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Vernalisation in Plants: Site,
Requirements, Mechanism and
Importance
by Srinibas Kumar Vernalisation
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Vernalisation in Plants: Site, Requirements, Mechanism and
Importance!
Temperature plays significant role in metabolic activities of plants.
Temperature is one of the important factors determining the
distribution of plants. Temperature also plays vital role in the
germination of seeds and subsequent flowering of plants.
Plants of temperate zone, as expected, germinate at a relatively low
temperature, whereas tropical plants germinate best at much higher
temperature. Development and flowering in many temperate plants
can be altered by subjecting moistened seeds to low temperature.
Many plants do not come to flower before they experience a low
temperature. These plants remain vegetative during warm season,
experience low temperature during winter, grow further and then bear
flowers and fruits. It was found by Lysenko (1928), a Russian
scientists, that the cold- requiring biennial plants can be made to
flower in one growing season by providing low temperature treatment
to young plants or moistened seeds. He called the effect of this chilling
treatment as vernalisation. Vernalisation is, therefore, a process of
shortening of juvenile or vegetative phase and hastening flowering by
a previous cold treatment.
An example of winter rye may be quoted here. When the seeds of this
variety of rye were germinated at 1°C for four weeks, the plants
flowered eleven weeks after planting, but at the same time seeds
germinated at 18°C did not produce flowering shoot in the same
duration (Fig. 5.5).
Another interesting fact about the effects of vernalisation came from
the work on biennial varieties of Hyoscyamus niger (henbane) by
Melchers and Lang (1948). This variety of henbane will flower only
when vernalisation is followed by long day treatment, and
vernalisation followed by short-day treatment fails to induce flowering
(see figure 5.6).
Still more interesting is the fact that annual variety of henbane does
not require cold treatment for flowering. The annual variety differs
from biennial ones in the possession of a single dominant gene which
functions as a substitute to vernalisation.
It is presumed that the said gene brings about direct production of the
precursor of flowering substance, which in the biennial variety
requires cold treatment. These findings indicate the possibility of
conversion of certain hormone precursor into a flower-inducing active
form which under the influence of appropriate day-length induces
flowering. Common examples of plants requiring vernalisation are
winter rye, winter wheat, winter oat, winter barley, pea, beet, cabbage,
Henbane, Chrysanthemum, Viola, Clover, etc.
Site of Vernalisation (Site of Perception):
Site of perception of cold stimulus is different in different plants. It
can be a germinating seed or metabolically active embryo (Secale
cereale), shoot apical meristem (Chrysanthmum) or vegetative parts
such as leaves (Hyoscyamus niger and other biennials).
Requirements of Vernalisation:
(i) Low temperature:
Vernalisation, unlike photoperiodism, is a cumulative process because
plants become gradually more and more effectively vernalized with
time upto as long as about two months. Full vernalisation requires up
to about 50 days of treatment between – 2°C and about 12°C. If
vernalisation is followed by high temperature treatment at about 40°C
for a minimum of two days, the vernalizing stimulus is lost. This is
known as devernalisation. Devernalised plants can, however, be
vernalized again.
(ii) Actively Dividing Cells:
Vernalisation does not occur in dry seeds. The seeds must be
germinated so that they contain an active embryo. For this the seeds
are moistened before exposing them to low temperature. In a whole
plant, an active meristem is required.
(iii) Water:
Proper protoplasmic hydration is must for perceiving the stimulus of
vernalisation.
(iv) Aerobic Respiration and
(v) Proper Nourishment
Mechanism of Vernalisation and Induction of Flowering:
The stimulus received by the actively dividing cells of shoot or embryo
tip travels to all parts of the plant and prepare it to flower. The
stimulus has been named as vernalin. Melchers (1936, 1937)
demonstrated in henbane plants (Hyoscyamus niger) translocation of
vernalisation stimulus takes place through a graft union (Fig. 5.7). If
leaf or stem of a vernalised plant is grafted on an un-vernalised
henbane plant the latter plant will flower.
The stimulus was found to be non-specific, i.e., can pass across a graft
between plants of different species. It was Melchers who, for the first
time, suggested that a substance which he called vernalin was
produced during the process of vernalisation. Attempts to isolate and
chemically identify vernalin have not succeeded as yet. However, Lang
et. at. (1957) have demonstrated that treatment with Gibberellic acid
(GA), a plant hormone, substitute for cold treatment in some species
of plants.
The formation of vernalin is not enough to bring about vernalisation.
In addition, a suitable day length is also necessary. It is postulated that
in the appropriate photoperiod, either vernalin is converted into
florigen or vernalin regulates the synthesis of florigen from precursors.
Florigen then induces the vegetative meristems to switch over to
reproductive development. This initiates the process of flower
differentiation.
Vernalisation simply prepares a plant to flower. It makes the plant
perceptive to the stimulus; however, it itself does not function as a
stimulus. Photoperiodism, on the contrary, not only provides the
stimulus for flowering but also induces it.
Advantages/Importance of Vernalisation:
(i) Vernalisation can help in shortening the juvenile or vegetative
period of plant and induce early flowering. It is applicable to not only
temperate plants but also to tropical plants, e.g., wheat, rice, millets,
cotton.
(ii) It increases yield, resistance to cold and diseases.
(iii) ‘Kernel wrinkles’ of Triticale can be removed by vernalisation.
(iv) It enables the biennials to behave as annuals.
(v) Plants can be grown in such regions where normally they do not
grow
Photoperiodism Process and Effects of
Light on Flowering of Plants
by Srinibas Kumar Photoperiodism
ADVERTISEMENTS:
Let us make an in-depth study of the photoperiodism
process and effects of light on flowering of plants.
Early twentieth century workers were of the belief that flowering in
plants is a phenomenon effected by nutrition.
Kraus and Kraybill (1918) observed in the case of tomato plant,
optimum nitrate and carbohydrates supply accelerated vegetative
growth, but with a poor nitrate supply both reproduction and
vegetative growth declined.
However, in the year 1906, a commercial variety of tobacco, Maryland
narrow-leaved, gave rise to a new mutant called Maryland mammoth.
This new variety showed vigorous vegetative growth during the
summer, but the plants did not set seeds before the cold weather set
in. W. W. Garner and H.A. Allard, plant physiologists, came forward to
investigate the cause.
Finally they observed that these plants always bloomed during the
short days of the winter months. The plants were made to flower even
during the months of summer by cutting down the light period to
seven hours a day (Fig. 5.1).
Gamer and Allard published their investigations in the year 1920.
They, however, concluded that flowering was caused by exposure to
days made up of short light and long dark periods. Since then it has
been known that an environmental factor of great significance in
controlling flowering is day length.
In tropical regions of earth there is very little change in day length
throughout the year, and the days and nights are about equal. In
temperate regions, day length changes from winter to summer, and
the long days coincide with the warmer season. Many tropical species
of plants when brought to temperate zone flower only when the days
are short, continued long days prevent the formation of flower buds.
Plants native of temperate zone have a variety of flowering habits.
Many temperate plants flower during spring when days are
moderately short. Others flower during the summer when the days are
long. Still others produce flowers during the short days of late summer
and early fall. This mechanism that enables plants to respond to day
length so that they flower at a specific time of the year is known as
photoperiodism. The length of the daily period of light to which a
plant is exposed is called photoperiod.
Plants are grouped according to their response to day length
into what are called:
(i) Short-Day Plants
(ii) Long-Day Plants and
(iii) Day-Neutral Plants
(i) Short-Day Plants (SDP):
These plants flower when exposed to day lengths shorter than or
below a certain critical maximum. The critical photoperiod, however,
varies from species to species. If these plants are exposed to day
lengths in excess of this critical point, they continue growing
vegetatively (Fig. 5.2 A) Common examples of short-day plants are
chrysanthemums, cock-lebur (Xanthium strumarium), tobacco
(mutant. ‘Maryland mammoth’, Nicotiana, tabaccum), soyabean
(Glycine max), and sugarcane (Sacchamm officinarum), etc. They
normally flower in the early spring or autumn.
(ii) Long-Day Plants (LDP):
These plants begin flowering when exposed to day lengths longer than
or above a certain critical minimum. Below the critical photoperiod,
these plants continue their vegetative growth (Fig. 5.2 B). The critical
photoperiod, in such plants also, varies from species to species. Some
common examples of long day plants (LDP) are barley (Hordeum
vulgare), spinach (Spinacea olemcea), radish (Raphanus sativus),
henbane (Hyoscyamus niger), onion (Allium cepa) and carrot (Daucus
carota), etc. They normally flower in late spring or early summer.
(iii) Day-Neutral Plants (DNP):
These plants flower after a period of vegetative growth, regardless of
the photoperiod. In other words, they are unaffected by the day or
night lengths, and flower around the year (Fig. 5.2-C). Some common
examples of day-neutral plants are cucumber (Cucumis sativus),
cotton (Gossypium hirsutum), tomato (Lycopersicum esculentum),
sunflower (Helianthus annuus), Maize (Zea mays) and some varieties
of pea, etc.
Photoperiodic Responses are Under the Control of Genes:
It is a matter of common observation that the critical day length of
both long-day and short- day plants tends to fall within a range of 12—
14 hours. Commercial flower growers can induce or delay flowering by
controlling the photoperiodic and temperature conditions in
glasshouses to meet the demands of the market. The photoperiodic
responses of plant are now considered to the under the control of
genes.
These can be modified by various methods to yield varieties
responding to required day lengths. For instance, scientists at the
National Botanical Research Institute, Lucknow have been able to
develop varieties of Chrysanthemum which can bloom in different
months of the year including summer.
Critical Photoperiod:
The critical photoperiod for long and short day plants greatly varies
from species to species. For instance. Chrysanthemum and Poinseltias
are both short-day plants, but Chrysanthemum form flowers when the
days are shorter than 14.5 hours, whereas Poinsettias produce flower
buds only when the days are less than 12.5 hours.
Spinach and rose mallow are long-day plants, but spinach flowers
when the days are longer than 14 hours, the rose mallow flowers when
they are longer than 13 hours. In other words, the short-day plants
flower only when the days are shorter than a critical photoperiod, and
the long-day plants flower only when the days are longer than the
critical duration.
Induction Period:
Induction period is the minimum period of exposure to a long day or a
short day which is required to induce flowering. Induction period
differs in different plants. For instance, Xanthium requires only one
cycle of day plus night, but most plants require about ten such cycles.
Long-Night and Short-Night Plants:
The Terms Long-Day and Short-Day Plants are actually misnomers.
Earlier when the photoperiodism was discovered, the duration of the
light period i.e. photoperiod was thought to be critical for flowering.
However, later researches, noted that in short-day plants (SOP), when
the long night period was interrupted by a brief exposure to light, the
plants failed to flower (Fig. 5.3).
From this observation, scientists concluded that what is critical or
essential for these plants to flower is long and un-interrupted dark
period rather than a short day length. A brief interruption of the dark
period with light nullified the effect of long night. So to be more
precise and appropriate, short day plants may be regarded as longnight plants.
Similarly long-day plants (LDP) respond to nights shorter than the
critical dark period. Curiously long day plants do not need an
uninterrupted dark night. Long-day plants are also regarded as shortnight plants.
Theory of Photoperiodic Action:
Attempts have been made to understand as to how day (or night)
length affects the plant so as to change the normal leaf primordia of
the stem apex into flowering primordia? As written above, in shortday plants it is the dark period rather than the light period which
affects induction of flowering, but in long-day plants, dark period is
not at all important and they can flower even in continuous day light.
This clearly indicates that there must be two different systems
operating in two groups of plants for the induction of flowering.
Role of Phytochrome, Florigen and Phytohormones in Flowering:
There are experimental evidence (Hendricks and Borthwick) that only
red light (wavelength 660 mμ) is effective in inhibiting flower
initiation in short-day plants, when the dark period towards midnight
is interrupted with this illumination. This wave length of light at the
same time accelerates the growth of stem and root and formation of
anthocyanin pigment.
It is more interesting to note that this inhibition of flowering in short
day plants can be reversed by treating the plants with far-red light
(wave length 730 mμ) (Fig. 5.4). This suggests the existence of a single
compound, phytochrome, responsible for photoperiodic action. The
phytochrome (probably) exists in two inter convertable forms P730
and P660. When P660 is illuminated with red light (660 mμ) it is
transformed to the P730 form.
The P730 form can be converted into the P660 form by far red light
(730 mμ). During night P730 is converted into P660 form and hence
at the end of day period the predominent form of phytochrome is P730
because sun light contains more red light of 660 mμ) wavelength.
In short day plants flowering is initiated when there is sufficient
accumulation of P660 and that is the reason why flowering is inhibited
in these plants when dark period is interrupted by red light of 660 mμ
wavelength which converts P660 form into P730 form (Fig. 5.4). But
the manner in which the flowering is initiated by the phytochrome is
not yet well understood.
The only facts known about this flowering substance are that it is
proteinaceous in nature and most likely acts as an enzyme which
initiates the formation of certain hormone or hormones which
ultimately bring about the conversion of vegetative primordia into
flowering primordia.
Florigen—A Hypothetical Flowering Hormone:
Evidence that a flowering hormone “florigen” exists in plants comes
from the work of Naylor (1952), who states that a plant can be made to
bloom by grafting on it a leaf from another variety, species, genus, or
even from another family. A certain parasitic plant which grows on the
roots of red clover is probably never exposed to light and yet it blooms.
It is assumed that this parasite obtains its stimulus for flowering from
its host.
1. The metabolism of florigen is believed to be phytochrome-mediated.
2. Florigen has never been isolated. It is a hypothetical hormone.
3. The florigen is translocated to the vegetative bud through phloem,
where it transforms vegetative but into flower bud.
4. Florigen is a sort of stimulus. Unlike other phytohomones, it is
neither growth promoter nor a growth inhibitor.
5. The seat of synthesis and the seat of action of florigen are leaf and
shoot tip respectively.
Chailakhyan (1968) demonstrated that the site of perception of light
for photoperiodic inductions (stimulus) are the green leaves. This is
evident from the fact that a plant from which all leaves have been
removed fails to flower even under the inductive light conditions.
Further confirmation was obtained from experiments with Xanthium,
a short-day plant, in which flowering occurred even when one-eighth
of a leaf was exposed to short days.
Photoperiodic induction received by a single leaf or its part in a plant
is considered enough to induce flowering. Further, a floral stimulus
from an induced leaf in a long or short-day plant can be transmitted or
trans-located to another non-induced plant by grafting. Besides, the
floral stimulus is not species-specific because grafting an induced twig
of Xanthium on to a vegetative soya bean plant can induce the latter to
flower.
The nature of the flower-producing stimulus has been widely debated.
Some plant physiologists have proposed the existence of a flowerinducing growth hormone, the florigen (Naylor 1952 and Chailakhyan
1968). The metabolism of florigen is believed to be phytochromemediated. Unfortunately, the florigen has never been isolated.
The florigen is trans-located via phloem to the vegetative bud
primordia which undergo transformation (morphological changes)
leading to the production of floral buds.
Four steps are involved in this transformation:
(i) Perception of the stimulus by phytochrome in the leaves
(induction);
(ii) Change to new pattern of metabolism in the leaves leading to the
production of flowering hormone, the florigen;
(iii) Translocation of florigen (the stimulus) to the bud primordia; and
(iv) Transformation of vegetative bud primordia into floral buds (the
response).
Role of Phytohormones in Flowering:
Researchers have indicated that flowering is also regulated by the
interplay of the phytohormones, the auxins, gbberellins, cytokinins
and ethylene. Application of hormones can substitute for the necessary
photoperiod and can initiate floral development in certain plants. It is
interesting to note that IAA (auxin) inhibits flowering in most of the
plants. An exception, however, is pineapple (Ananas). Gibbereuic acid
(GA) can substitute photoperiodic induction in many long day plants.
It, however, is almost ineffective in short-day plants except a few such
as Impatiens balsamia (Balsam plant).
Significance/Practical Importance of Photoperiodism:
1. Photoperiodism determines the season in which a particular plant
shall come to flower. For example, short-day plants develop flowers in
autumn-spring period (e.g., Dahlia, Xanthium) while long-day plants
produce flowers in summer (e.g., Amaranthus).
2. Knowledge of photoperiodic effect is useful in keeping some plants
in vegetative phase (e.g., many vegetables) to obtain higher yield of
tubers, rhizomes etc., or keep the plant in reproductive phase to yield
more flowers and fruits.
3. A plant can be made to flower throughout the year by providing
favourable photoperiod.
4. Helps the plant breeders in effective cross-breeding in plants.
5. Enable a plant to flower in different seasons thus fruits can be
produced during their offseason by controlling photoperiod
Differences between Photoperiodism
and Vernalisation
by Srinibas Kumar Photoperiodism
ADVERTISEMENTS:
The upcoming discussion will update you about the
differences between photoperiodism and vernalisation.
Photoperiodism:
1. It is a process of stimulating flower induction by exposing the plants
to appropriate photoperiods.
2. It provides both stimulus and induction of flowering.
3. The stimulus is perceived by only the green leaves.
4. The stimulus itself induces flowering.
5. It is mediated through florigen.
6. Exposure to 2 to 3 appropriate photoperiods is enough to induce
flowering.
7. Photoperiodic induction cannot be reversed or nullified by exposure
to unfavourable photoperiods.
8. Gibberellic acid can replace exposure to long photoperiods in long
day plants only,
Vernalisation:
1. It is a process of preparing the plants for perceiving stimulus for
flower induction by cold treatment.
2. It only prepares a plant for perceiving the flowering stimulus. It
does not induce flowering.
3. The stimulus is perceived by meristems, embryos or leaves.
4. The stimulus itself does not induce flowering. It must be followed by
appropriate photoperiod and temperature.
5. It is mediated through vernalin which induces the synthesis of
florigen.
6. Exposure to low temperature (between — 2°C to 12°C) for about 50
days is needed to induce flowering.
7. Plants can be devernalizd i.e., vernalization can be reversed or
nullified by exposure to high temperature (about 40 °C).
8. Gibberellic acid can replace cold treatment to induce vernalization