The formation of flower or an inflorescence occurs during and after the transition of shoot apex to flowering phase. The introduction of scan electron microscope has greatly advanced the study of development of floral organs in a flower and floral primordia in an inflorescence. In an inflorescence apex a series of small, leaf-like primordia of involucral bracts appear first.
Subsequently the floral primordia are formed in acropetal sequence. Floral primordia gradually develop over the whole surface of the inflorescence apex. In the apex of an individual flower the floral organs, in most cases, appear in distinct acropetal sequence (Fig. 30.3). The recently formed organ is closest to the reproductive apex.
Exceptions are noted in certain genera and families where basipetal or centrifugal development of the primordia of stamen is noted in the zone of stamens. Ex. Paeonia, Bixaceae, Dilleniaceae, Tiliaceae etc. There are many other exceptions that are mentioned in the literature of Fahn (1997).
The apical meristem of an individual flower and inflorescence consists of a central zone of cells covered by an outer zone of cells. The cells of the central zone have large vacuoles. The cells of outer zone are smaller than the cells of central zone.
The cells of outer zone are rich in protoplasm and they divide anticlinally. It is noted that the number of layers of cell in the outer zone of reproductive apical meristem is larger than that is present in apical meristem of vegetative apex.
The various lateral members of a flower like sepal, petal, stamen and carpel originate from the second and third layers of apical meristem, present in the reproductive apex of an individual flower. Ex. Nuphar, Aquilegia etc. In Portulaca the second layer of apical meristem gives rise to the lateral organs of a flower by periclinal divisions.
Leaf, the other lateral member of vegetative shoot apex, also shows similar type of development. This led Cutter (1971) to interpret that all the lateral members of the shoot apex are ‘different manifestations of a leaf-like structure’.
In angiosperm there exist a number of variations in the development of flower. No single example can be cited as a typical for the whole group. Boke in 1948 illustrated the development of flower of Vinca rosea (Fig. 30.4). In the reproductive apex the sepals are formed in spiral sequence. After the evocation of sepals the apex enlarges.
Petal primordia (five in number) are formed on the enlarged apex. The petals unite during development. The stamen primordia originate from the basal region of corolla tube. After the formation of stamen the floral apex reduces considerably. The second layer of tunica and the peripheral layers of corpus of floral meristem divide periclinally and form the primordia of sepals, petals and carpels.
The filament of stamen is formed as a result of intercalary growth at the base of stamen primordium. The diameter of floral meristem increases again at the end of stamen formation. The cells at the periphery of the meristem form a meristematic ring. On the two opposite sides of meristemaic ring carpel primordia are formed.
The floral members and vegetative leaves exhibit similar growth pattern. Sepals and petals undergo apical, marginal and intercalary growth. The apical growth is of shorter duration. Increased activity is observed in plate meristem. Marginal growth occurs in stamen and carpel. Thus, the leaf and the lateral members of a flower exhibit similar mode of growth and origin.
The shoot apical meristem forms diverse type of lateral organs. Normally it forms leaves and after switching over to floral meristem it forms all the floral organs. Naturally it is assumed that the shoot apical meristem undergoes various changes during ontogeny. Wardlaw (1961) suggested that in the apical meristem during development there is a ‘sequential evocation of genes’. These genes, when expressed, form the different lateral organs.
The expression of each gene is dependent on the preceding one. Heslop-Harrison (1964) holds more or less same opinion that each set of gene expresses to develop each phase of flower development. The expression of each set is dependent on the preceding act. Heslop-Harrison considers that each set of floral gene becomes depressed during ontogeny thus allowing the other set of genes to express.
The sequential evocation of genes causes the floral meristem to pass through different physiological states. The successive different physiological states regulate the formation of each kind of floral organ. Microsurgical experiments were designed to test the formation of various physiological states and consequently the formation of floral organs.
The young floral meristem of Primula bulleyana was bisected. It was observed that new floral apices developed from each half apex. The floral meristem was bisected at various stage of development. It was observed that when the floral meristem was bisected before sepal formation, complete flower developed from the half apices. In the later stages of development the flower develops lacking the organ adjacent to which incision was made.
It is previously mentioned that expression of each set of gene is dependent on the preceding one in a floral meristem. In this sense the petal forming genes will express later than the sepal forming genes. In other words the peripheral whorl of sepals arises first and it then induces to form the next inner whorl, i.e. petal, which in turn induces to form the next inner whorl—stamens and so forth.
Microsurgical experiments support the above view. The different floral organ forming peripheral areas on the floral meristem of Nicotiana tabacum were resected and the fate of inner floral organs were noted. It was observed that resection at sepal forming areas prevented the formation of petals, stamens and carpel.
In another observation when the petal promordia were resected there are no formation of stamens and carpel. Despite the elegance of the microsurgical experiments, the results obtained on the basis of molecular genetics reveal that the formation of petal or carpel is not dependent on the preceding one. Studies on ABC model of flowering (discussed later) reveal that any type floral organ can be induced to develop in any whorl at will.
Though the conclusion derived from microsurgical experiments is inconsistent with the finding based on molecular genetics, it is consistent with the fact that the development of floral organs in the inner whorl is dependent on the formation of organs in preceding whorl regardless the type of organ produced in that whorl.
Two major factors of the environment control the induction of reproductive growth in plants. These are temperature and photoperiod. They are described in a tremendous bulk of Internet Websites and literatures and will be mentioned here in brief.
Some amount of vegetative development precedes the formation of flowering. A plant must attain certain age before it is ‘ripe to flower’. Klebs introduced the term ‘ripeness-to-flower’ in 1913. Later this term is used in the study of vernalization and to denote the time when a plant becomes sensitive to photoperiodic induction. The age varies markedly with species. In the seedling stage plants like Chenopodium rubriim, Pharbitis nil etc. can be induced to flower.
Xanthium strumarium and Hyoscyamus Niger need a few weeks of age when they can be induced to flower. The forest trees need tens of years of vegetative growth before the formation of any flower. Environmental stimulus causes the transition to flowering of the vegetative shoot apex. When a plant is ‘ripe to flower’ a period of exposure to low temperature is required for flowering.
The stimulus of transition to flowering of vegetative apex by exposure to low temperature is called vernalization. Vernalization induces or accelerates flowering by low temperature. Normally annuals are vernalized at seedling stage and biennials perceive the cold temperature stimulus after the first season of growth (ex. Digitalis, Apium graveolens etc.). The spring-flowering species like Viola and Primula perceive the low temperature stimulus during the preceding winter.
Vernalization is well studied in cereal crops like wheat, rye (Secale cereale) etc. These crops have several varieties. There exists marked difference in reproductive behavior between the varieties. Vernalization may be illustrated by the following example. The seeds of winter variety of wheat when sown in autumn, they flower in early summer.
The seeds germinate and the seedlings attain some growth before winter. At this stage the seedlings consist of large number of leaves that remain close to the ground in the form of a mat.
This stage of wheat seedling is referred to as rosette stage. The growth is minimum at this stage in winter. Normal growth is resumed with the advent of spring and the crop is produced following flowering in summer.
However if the seeds of winter variety of wheat are sown in late spring, they will grow up to the rosette stage. These plants fail to form flower. In contrast, the spring variety of wheat shows a different reproductive behavior. The seeds of spring variety of wheat when planted in spring, they grow and produce crop following flowering in early summer.
The seeds of winter variety can be converted to spring variety by vernalization treatment in the following way. The seeds of the winter variety were moistened with water and allowed to germinate. The germinated seeds were then stored for a few weeks at 0° to 5°C. Such treated seeds when sown in the spring, they grow and produce crop following flowering in the early summer.
The rosette stage is omitted and the seedling responded like spring variety. The winter variety of wheat needs low temperature (0°-5°C) for vernalization but the plants like millet, sorghums, maize, soybeans etc. need a high temperature of 20°-30°C for 5-10 days.
The growing tip consisting of the meristematic tissues is the site of vernalization. Certain plant like Arabidopsis thaliana can be induced to flower at the stage of seed. Researches reveal that the embryo is the seat of response of vernalization in a seed. The effect of vernalization is transmitted within the plant.
The following experiment with Chrysanthemum reveals it (Fig. 30.5). A plant was grown and the apex was treated with chilled temperature. The treated apex was allowed to grow until it forms two more leaves. The apex and the terminal leaf were removed.
The axillary bud, which is present in the second leaf from tip, was allowed to grow until it forms two more new leaves. The apex and the terminal leaf were removed and the axillary bud present in the next leaf was allowed to grow to form two more new leaves.
The apex and the terminal leaf were again removed and this process was repeated until seven pairs of leaves were produced. It is to note that all the seven pairs of leaf are developed from the original apex that is treated with cold temperature. The terminal bud and leaves of the 7th pair were not removed. The terminal bud was allowed to grow. It was observed that it produced flower.
Once an apical meristem is vernalized it seems to be permanent. All growth forms from it are also vernalized. The effect of vernalization in Chrysanthemum is confined to the treated apex only.
Flowering is not induced in non-chilled apices of the same Chrysanthemum plant. This argues against the presence of a transmissible stimulus as a result of induction of vernalization. In favour of the presence of transmissible stimulus the following grafting experiment is cited.
Hyoscyamus niger was vernalized and a leaf from it was grafted on to unvernalized Hyoscyamus. It was observed that in unvernalized Hyoscyamus flowering was induced. The effect of vernalization was transmitted to the unvernalized Hyoscyamus that led to the induction of flowering. The transmissible stimulus is common to more than one species. It was revealed from the experiment that the grafting of vernalized leaf of Hyoscyamus was successful on Petunia and Nicotiana.
Later researches suggest that the effect of vernalization is mediated through a hormone. Melchers (1936) postulated the existence of a flower-forming hormone called vernalin on the basis of the transmission of vernalization stimulus across a graft union between vernalized and unvernalized Hyoscyamus plant.
Vernalin is hypothetical hormone that has not been extracted as yet even in crude form. Florigen is hypothetical flowering hormone. Cajlachjan (1936) coined the word ‘florigen’ for the hypothetical unisolated flowering hormone. Vernalin is assumed to be the precursor of florigen (Fig. 30.6) or catalyser of its formation.
Lang (1957) first reported that exogenous gibberellic acid (GA) could cause flowering in vernalized-requiring plants under non-inductive environmental condition and in many species of long day plants. Chailakhyan (1970) emphasized the role of GA on the flowering of many plants.
Precursors of GA are produced and accumulated during vernalization. These precursors change into gibberellins in long days. So it is regarded that the precursors of gibberellins are hormone, which is assumed to be the vernalin. Vernalin is produced on the meristematic shoot apex and seems to move only with dividing cells.
After vernalization if a plant gets definite photoperiod flowering occurs. Garner and Allard (1920) termed the response of plants to the length of day as photoperiodism. Garner and Allard classified angiosperms into three main groups according to day-length that affects the ways of flowering.
These groups are referred to as short-day plant (SDP), long-day plant (LDP) and day-neutral plant (DNP). SDP requires day length that is reduced to a certain critical value. In LDP flowering occurs only if the day-length exceeds a certain critical value. In DNP the day-length does not exert any effect on the time of flower initiation.
Though the SDP, LDP and DNP are classified on the basis of critical day-length, it is regarded that photoperiodism is a response to the duration and timing of light and dark periods. Photoperiodism occurs in natural 24-hours light-dark cycles and so the duration and timing of light and dark periods are referred to ‘critical day-length’ and ‘critical night-length’.
In LDP critical day-length is the minimum day-length required for flowering. In SDP, the critical day-length is the maximum day-length on which flowering can occur. In contrast the critical night-length in SDP is the minimum period of darkness required for flowering, whereas in LDP the critical night-length is the maximum period of darkness required for flowering.
So in LDP, for flowering, the day length must be longer than critical day-length and the dark period must be shorter than the critical night-length. On the contrary in SDP, for flowering, the day length must be shorter than the critical day-length and the dark period must be longer than the critical night-length.
So it is the day lengths shorter or longer than the critical day-length will determine whether flowering will occur. The distinction between SDP and LDP in not based on the absolute values of the critical day-length. The following illustration will explain the above discussion. Xanthium strumarium is a short-day plant.
The critical day- length of this plant for flowering is about 15.5 hours. Hyoscyamus niger is a long- day plant. The critical day-length of this plant for flowering is about 11 hours. When 12 to 14-hour of photoperiods in 24-hour light-dark cycles was given to Xanthium strumarium and Hyoscyamus niger, flowering occurred in both species.
Later researches reveal that the single most important controlling factor for flowering is the length of the uninterrupted dark period. For example, to stimulate flowering in the short-day plant Xanthium strumarium 8-hour day and 16-hour night are required. Such appropriate photoperiod is referred to as inductive day or night.
In an experiment with Xanthium a brief flash of light interrupts the continuous 16-hour inductive dark period. This inhibits flowering. On the contrary it is observed that when a LDP is kept under SDP condition, a brief flash of light during the continuous 16-hour dark period induces flowering (Fig. 30.7).
Using action spectrum technique it is possible to determine the wavelengths of light that are effective in the night interruption phenomenon. Experiments with monochromatic light have revealed that red light in the region of 660 nm wavelength is most effective in the night interruption phenomenon.
The effect of red light can be counteracted with the subsequent application of ‘far-red’ light in the region of 730 nm wavelengths. The effect of red and far-red light is depicted in Fig. 30.8. In SDP when the long inductive night was interrupted by red light the flowering was inhibited, while by the subsequent application of far-red after red promoted flowering.
The experimental plant may be exposed several times to red followed by far-red and accordingly the inhibition and promotion of flowering is observed. The effect of light on flowering is always dependent on the last quality of light exposure, i.e. when it is far-red flowering occurs, but when it is red, flowering is inhibited.
The above behavior was later interpreted on the basis of the presence of phytochrome. Phytochrome is a pigment and it is involved in floral initiation. It is a water-soluble chromoprotein and bluish in color. It is a dimeric chromoprotein and consists of two chromophores, each of which consists of linear tetrapyrrole compound covalently linked to a cysteine residue.
Phytochrome exists in two states that are referred to as Pr and Pfr. Each form is photointerconvertible, i.e. each form is capable of being converted to the other form by absorbing light of appropriate wavelength. Pr form absorbs light maximally in the red region of 660 nm wavelength. Pfr absorbs light maximally in the far-red region of 730 nm wavelength and it serves as the photoreceptor.
The ‘red, far-red reversible photoreactions’ of flowering occur through these interconvertible forms of phytochrome. It is to note that both Pr and Pfr also show absorption in the blue region at about 370 nm and 400 nm respectively but their action spectra for the photo-transformations are restricted to 660 nm and 730 nm respectively.
Pr is metabolically inert form. The active form of phytochrome is Pfr. Phytochrome is synthesized in Pr form. It is red-light absorbing form and red light converts Pr to Pfr. Pfr may then exert its action by promoting flowering. Pf is the far-red absorbing form. Pfr is reverted to Pr by far-red. Pfr may also revert to Pr in the dark. Pfr may be destroyed without exerting its action (Fig. 30.9).
Evidence suggests that during photo-transformation of Pf to Pfr in one chromophore, a proton migrates from pyrrole ring to its ethylidene group. This causes a shifting in double bond in the pyrrole ring 1 (Fig. 30.10). Changes also occur in the protein portions. It is regarded that protein accounts Pr being inactive and Pfr being active.
Phytochrome has effect on the permeability of plasma membrane. For example, the exposure of far-red light to Mimosa pudica at the end of the day inhibits the sleep movements of leaves.
The effect of far-red can be nullified by the application of red light. It is now known that the turgidity and flaccidity of pulvinus —the swollen base of compound leaf of Mimosa is responsible for seismonasty, i.e. the sleeping movements of leaves of Mimosa.
Evidence suggests that the movement of potassium ions controls the turgor of cells present in the pulvini. Hence it is regarded that phytochrome regulates the permeability of plasma membrane.
So far as transition to flowering is concerned, it is suggested that phytochrome changes the permeability of plasma membrane of cells present in the shoot apex. As a result the entry of growth hormones like gibberellins, cytokinin etc. is regulated. The level of growth regulators determines the response to flowering.
The leaf perceives the light stimulus. A single leaf of Xanthium, more precisely one eighth of one leaf is enough to cause the plant to flower. Evidence indicates that the stimulus developed as a result of inductive photoperiod is transmissible. The site of perception and the site of responses are distantly placed.
This led to postulate the hypothetical flower forming hormone —’florigen’. The concept of florigen complex is now based on the following four groups of hormones and physiologically active compounds —indole acetic acid, phytokinins, gibberellins and anthesins. Gibberellins and anthesins form an integrated system of metabolism that precedes flowering.
Gibberellins are necessary for the formation and elongation of stem and anthesins are necessary for flower formation. The above mentioned four groups of hormone influence flowering. Lang (1965) after extensive grafting experiments concluded that the florigen complex is same in LDP, SDP and DNP.