Photoperiodism and Vernalization

Flowering in a plant occurs at a particular time of the year and controlled by many morphological and environmental conditions. Two important controlling factors are photoperiod or light period, i.e., photoperiodism, low temperature i.e., vernalization.

(1) Photoperiodism (Light period) : The effects of photoperiods or daily duration of light periods (and dark periods) on the growth and development of plants, especially flowering is called photoperiodism. The role of photoperiodism in the control of flowering was demonstrated for the first time by W.W Garner and H.A. Allard (1920). They observed that Maryland Mammoth variety of tobacco could be made to flower in summer by reducing the light hours with artificial darkening. It could be made to remain vegetative in winter by providing extra light. On the basis of length of photoperiod requirements of plants, the plants have been classified into following categories.

(i) Short day plants (SDP) : These plants initiate flowering when the day length (Photoperiod) become shorter than a certain critical period. Most of winter flowering plants belong to this category e.g., cocklebur (Xanthium), Chrysanthemum, sugarcane, tobacco (Mutant Maryland Mammoth), soyabean, strawberry etc.,

(ii) Long day plants (LDP) : These plants begin flowering when the day length exceeds a critical length. This length too differs from species to species. The long day plants fail to flower, if the day length is shorter than the critical period. e.g., spinach (Spinacea oleracea), henbane (Hyoscymus niger), radish, sugar-beet, wheat, lattuce, poppy, larkspur, maize etc.

(iii) Day neutral plants : These plants can flower in all possible photoperiods. The day neutral plants can blossom throughout the year. e.g., cucumber, cotton, sunflower, tomato, some varieties of pea, etc.

(iv) Intermediate plants : These plants flower only under day lengths within a certain range usually between 12-16 hours of light but fail to flower under either longer or shorter photoperiods. e.g., Mikania scandens, Eupatorium hyssopifolium and Phaseolous polystacous. 

(a) Amphiphotoperiodic plants : Such plants remain vegetative on intermediate day length and flower only on shorter or longer day lengths. e.g., Media elegans.

(b) Short long day plants : These plants require short photoperiods for initiation of flowering and long photoperiods for blossoming. e.g., Triticum vulgare, Secale cereale.

(c) Long short day plants : These plants require long photoperiods for initiation of flowering and short photoperiods for blossoming. e.g., Bryophyllum, Cestrum.

Critical period : Critical photoperiod is that continuous duration of light, which must not be exceeded in short day plants and should always be exceeded in long day plant in order to bring them to flower. There is no relation with the total day length. Thus, the real distinction between a SDP and LDP is whether flowering is induced by photoperiods shorter or longer than the critical period. The critical day length for Xanthium (a short day plant) is 15. 6 hours and that for Hyoscymus niger (a long day plant) is about 11 hours, yet the former is SDP as it flowers in photoperiods shorter than its critical value, whereas the latter is LDP requiring photoperiods longer than its critical value. Both Xanthium and Hyoscymus niger flower with 14 hours of light per day. Thus, day length in which a plant flowers is no indication of its response class in the absence of further information.

Skotoperiodism (Dark period) : When photoperiodism was discovered, the duration of the light period was thought to be critical for flowering. Subsequently, it was found that when the long night period was interrupted by a brief exposure to light, the short day plants, failed to flower. Thus, for flowering, these plants require a long night or critical dark period rather than a short day length. Similarly, long day plants respond to nights shorter than the critical dark period. Curiously, they do not need an uninterrupted dark period. Therefore, a short day plant is also called long night plant and a long day plant as a short night plant.

In the night interruption experiments, when the short day plants were exposed to a flash of light before achieving a critical dark period, flowering was prevented. It is called light break reaction.

Mechanism of photoperiodism

Photoreceptor : The chemical which perceives the photoperiodic stimulus in leaves is phytochrome. The wavelengths of light are absorbed by the leaves. This becomes evident by the fact that defoliated (leaves removed) plant does not flower. Presence of even a single leaf is sufficient to receive required amount of photoperiod. Partially mature leaves are more senstitive to light while very young or mature leaves are much less sensitive to photoperiodic induction.

Garner and Allard’s early work led to the discovery, isolation and much of the characterization of the pigment responsible for absorbing light involved in photoperiodic phenomenon of plants. Borthwick, Hendricks and their colleagues later termed this pigment phytochrome. Pigment was isolated by Butter et al. (1959). This pigment controls several light dependent developmental processes in plants besides flowering, phytochrome exist in two interconvertible forms. The red (660nm), absorbing form Pr and the far red (730 nm), absorbing form Pfr. Pr is converted to Pfr on absorbing red light. Pfr is converted to Pr rapidly absorbing far red light or slowly in darkness. The slow conversion to red absorbing form is under thermal control. During the day when white light available, Pfr accumulates in the plant. This form of phytochrome is inhibitory to flowering in short day plants and stimulatory to flowering in long day plants. In evening, Pfr undergoes thermal and spontaneous decay to change into Pr. This pigment is stimulatory to flowering in short day plants and inhibitory to flowering in long day plants.

Therefore, in SDP interruption of dark periods with a flash of red light converts Pr into Pfr and flowering is inhibited.

Importance of phytochrome : Phytochrome is located in plasmamembrane. Phytochrome far red (Pfr) form is considered to be biologically active form and is responsible to initiate a number of physiological process such as.

(1) Elongation of stem and leaves.

(2) Plastids morphology and differentiation of stomata.

(3) Seed germination.

(4) Photoperiodism and transpiration.

The florigen complex (Flowering hormone): When the proper amount of light is perceived by leaves, they produce a chemical (flowering hormone), which undergoes stabilisation in dark. Later on, this chemical passes to shoot apex and causes its differentiation into flowering shoot.

Chailakhyan (1936) a Russian investigator on photoperiodism, proposed that it be called 𠆏lorigen’. According to him (1958) the 𠇏lorigen complex”, the true flowering hormone includes two groups of substances formed in leaves :

Gibberellins : Which are necessary for formation and growth of stem.

Anthesins : Substances which are necessary for flower formation.

Photomorphogenesis : When plants are grown in continuous darkness they become etiolated i.e., such plants are longer, weaker, having yellowish half opened leaves, while light grown plants do not show such conditions. When etiolated plants are kept in light they gradually develop green colour and become normal. The effect of light in reversing etiolation involves two kinds of action one the biochemical level for the synthesis of the chlorophyll and secondly at the level of morphogenesis light acts to promote expansion of the leaves and inhibits elongation of the internodes. This phenomenon is called photomorphogenesis and is independent of the direction of light.

The action spectrum of photomorphogenesis reveals that plants are most sensitive to red light, but blue light is ineffective.

(2) Vernalization : Russian agronomist Lysenko coined the term vernalization (1929-30). According to him vernalization may be defined as the method of inducing early flowering in plants by pretreatment of their seeds at low temperatures. Chourad (1960) has defined it as the acquisition or acceleration of the ability to flower by chilling treatment. The low temperature requirement for flowering was first noticed by Klipport (1857) while working with winter varieties of cereals such as wheat, barley, oat and rye. He observed that, these varieties when sown in spring failed to flower the same year but grow vegetatively. Such winter varieties, when sown in the autumn, they flowered in spring of the same year.

Site of vernalization : The stimulus of vernalization is perceived only by the meristematic cells such as shoot tip, embryo tips, root apex, developing leaves etc.

Requirement of vernalization :

(i) Low temperature : Low temperature requiredਏor vernalization is usually 0-4^oC is most of the cases. The chilling treatment should not be immediately followed by high temperature (i.e., about 40^oC), otherwise the effect of vernalization is lost. This phenomenon is called de-vernalization.

(ii) Duration of low temperature treatment : It varies from species to species from a few hours to a few days.

(iii) Actively dividing cells : Vernalization stimulus is perceived only by actively dividing cells. Therefore, vernalization treatment can be given to the germinating seeds or whole plant with meristematic tissues and other conditions.

(iv) Water : Proper hydration is must for perceiving the stimulus of vernalization.

(v) Oxygen : Aerobic respiration is also a requirements for vernalization. The stimulus has been named as vernalin (reported by Mechlers).

Process of vernalization : Usually vernalization treatment is given to the germinating seeds. The seeds are moistened sufficiently to allow their germination. They are then exposed to a temperature of 0-4^oC for a few weeks and sown to the fields. Lysenko developed the process of vernalization it is completed in two stages.

(i) Thermostage : Germinating seeds are treated with 0-5^oC in presence of oxygen and slight moisture. The seed dormancy is broken.

(ii) Photostage : The stage is very essential to initiate the reproductive phase. After vernalization plants must be subjected to a correct photoperiod in order that they may produce flowers.

Importance of vernalization

(i) Vernalization is believed to overcome some inhibitor and induce synthesis of growth hormones like gibberellins.

(ii) It reduces the vegetative period of plant.

(iii) It prepares the plant for flowering.

(iv) It increases yield, resistance to cold and diseases.

(v) Vernalization can remove kernel wrinkles in wheat.

(vi) Vernalization is beneficial in reducing the period between germination and flowering. Thus more than one crop can be obtained during a year.

Senescence and Death

Plant and their parts develop continuously from germination until death. The production of flowers, fruits and seeds in annuals and biennials leads to senescence. The latter part of the developmental process, which leads from maturity to the ultimate complete loss of organization and function is termed senescence. Several workers equate ageing and senescence as same process. Ageing is a sum total of changes in the total plant or its constituents while senescence represents degenerative and irreversible changes in a plant. The study of plant senescence is called phytogerontology. 

Types of senescence : Plant senescence is of four types- whole plant senescence, shoot senescence, sequential senescence and simultaneous senescence. The last three are also called organ senescence.    

(1) Whole plant senescence : It is found in monocarpic plants which flower and fruit only once in their life cycle. The plants may be annual (e.g., rice, wheat, gram, mustard etc.), biennials (e.g., cabbage, henbane) or perennials (e.g., certain bamboos). The plant dies soon after ripening of seeds.

(2) Shoot senescence : This type of senescence is found in certain perennial plants which possess underground perennating structures like rhizomes, bulbs, corm etc. The above ground part of the shoot dies each year after flowering and fruiting, but the underground part (stem and root) survives and puts out new shoots again next year. e.g., banana, gladiolus, ginger etc.

(3) Sequential senescence : This is found in many perennial plants in which the tips of main shoot and branches remain in a meristematic state and continue to produce new buds and leaves. The older leaves and lateral organs like branches show senescence and die. Sequential senescence is apparent in evergreen plants e.g., Eucalyptus, Pinus, Mango.

(4) Simultaneous or synchronous senescence : It is found is temperate deciduous trees such as elm and maple. These plants shed all their leaves in autumn and develop new leaves in spring.�use of this shedding of leaves, autumn season is also called fall. e.g., Dalbergia, Elm, Mulberry, Poplar.

Theories of senescence

(1) Wear and tear : According to this theory, senescence occurs due to loss of activity and cells undergo wear and tear due to disintegration of organelles.

(2) Toxicity : It is viewed that senescence takes place due to accumulation of toxic and deleterious substances in all.

(3) Loss of metabolites : It is assumed that senescence leads to gradual depletion of essential metabolites in a cell.

(4) Genetic damage

Characteristics of ageing and senescence

(1) There is general decline in metabolic activities, decline in ATP synthesis and also decreased potency of chloroplast.

(2) Decrease in RNA and DNA

(3) Decrease in semipermeability of cytoplasmic membranes.

(4) Decrease in the capacity to repair and replace wornout cells.

(5) There may be accumulation of chromosomal aberrations and gene mutations with advancing age as a result of these changes protein synthesis becomes defective.

(6) Increased production of hydrolytic enzymes such as proteases and nucleases.

(7) Deteriorative change in cell organelles and membranes.

(8) Decrease in the internal content of auxin and cytokinins and increase in the production of abscisic acid or ethylene.

Importance of senescence : Biologically senescence and death have following advantages :

(1) It maintains efficiency since the old and inefficient organs are replaced by young efficient part like leaves, buds, flowers and fruits. etc.

(2) During senescence, the cellular breakdown results in release of many nutrients including amino acids, amides, nucleotides, simple sugars and minerals. The same are withdrawn from the senescing organs into the main trunk and later utilised in the growth and developed of new parts.

(3) Shoot senescence is a mechanism to help the plants perennate during the unfavourable periods.

(4) Simultaneous or synchronous leaf fall occurs in autumn prior to winter. It reduces transpiration, which is essential for survival in winter, when the soil in frozen and roots can not absorb water.

(5) Litter of fallen leaves and twigs is an important source of humus and mineral replenishment for the soil.

Abscission

The process of shedding of leaves, fruits or flowers by a plant is called abscission. The shedding of plant parts takes place by the formation of a special layer of cells called abscission layer, within the region of attachment. The middle lamella between certain cells in this layer in often digested by polysaccharide hydrolyzing enzymes such as cellulase and pectinases.