In this article we will discuss about the process of transition in flowers, explained with the help of suitable diagrams.
Reproductive apex exhibits plastochron size fluctuations during the development of floral parts/primordia. The apex may widen and become slightly flattened.
In Liriodendron the apex becomes conical and elongated. Smith (1966) reported the horizontal and vertical enlargement of the apex in Carex. The apex of Chrysanthemum, during the formation of capitulum, increases in area nearly 400-fold within a few hours.
During transition to flowering apex the above-mentioned changes in shape is linked with the rise of mitotic index in the shoot apex. The increased mitotic activity is observed in the cells present between the rib meristem and central mother cell zone. Gradually mitoses spread over the zone of central mother cell.
As a result cells are added to tunica. In the study of transition to flowering, plants are induced to flower. In an experiment with Xanthium, which is one of the most thoroughly investigated angiosperm leaves and flowers; it is observed that a significant increase in mitotic index occurs within 24 hours after the beginning of inductive treatment.
In one occasion the mitotic index increased within 16 hours after the beginning of the inductive treatment. Bhar et al. (1969) observed the increase of mitotic index in Pharbitis nil on the second day after inductive treatment. Several species of short day plant exhibit increased rate of mitosis following induction.
The cells, situated below the central zone and distal to rib meristem, show increased mitotic activity. Apart from mitosis the amount and distribution of endoplasmic reticulum change. This was observed in Chenopodium, 3 hours after the end of the first inductive long night. The enzyme phosphatase and the number of dictyosomes per cell increase.
[Mitotic index is the percentage of cells where nuclei are in mitotically dividing stage in the total number of cells present at the time of fixing. Fixing is a process of immediate killing of tissues and to preserve the contents of cells as closely as possible to their living state. Mention may be made of the following chemicals that are frequently used as cytological fixatives: Ethyl alcohol, Nawaschin-A, Nawaschin- B, Liwitsky’s fixative, Carnoy’s fixative, Bouin’s fixative etc.]
The typical tunica-corpus organization of vegetative shoot apex, advanced by Schmidt in 1924, becomes modified during the transition to flowering. In the floral apex Gregoire recognized two zones. The outer zone consisted of darkly staining cells and this was named as mantle. The inner zone was composed of large parenchyma cells and that was named as core.
These two zones cannot be analogized with the usual tunica-corpus organization. So, according to Gregoire, the outer and inner zones were respectively named as meristematic mantle and parenchymatous core (Fig. 30.1). The large cells of core become highly vacuolated during transition. The association between mantle and vacuolated large cells of core are termed as mantle-core type of organization.
Mantle-core type of organization gradually becomes established during transition to flowering. Thus a mature floral apex shows outer mantle and inner parenchymatous core. The mantle is composed of dense cells and consists of tunica and a large part of the corpus. The parenchymatous core is largely derived from the rib meristem situated below the apex.
Before transition to flowering the apex of Calendula officinalis is dome-shaped with two-layered tunica. The apex is gradually transformed to a broadly based cone by horizontal and vertical growth. The rib meristem is conspicuous. The apex loses the organization of vegetative apex. At this stage the mantle-core organization is yet to be established. Calendula has capitulum inflorescence.
Before flowering begins the inflorescence apex assumes the characteristic shape by the increase of central parenchymatous core. Primordia of involucral bracts appear first on the inflorescence apex. After further growth primordia of floret appear on the periphery of inflorescence apex in acropetal sequence. Gradually floral primordia develop over the whole surface of inflorescence apex (Fig. 30.2).
The shoot apex of Xanthium has single-layered tunica. The cells present on the outermost layer of corpus are large. The rib meristem is conspicuous. Xanthium plants were given inductive photoperiod to study the transition of vegetative apex to reproductive apex. Xanthium is short day/long night plant.
The vegetative apices of Xanthium plant were treated with one inductive long night. The apices showed a change in form two days after induction. Increased mitotic index was observed in the cells present between the rib meristem and central zone. In these cells the ribonucleic acid content is also increased.
The mantle-core organization is established seven days after induction. Primordia of floret developed ten days after induction. The above-mentioned changes are also observed in the development of a single flower in the floral apex of Nuphar lutea of the family Nymphaeaceae. In the floral apex the mantle-core type of organization is gradually established. The floral organs are formed on the reproductive apex.
Buvat (1955) interpreted the vegetative shoot apex to consist of anneau initial and meristeme d’attente. The former is the lateral and sub terminal meristem, and considered as the initiating ring to cause the vegetative growth of the shoot apex. The anneau initial or initiating ring has the main histogenic role in the development of apical structure.
Meristeme d’attente is the terminal meristem and the central cells of vegetative shoot apex compose it. This meristem remains dormant during the vegetative phase of development.
It is considered to be without any histogenic role in the development of apical structure. Meristeme d’attente, also called waiting meristem, supposedly becomes active in the formation and development of inflorescence apex and terminal flower at the time of flowering.
The above interpretation regarding the apical zonation is based on the study of mitotic index within the apical region and on the observations of cytological characteristics of the apical cells.
The vegetative meristems have the following characteristics:
(1) Produce foliage leaves continuously;
(2) Possess vacuolated cells;
(3) The tunica cells contain little ribonucleic acid in the cytoplasm, and
(4) The corpus cells have small nucleolar volume.
The following features are present in the reproductive apices:
(1) The tunica cells have increased nucleolar volume;
(2) The central cells have greater activity and
(3) The central axial cells have more ribonucleic acid.
In a study with Lupinus, Buvat (1952) reported that the cells in tunica became more meristematic in appearance at the onset of flowering. At this time the cells present at the summit of corpus also become more meristematic in appearance.
During this period the cells of tunica and corpus contain homogenous condriome and numerous small vacuoles. The apex of Aster sinensis shows cytological differences after the formation of third leaf primordium.
At this time the cells situated at the summit of tunica and corpus differ markedly from the laterally situated cells that form the anneau initial. The cells at the summit of tunica and corpus possess small nucleoli, large vacuoles and slightly different chondriome in comparison to the cells present in the anneau initial.
Nougarede et al. (1964) reported the existence of ‘intermediate’ phase in the shoot apex. This phase occurs in between vegetative phase and reproductive phase. This phase is observed in those plants that remain or induced to remain vegetative over a considerable period of time. During intermediate phase meristeme d’attente and anneau initial show some changes in their appearance, zonation and function.
The apex of plants having intermediate phase is called intermediate apex. In this apex the activity of meristeme d’attente may begin, ex. Amaranthus retroflexus. A. retroflexus is a short day plant. These plants were maintained in long day conditions by photoperiodic inductive treatment of 16 hours of light for 60 days.
The apices of such plant show the combined characteristics of vegetative meristems and reproductive apices that are mentioned in the previous paragraph. Cutter (1971) opines that the intermediate apex represents a compromise between vegetative and reproductive apices. Heslop-Harrison et al. (1969) considers the intermediate stage of Cannabis sativa as a very slow transition to flowering.
The characteristic of intermediate apex is attributable to gibberellin that increases in apical tissues. It is observed that plants, treated with gibberellin, have growth only on the central zone in the apex. It is to note that it is the environmental conditions, to which the plants have previously been subjected, determine the transition to flowering.
The above discussed structural, anatomical and cytological changes in the transitional apex are no doubt accompanied and preceded by certain biochemical and physiological changes. A few changes are mentioned below.
Physiologists have shown that synthesis of ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) is the necessary preliminary to flowering. The cytoplasmic RNA content is usually restricted to the flanks of vegetative shoot apices. It increases and uniformly spreads throughout the apices when the apices are induced to flower.
Chenopodium album. C. album is a short day/long night plant.
In this plant the concentration of RNA increases after two long inductive nights and again after four long inductive nights. This increase of RNA content is also observed in Pharbitis and Xanthium within one day after one long inductive night. The increase of RNA is a photoperiodic phenomenon. It is revealed by the fact that when the long inductive night of Xanthium is interrupted with a few minutes of light the RNA content is not increased.
[The presence of RNA is detected by pyronin. Pyronin is a dye that stains RNA. Sections of shoot apex were stained with pyronin. Pyronin combines with RNA and the latter stains pink. Extracting RNA from sections with the enzyme ribonuclease can test the specificity of the dye pyronin. These sections show no stain of RNA. The layer where RNA is present can be detected with the dye pyronin. Another dye pyronin – methyl green was evolved for localization of RNA and DNA in a tissue. The more highly polymerized DNA combines with methyl green and so the DNA stains green. The cytoplasm and nucleolus stains pink. The less polymerized RNA also stains pink. To locate RNA in these sections, RNA is extracted form some parallel sections and the sections were stained with pyronin. A comparison between the test section and parallel section will locate the presence of RNA.]
Following floral induction Barbar et al (1968) demonstrated changes in the content of soluble protein in the vegetative apex. It was observed that after four inductive short days the concentration of total protein was much greater than the vegetative apices. Following five inductive short days there was decrease in staining for histones.
Corson (1969) demonstrated that in Datura stramonium the vegetative apex enters the transition phase when 7 or 8 leaf primordia are formed. The apex shows four zones on the basis of staining property. The summit one, two peripheral flank zones and one central zone compose the four zones of the apex as is revealed in longitudinal sections of vegetative and transition shoot apices.
Corson analyzed the mitotic index of the different zones. The mitotic index of the summit zone of vegetative apex was significantly lower than the other three zones. The mitotic index in the summit and central zones of transitional apices increased significantly. The increase of mitotic index was accompanied with the increase in RNA, total protein and phospholipid.