Bonner (CIT) 1938

PHOTOPERIODISM REDEFINED

K. Hamner (U of Cal.) and J. Bonner (CIT) 1938:◦ Short Day Plants

uninterrupted darkness must be of a certain duration.

(so much darkness or more)

◦ Long Day Plants uninterrupted darkness must be less than a certain

maximum value. (so much darkness or less)

◦ Day Neutral Plants Flowers at a certain level of maturity or in response

to some environmental factor other than the photoperiod.

The control of flowering

Flowering

“Florigen” hormone

Flower buds

Photoperiod mechanism in the leaves

Change in day length

• the flowering signal is generated in the leaf

• the signal goes one way: from the leaf to the apex

• Grafting transmittable

The flowering signal: florigen

?

vegetative or reproductive growth?

SAM

Florigen

Florigen

Florigen

Exp. 1: The leaf or apex of Perilla (a short day plant) was exposed to different daylength.

Exp. 2: Grafting experiment with Perilla

Do plants really measure the length of the daylength?

• Xanthium flowers when the dark period exceeds 8.5 hours.

Hamner and Bonner (1938): Xanthium strumarium, a SD plant with CD = 15.5 hours

• Short interruption of dark period, even by a pulse of light as short as 1 minute delays flowering.

• The relative length of dark is not the determining factor.

Long and Short Day Plants May Flower at the Same time

For plants with a critical night length, a short flash of light in the middle of the night would make the plant behave as if it had been exposed to a long day

The night break phenomenon

The quality of the light

The wavelength of the light used is important

StimulatesInhibits670 – 680nmRed light

ReversesStimulates>700nmFar red light

LONG-DAYSHORT-DAYWavelengthColour

This indicated that there should be a pigment that absorbs red light (in other words this pigment should be blue-green)

This pigment is the mechanism capable of recognising changes in day length

PHYTOCHROME

The pigment

PHOTOPERIODISM H. A. Borthwick and S. B. Hendricks 1950’s

US Ag

Phytochrome exists in two versions which are inter-convertible

PR that absorbs red light PFR that absorbs far red light

The photoperiod mechanism

PR

RED LIGHT

FAR RED LIGHT

PFR

In the short-day plant

PFRPR builds up

Darkness (slow)Far red light (fast)

Short-day plants

FLORIGENActivated

FLOWERING

In the long-day plant

PFR builds up PR

SunlightRed light

Long-day plants

FLOWERING

FLORIGENActivated

Summary

SunlightRed light

Darkness (slow)Far red light (fast)

PFR builds up

Long-day plants

FLOWERING

FLORIGENActivated

PR builds up

Short-day plants

FLORIGENActivated

FLOWERING

VERNALISATION from Latin: vernus, of the spring

Discovered by Trofim Denisovich Lysenko (1898-1976)

Cold/low temperature perceived by the shoot apex. Two phenomena : the growth and the development

Merely prepares the plants for flowering

Overview

Plants differ in age at which they become sensitive to vernalization. E.g. winter annuals(winter rye) respond tolow temperature early in their life cycle (can be vernalized before germination, imbibedseeds)

Vernalization results in competence to flower at the shoot apical meristem

Imbibed Rye seeds exposed to 5ºC for different lengths of time, then immediately treated for 3 d at 35º

Most biennials (grow as rosette during first season and flower in following summer; parsley, carrot) must reach minimal size before they become sensitive.

Effective temperature of vernalisation: 0-10ºC;Effect of cold increases with duration of cold treatment, requiring several weeks of exposure (varies with species) .Vernalization can be lost after exposure to high temperature (devernalization); the longer the exposure to low temp., the more permanent the vernalization effect.

.

Thermostage • First stage and referred to as Lysenko

stage.• Condition for successful passage

* temperature* moisture* aeration* time

Theory

Photostage Photoperiodisme Pronounced effect of the relative length of day and night on the production of flowers.

Third stage Necessary for seed formation

.

The seeds are allow the germinate

Given cold treatments (0-5⁰)Sown

Process

Low temperature Themoinduced condition

Venralin Florigen (The flowering signal)

Mechanism

No vernalization Vernalization

.Vernalization induces flowering in winter annual ecotypes of Arabidopsis thaliana

Plants are genetically identical.

Exposed as a seedlingto 4ºC for 40 days.

Winter annuals require vernalization to flower while rapid-cyclers do not require vernalization.

Arabidopsis accessions have adopted different reproductive strategies

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