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🌿 Transmission of Stimulus

Transmission of Stimulus.

This lesson provides exam-focused context on key concepts in crop physiology and connects the section topics for quick revision.

Photoperiodic Induction

The influence of the length of day and night on the initiation of flowering is called

photoperiodic induction or photo induction .

Plants may require one or more inductive cycle for flowering. An appropriate

photoperiod in 24 hours cycle constitutes one inductive cycle. If a plant which has received

sufficient inductive cycle is subsequently placed under unfavourable photoperiod, it will still

flower.

Flowering will also occur if a plant receives inductive cycles after intervals of

unfavourable photoperiods (i.e. discontinuous inductive cycle). This persistence of

photoperiodic after effect is called as photoperiodic induction.

  • An increase in the number of inductive cycles results in early flowering of the plant.

For instance, xanthium (a short day plant) requires only one inductive cycle and

normally flowers after about 64 days. It can be made to flower even after 13 days if it

has received 4-8 inductive cycle. In such case number of flowers is also increased.

  • Continuous inductive cycles promote early flowering than discontinuous inductive

cycle.

Some of the examples of plants which requires more than one inductive cycle for

subsequent flowering are,

Biloxi soybean (SDP) - 2 inductive cycles

Salvia (SDP) - 17 Inductive cycles

Plantago (LDP) - 25 Inductive cycles

Critical day length

Maryland mammoth tobacco and xanthium are short day plants, but the Maryland

mammoth tobacco is induced to flower when the photoperiod is shorter than 12 hours (12L

/12D) whereas, xanthium is induced to flower when the photoperiod is shorter than 15.5

hours (15.5L /8.5D). The photoperiod required to induce flowering is referred to as the

critical day length . Hence, the critical day length for Maryland mammoth tobacco and

xanthium are 12 and 15.5 hours respectively. A short day plant is one that flowers on

photoperiods shorter than the critical day length.

Long day plants, on the other land, are induced to flower on photoperiods longer than

critical day length. For example, the critical day length for Hyoscyamus niger is 11 hours

(11L /13D) and it is induced to flower on photoperiods longer than 11 hours.

Suppose, xanthium and Hyoscyamus niger are exposed to a photoperiod of 14 hours

of light and 10 hours of darkness (14L/10D), flowering will be induced in both plants.

Xanthium, a short-day plant, will flower because 14L /10D photoperiod is shorter than

critical day length of 15.5 hours. Hyoscyamus, a long-day plant, will flower because

14L/10D is longer than the critical day length of 11 hours.

Perception of photoperiodic stimulus and presence of a floral hormone

  • Photoperiodic stimulus is perceived by the leaves and a floral hormone is produced in

the leaves which are then translocated to the apical tip, subsequently causing

initiation of floral primordia.

  • Photoperiodic stimulus perceived by the leaves can be shown by a simple experiment

on cocklebur (xanthium), a short day plant. Cocklebur plant will flower if it has

previously been kept under short day conditions. If the plant is defoliated and kept

under short day condition, it will not flower. Flowering will also occur if all the

leaves of the plant except one leaf have been removed.

  • If the cocklebur plant whether intact or defoliated is kept under long day condition it

will not flower. But if even one of its leaves is exposed to short day condition and the

rest are under long day condition, flowering will occur.

  • The photoperiodic stimulus is transmitted from one branch of the plant to another

branch. For example, if in a two branched cocklebur plant one branch is exposed to

short day and the other to long day photoperiod, flowering occurs on both the

branches.

  • Flowering also occurs if one branch is kept under long day conditions and other

branch from which all the leaves except one have been removed is exposed to short

day condition. However, if one branch is exposed to long photoperiod and the other

has been defoliated, under short day conditions, flowering will not occur in any of the

branches.

Flowering stimulus: Florigen

The flowering stimulus is produced in leaves and translocated to apical and lateral

meristems where flower formation is initiated. Chailakhyan (1937) called the flowering

stimulus or flowering hormone as Florigen.

Flowering stimulus is similar in long day plants and short day plants. This can be

proved by a grafting experiment and can be translocated from one plant to another.

Maryland mammoth tobacco, a short day plant and Hyoscyamus niger, a long day

plant, are grafted so that the leafy shoots of both the species are available for experiment. If

the grafted plants are exposed to either long day a short day conditions, both partners flower.

If grafting union is not formed, the flowering stimulus is not translocated from one partner to

another partner.

Theories of Flowering

  1. Bunning’s hypothesis

  2. Chailakhyan’s hypothesis

Bunning’s hypothesis:

Bunning (1958) assumes the presence of endogenous rhythms (Oscillator which

consist of two half cycles. The first half cycle occurs in day and is called photophilous

phase . During this, anabolic process predominates including flowering in plants. The other

half cycle is dark, sensitive and is called skotophilous phase . In this, catabolic process

(dehydration of starch) predominates.

SD plants have a critical day length of 9 hours. This period falls within the

photophilous phase. Light during scotophil phase will inhibit photo process initiated during

photophase. The L.D. plants have a critical day length of 15 hours and some light falls in the

skoto philous phase. Under these conditions in L.D. plants will flower. In S.D. plants

oscillator is present close to skoto philous phase, while in L.D. plants it is close to photo

philous phase .

Chilakhyan’s hypothesis:

This hypothesis assumes that flowering hormone – florigen is a complex of two types

of substances – gibberellin and anthesins. Gibberellin is essential for growth of the plant

stems and anthesins are required for flower formation.

According to him, flowering in all annual seed plants requires two phases: (i) Floral

stem formation phase (ii) Flower formation phase . First phase involves increased

carbohydrate metabolism and respiration with increased content of GA in leaves. Second

phase requires intensive nitrogen metabolism, higher content of anthesins in leaves and

nucleic acid metabolites in stem buds.

Long day conditions favour the first phase while short day conditions favour second

phase. In long day plants gibberellins are critical, while anthesins are critical in short day

plants. However, anthesin is hypothetical; it has not been isolated as yet.

VERNALISATION

The cold treatment given to plant buds, seeds or seedlings for promoting early

flowering is known as Vernalisation . In short, the chilling treatment for induction of early

flowering is called Vernalisation.

Besides an appropriate photoperiod, certain plants require a low temperature

treatment during their early stages of the life for subsequent flowering in the later stages.

This low temperature treatment requirement was termed vernalization by Lysenko (1928).

Due to vernalization, the vegetative period of the plant is cut short resulting in an early

flowering. In nature, vernalisation takes place in the seed stage in annuals like winter rye

( Secale cereale ). The biennials and many perennials respond to cold treatment at a very late

stage. E.g. Henbane, apples etc.

Perception of cold stimulus and presence of floral hormone

The cold stimulus is perceived by the apical meristems. The perception of the cold

stimulus results in the formation of a floral hormone which is transmitted to other parts of the

plant. In certain cases, the cold stimulus may even be transmitted to another plant across a

graft union.

For instance, if a vernalized henbane plant is grafted to an unvernalized henbane

plant, the later also flowers. This is due to the induction of the plant to produce a hormone

named as Vernalin by Melchers (1939).

Conditions necessary for vernalization

1. Age of the plant

The age of the plant is an important factor in determining the responsiveness of the

plant to the cold stimulus and it differs in different species. In cereals like winter wheat, the

vernalization is effective only if the germinating seeds have received cold temperature

treatment for sufficient time.

While in the case of biennial variety of henbane ( Hyoscyamus niger ), the plant will

respond to the cold treatment, only if they are at rosette stage and completed at least 10 days

of growth.

2. Appropriate low temperature and duration of the exposure

Most suitable temperature for vernalizing the plants ranges between 1-6°C. The

effectiveness of low temperature treatment decreases from 0 to 4°C. Low temperature at

about -6°C is completely ineffective. Similarly at high temperatures from 7°C onwards, the

response of the plants is decreased. Temperature of about 12-14°C is almost effective in

vernalizing the plant. Besides an appropriate low temperature, a suitable duration of the cold

treatment is essential for vernalization. Depending upon the degree of temperature and in

different species this period may vary, but usually the duration of the chilling treatment is

about one and half months or more.

3. Oxygen

The vernalization is an aerobic process and requires metabolic energy. In the absence

of O2, cold treatment becomes completely ineffective.

4. Water

Sufficient amount of water is also essential for vernalization. Vernalization of the dry

seed is not possible.

Mechanism of Vernalization

There are two main theories to explain the mechanism of vernalisation.

Phasic developmental theory

This theory was proposed by Lysenko (1934) as follows.

(i) The growth (increase in size) and development (i.e. progressive change in the

characteristic of the new organs) are two distinct phenomenons.

(ii) According to this theory, the process of the development of an annual seed plant

consists of a series of phases which must occur in some predetermined sequence.

(iii) Commencement of any of these phases will take place only when the preceding

phase has been completed.

(iv) The phases require different external conditions for the completion such as light

and temperature.

(v) Vernalization accelerates the thermo phase i.e. that phase of development which

is dependent upon temperature.

Thus, in winter wheat, low temperature is required for the completion of first thermo

phase. After this, the next phase that is dependent upon light (photo phase) starts.

Vernalization of winter wheat accelerates the first thermo phase so that there is an early

swing from vegetative to reproductive phase or flowering.

Hormonal theories

It has already been described that vernalization probably involves the formation of a

floral hormone called as vernalin . Based on this fact, many hypothetical schemes have been

proposed by different workers from time to time. The first hormonal theory proposed by

Long and Melchers (1947) is schematically shown below.

D

According to this scheme, the precursor A is converted into a thermo labile

compound B during cold treatment. Under normal conditions B changes into C which

ultimately causes flowering. But at higher temperature B is converted into D and flowering

does not take place (devernalization).

Devernalization

The positive effect of the low temperature treatment on the vernalization of the plant

can be counteracted by subsequent high temperature. This is called devernalization. The

devernalized plant can again be vernalized by subsequent low temperature treatment.

Vernalization and Gibberellins

The gibberellins are known to replace the low temperature requirement in certain

biennial plants such as henbane, where the plant normally remains vegetative and retains its

rosette habit during the first growing season and after passing through the winter period

flowers in the next season. The gibberellins cause such plants to flower even during the first

year.

Significance of vernalization

  1. Vernalization shortens the vegetative period of the plant

  2. It increases cold resistance of the plants

  3. Vernalization increases the resistance of plants to fungal diseases.

  4. It is a physiological process that substitutes or compensates the effect of thermo

phase.

  1. In biennials, vernalisation induces early flowering and early fruit setting.

  2. A non vernalised shoot apex can be induced to flower by grafting the plant with a

vernalised plant.

Summary Cheat Sheet

  • Review each concept section above in sequence to connect definitions, processes, and applied crop-physiology outcomes.
  • Focus on high-yield terms, pathways, and condition-dependent responses for exam-ready recall.
  • Use the listed examples, comparisons, and cycles as rapid-revision anchors before practice questions.

References

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