Unit IV: Plant Responses

Concept of Florigen

Julius Sachs in the 19th century was probably the first person to support the idea that ‘flower-forming substances’ are present in flowering plants. He observed that leaf cuttings from flowering Begonia plants produce adventitious shoots which quickly flower, whereas leaf cuttings prepared from vegetative plants regenerate only vegetative shoots. Many years later, after the discovery of photoperiodism.

It was conceived that the leaves are the receptors of photoperiod and so it became obvious that some information is transmitted from the leaves, which causes a floral response in the meristem. In 1937, Chailakhyan proposed that the signal generated in the leaf is a substance of hormonal nature and named it ‘florigen’.

The major evidence for the existence of florigen is derived from grafting experiments in which a receptor plant in non-inductive condition is induced to flower by graft union with a previously induced donor plant. In such grafting experiment, donor and receptor plants may either be of the same species or of two different species or genera.

In an extreme case, the two partners belonging to different photoperiodic groups (SDP, LDP and DNP) can be used as donor and receptor.

These results support the concept of a transmissible floral stimulus which seems to be physiologically similar in all photoperiodic plants. In grafting experiments, in which the donor is either the stock or the scion, the movement of the floral stimulus may be in both the upwards and downwards directions.

In 1937, Melchers reported transmission of a flower stimulus in biennial Hyoscyamus formed as a result of low temperature vernalization and called it ‘vernalin’ as the product of vernalization. There was, however, no evidence of transport of a stimulus formed as a result of cold treatment alone indicating lack of mobility of the product of vernalization. Melchers assumes that vernalin is the physiological precursor of florigen.

All these results generally support the idea that the floral hormone is the same in all higher plants. At the same time, some doubts are there about the existence of a universal floral hormone common to all plants because grafts can only be made between compatible species and it is difficult to visualize transmission of the floral stimulus in the incompatible grafts.

(a) Transport of the Floral Stimulus:

The physical and chemical nature of the postulated floral hormone florigen is not known. So, Indirect studies have been made to determine the time, velocity and pattern of transport of the floral hormone because the main characteristic of a hormone is translocation.

In experiments involving defoliation at various intervals, it has been proved that the hypothetical floral hormone is formed in the induced leaves and moves out of the leaves soon after it is formed and trans located over a short period following induction.

Another experiment involving the technique of sequential defoliation as a means of removing the source of florigen was designed to understand how the floral response passes several points of known distance along the translocation pathway.

Using this method, Evans and Wardlaw obtained a velocity of 1 to 2 cm/h in the LDP Lolium, whereas King et al. (1968) obtained a much higher value of 30 cm/h in the SDP Pharbitis.

Regarding the pattern of transport, Lang has indicated that the floral hormone moves only through living tissue. Initially the movement probably occurs from cell to cell through the mesophyll of the leaf blade until loaded into the phloem.

The phloem tissue is the path of further transport in the petiole and stem. Assimilates are known to move in the phloem by mass flow from sources to sinks. Florigen in thought to move passively with assimilates and can move both acropetally and basipetally, i.e., its movement is non-polar as is the flow of assimilates.

(b) Nature of Florigen:

The florigen theory can only be tested by isolation of the hormonal principle. But even after several years of extensive studies, physiologists studying flowering have no idea of its chemical structure. In Bonner’s laboratory, several attempts were made to isolate flower-inducing activity from Xanthium and this material was applied to cause floral initiation in vegetative plants.

Sometimes, extracts from flowering plants have been shown to produce positive effects in inducing flowering in some vegetative rosette LDP. But the situation seems to be problematic because exogenous gibberellic acid (GA₃) has exactly the same effects and gibberellin-like materials are present in these extracts, so their activity can be ascribed to the presence of gibberellins.

The proponents of the florigen theory are not in favour of considering these compounds as floral hormones because the GAs are not the universal promoters of flowering.

In 1964 Lincoln and co-workers prepared a crude extract from flowering branch of Xanthium but their efforts to purify the active principle did not meet with any success as it led to loss in activity during the purification process.

The active material is highly water-soluble containing a carboxyhc acid and so it has been referred to as ‘florigenic acid’. Following a different approach. Cleland could identify the flower-inducing principle as salicylic acid but failure to induce flower initiation with this chemical makes it unlikely that this compound is the floral hormone.

In an attempt to identify the florigen nature, a variety of known chemicals including cyclic AMP, prostaglandin, DNA and different plant hormones have been used in treating vegetative plants. The outcome of this work is so far inconclusive because no single compound was found to exhibit universal florigenic activity which is effective in all higher plants.

Role of Phytochrome in Flowering

Phytochrome is a blue proteinaceous plant pigment. It is present in the plasma membrane of the cells of leaves and shoot apex. It was discovered by Butler in 1959.

Role of Phytochrome in Flowering of SDPs:

• Plants which requires less than 10 hours day length for the initiation of flowering are called Short Day Plants.

• Such plants usually require 8-10 hrs light period for flowering. E.g: Rice, coffee, tobacco, soyabean.

• They need more than 12hrs dark period for normal flowering.

• They need a continuous dark period of 14-16 hours for flowering. These plants never produce flowers when the day length exceeds certain critical value or dark period is interrupted by flashes of light.

• In SDP darkness is important for flowering.

• In many SDPs, if dark period is interrupted with a brief exposure (about 1 hr) to red light, the Pr is converted into Pfr form. Because of the accumulation of Pfr, flowering is inhibited. If far red light is given for a brief period after red light treatment, the Pfr is converted into Pr and the plant produces flowers.

• SDPs require higher Pr:Pfr ratio for flowering.

• During winter months more of far red light is received on the surface of earth as compared to portions of red light reaching earth ground. This converts much of Pfr form into Pr form, inducing flowering in SDPs.

• In summer months however the reverse ratio is observed due to more portion reaching earth keep SDPs non-flowering.

Role of Phytochrome in Flowering of LDPs:

• Plants which require more than 14 hours day length for the initiation of flowering are called Long Day Plants. They usually need 14-16 hrs light period for flowering. E.g Pea, sugar beet, radish, cabbage, wheat.

• The LDPs requires night 8-10 hours dark period for flowering.

• If the LDPs get a photoperiod of less than 14hrs light and more than 8 hrs dark period, they fail to flower. In LDPs, light is important for flowering.

• The long dark period inhibits flowering in LDPs. When the LDPs is interrupted by a brief flash of light, flowering is initiated in the LDPs.

• In LDPs, the role of phytochrome is more complex so that a blue-light photoreceptor is also required for the control of flowering.

• LDPs require higher Pr:Pfr ratio for flowering.

• During summer months more of red light is received on the surface of earth as compared to portions of far red light reaching earth ground. This converts much of Pr form into Pfr form thus inducing flowering in LDPs.

• In winter months, however the reverse ratio is observed due to more far red portion reaching earth keep LDPs non- flowering.

Circadian rhythms of change in Pr and Pfr concentrations are observed in both SDPs as well as LDPs in relation to light and dark periods. It also confirms that phytochrome takes part the in photoperiodism in plants.

During day time, Pr is converted into Pfr and it get accumulated in the plant.

It inhibits flowering in SDPs but initiates flowering in LDPs.

During dark period, Pfr gradually changes into Pr form. It stimulates flowering in SDPs and inhibits flowering in LDPs.

The SDPs needs a long dark period. During the long period Pfr is converted into Pr form which initiates flowering. If the long dark period is interrupted with red light, flowering is inhibited. This is because in red light Pr form is converted into Pfr form, which inhibits flowering.

If the interruption of dark period by red light is followed by far red light, the flowering is initiated. This is because in far-red light Pfr is converted into Pr form to initiates flowering.

The nature of light of light to which plants are plants are exposed at the last time shows the maximum response. If red and far red lights are given successively, the last light treatment shows the flowering response in plants.

Vernalization

The precise definition of vernalization is not universally accepted. The term is best defined as the specific promotion of flowering by a cold treatment given to the imbibed seed or young plant. In certain seeds, and buds low temperature is required to break their dormancy but these responses are not included in our definition since they do not relate to flowering.

Temperature also affects floral initiation in some species but these are easily distinguishable from vernalization, which is an inductive process. After vernalization, floral buds are initiated after a specific photoperiod or when the plant is brought back to a specific temperature.

However, in some species of Brassica the biochemical changes that occur in response to cold and specific photoperiod are comparable. In general long day plants require vernalization and these may be annuals, biennials, or even perennials. If these plants are not exposed to cold, their flowering is delayed or may even fail.

In fact the effect of low temperature enhances with the exposure of exposure until there is saturation of response at a duration which is variable with species. The effective temperature varies widely e.g. from freezing to 10°C. At sub- optimal low temperatures, once the duration of exposure is enhanced, vernalization is complete.

Vernalization implies the conversion of winter varieties to the summer varieties by cold treatment. The low temperature treatment shortens the vegetative period and promotes flowering. Russian geneticist T.D. Lysenko observed that artificial cold treatment of seeds of winter wheat permitted them to behave like spring wheat plants in spring. This process is called vernalization.

In the annual plant, growth is started in the spring, flowers are developed in the summer and fruits and seeds are produced in the fall of the same year. The flowering is primarily under the control of photoperiod and the influence of temperature is secondary. Biennials on the other hand need a cold winter before flowering in its second growing season.

Without subjecting to a cold treatment, the majority of these plants maintain their vegetative stage. Hence they have a qualitative or an absolute requirement for cold e.g. Hyoscyamusniger. However, in Secalecereale the requirement is quantitative or facultative. For instance, under continuous light unvernalized winter rye will flower in 15 weeks, and if vernalized flowering is observed in 7 ⅟₂ weeks. Obviously in this variety, vernalization shortens the time to flower.

For most biennials an “artificial” cold treatment followed by the correct photoperiod and temperature treatment will cause flowering during the first growing season. Thus in a biennial species, the flowering may be induced in the same time period as required for flowering of annuals.

The stage of vernalization also varies. For example, Secalecereale is vernalized in the seed stage while Hyoscyamus has to be vernalized when the plant is 10 days old and in the rosette stage. This is done by soaking and sprouting the seed in winter and keeping them in a vegetative stage by a low temperature or freezing until it can be shown in spring.

Flowering of many biennials is promoted by LD treatment following vernalization as in Hyocyamusniger. Other biennials are day neutral following low temperature treatment. In perennials grasses flowering is promoted by SD treatment following the low temperatures.

Thus, many different kinds of plants are stimulated to flower by cold periods which may have qualitative or quantitative effect. The flowering of some species also is promoted by a suitable day length.

Site of Vernalization:

Whenever vernalization occurs in the mature plants, the receptor of stimulus is the stem apex. In other words, the meristematic cells in the bud respond to the cold treatment. Thus, if a hormone-like substance is produced following vernalization, this occurs in the same cells in which it acts.

This fact is borne by the following experiment. If a plant is vernalized and its meristem is grafted into an unvernalized plant, the latter will flower, indicating as if it had been vernalized.

Conversely, if a meristem of an unvernalized plant is grafted onto vernalized plant where the vernalized meristem was removed, the transplanted, (unvernalized) meristem remains vegetative. Obviously vernalization is restricted to the meristematic tissues themselves.

Some workers have reported that excised leaves or isolated roots of Lunarisbiennis could also be vernalized. On close examination it was observed that new buds were formed on the leaf petioles.

Further these meristematic tissues had to appear before the vernalization was effective. It was proposed that vernalization requires not only the presence of a meristem but also the actively dividing cells. Indeed, replicating DNA was necessary for the perception of vernalization, no matter what their location in the plant was.

It is also suggested that the vernalization process requires energy. Thus, low temperature is effective only in the presence of oxygen, energy substrate (sucrose) and water contents at 40%. The percentage of flowering depends upon the duration of cold treatment, temperature used and the age of plant which varies with species.

New proteins appear following cold treatment. There is also synthesis of new mRNA. Gregory and Purvis have shown that embryo by itself can also be vernalized. In short receptor or perceptive mechanism of vernalization seems to be present in different parts of different plants.

Nature of Vernalization Process:

According to Chailakhyan, possibly two substances are involved in flower formation, one is GA or GA-like material and the other is a different compound. According to him, low temperature and LD requiring plants lack sufficient GA but have enough of the flower-inducing hormone, while SD plants contain sufficient GA but lack the flower-inducing hormone.

This point of view is supported by Melchers studies where a non-cold requiring (SD) tobacco plant (Maryland Mammoth) was grafted to a non-induced cold requiring long day Hyoscyamus plant and this induced the latter to flower.

Seemingly both the species contained one of the essential substances for the flowering process but was dependent on the other plant with which it was grafted for the second. This succeeded for the Hyoscyamus but not for the tobacco.

O.N. Purvis, the British physiologist, has proposed the scheme of flowering in cereal plants:

In this scheme, B is a compound that is part of a reaction system leading to flowering. D is a flowering hormone and C is an intermediate capable of initiating early stages in flowering initiation. The reaction system from B to D is controlled by photoperiod and leads to the production of floral hormone D.

In spring rye (an annual) B is either present in the embryo or is produced from A at normal temperature. In winter rye (biennial) B is insufficient. Exposure to low temperature accelerates its production. In the spring rye or vernalized winter rye, B accumulates in abundance.

Under long day conditions, B is slowly converted to C and C is rapidly converted to D, the flower hormone. Ultimately D reaches a critical stage and flowering ensues, while under short day condition the reaction C to D is inhibited thus forcing the back reaction, C to B to A to occur keeping the plant vegetative.

Application of Vernalization:

The shortening of vegetative period by vernalization in winter cereals enables to have the yield in the first year and helps them to escape drought in the regions having late summer drought.

Late varieties of wheat and oat can be cultivated in northern latitudes with short summer as vernalization helps fruiting in the short summer. Vernalization also increases yield of crop plants, removes wrinkleness of the grains and also enables to obtain flowering out of season.

In India several crops have been vernalized with varied success. For instance in rice, late Professor S. M. Sircar induced some delayed flowering following vernalization. Earlier Kar and Adhikary observed that presowing and high temperature induced early flowering in some varieties.

Presowing cold treatment of Corchoruscapsularis seeds caused delayed seed germination but produced high chlorophyll contents in the leaves and more healthy plants. When the seeds were subjected to prolonged low temperature it caused early flowering and also fruiting.

Some interesting results have been obtained on vernalization of gram, pea, mustard and linseed at different centres. The treatments were either given to the seeds, just emerged seedlings or even sprouted seeds.

The role of vernalization in reducing kernel shrivelling in triticale, a new synthetic genus and a polyploidy hybrid produced by cross breeding wheat and rye, has been shown. The basic mechanisms which regulate the kernel development were correlated with the vernalization response.

The lowering of α-amylase activity with high accumulation of endosperm starch leading to the reduction in kernel shrivelling could be obliterated through vernalization.

Plant Tropic Movements

1. Thigmotropism (Haptotropism):

Growth movements made by plants in response to contact with a solid object are called thigmotropism. These are curvature movements and are most apparently seen in tendrils and twiners. In most plants the curvatures of the tendrils which follow contact with a support are mostly the result of increased growth on the side opposite the stimulus.

2. Phototropism:

This kind of movement is induced by light. Not all plants and not all parts respond in the same way to this stimulus. In general, the stem mostly grows and turns towards the source of light, while the roots away from it. As shown in fig. 7.10 the leaves also positively respond toward the source of light. The leaves, however, take up such a position in which the broad surface of the blade is at right angles to the light rays. A stem is, therefore, said to be positively phototropic, a root negatively phototropic, and a leaf transversely phototropic or diaphotropic. Phototropism is also known as heliotropism.

In certain plants, such as Arachis hypogea (ground nut) more complex changes occur within a short period of time. The flower-stalks of this plant initially show positive phototropism until they have produced flowers. Soon after fertilization the stalks curl up and eventually bury the developing pods under the soil, thus showing negative phototropism.

3. Geotropism:

Any reaction to the stimulus of earth’s gravity is called geotropism. The effects of gravity on plants are not like those of light and temperature because it is both continuous in action and constant in strength. Primary roots and certain other portions of the root system tend to grow directly toward the centre of gravity and hence called positively geotropic.

Stem mostly grows away from the centre of gravity and is thus negatively geotropic. However, stems in prostrate plants have lost their negative geotropism and even develop into root stock or tubers which behave exactly like roots. Most of the leaves take up their positions at right angles to the centre of gravity and are, therefore, called transversely geotropic or diageotropic. Geotropism is of three types: orthogeotropism (e.g. primary root), plageotropism (e.g., secondary roots) and diageocropism (movement of tertiary roots)

4. Thermotropism:

Some of the plant organs markedly respond towards fluctuating atmospheric temperature. In response to this kind of stimulus plant parts exhibit curvature movements in order to take some advantageous position. Such movements are called thermotropism.

5. Chemotropism:

Certain chemical substances are responsible to bring about curvature movements in plant organs. For instance, movement of Pollen tube towards ovary due to absorption of calcium and borate from style of carpel; movement of tentacles in Drosera, closing of lid of Nepenthes due to nitrogenous food, and penetration of haustoria of parasite into host body etc.

6. Hydrotropism:

The paratonic curvature movements of growth in relation to the stimulus of water are called hydrotropic movements. The tropic response to the stimulus of water is called hydrotropism. The roots show positive hydrotropic response, i.e., they bend towards the water. Hydrotropism is stronger in roots compared to geotropism.

Plant Nastic Movements

1. Seismonastic Movements:

These movements are brought about by mechanical stimuli such as contact with a foreign body, fast wind and rain drops etc. Seismonastic movements are seen in stigmas, stamens and leaves of many plants. For instance, movements of leaf lets in Mimosa pudica (Sensitive plant), Biophytum sensitivum and Neptunia, etc.

Stigma lobes in certain plants such as Mimulus, Martynia and Bignonia, etc., encircle the pollengram as soon as it falls over them Stamens of Berberis, Portulaca and Opuntia respond instantly when touched by the body to any insect. The extent of the seismonastic movement depends upon such factors as the intensity of the stimulus, the vigour and age of the plant, and the time elapsed since the last stimulus.

2. Photonastic Movements:

These movements are induced by fluctuations in the intensity of light Such movements are exhibited by flowers of several plants. Many flowers open with the increasing illumination of the day and close up with the decrease in light intensity. Flowers of Cestrum nocturnum open at night, and close up with the dawn of the day.

3. Thermonastic Movements:

Such movements are brought about by changes in temperature. Many of the flower movements are thermonastic. Such flowers open with a rise and close with a drop in temperature. Sometimes thermonastic movements are associated with photonastic movements. In both types of responses the mechanism may be either differential growth, or changes in turgor on upper and lower sides of the petiole, leaf blade, or perianth part.

4. Nyctinastic Movements:

These movements are commonly called ‘sleeping movements’. Some authors have classified such movements under the category of photonastic or thermonastic movements. These movements are induced by alternation of day and night. The leaves of some plants like Enterobium (Fig. 7.15), Clover and Oxalis, growing approximately horizontal during the day, begin to droop and close toward evening and do not rise again until the next morning.

Conclusion:

Various kinds of movements are exhibited by plants and their organs. Smaller plant organisms and naked protoplasmic bodies show movements of locomotion where as higher plants, being fixed in a position, show movements of curvature. All such movements may be tactic, tropic or nastic.