The following points highlight the top six theories of flowering. The theories are: 1. C/N Relationship Theory 2. Trace Element Nutrition Theory 3. Water Stress Theory 4. Florigen Theory 5. Floral Inhibitor Theory 6. Plant Age Theory.
Theory # 1.
C/N Relationship Theory:
Before the discovery of photoperiodism and vernalization, a general observation made early in this century was like this luxuriant vegetative growth is usually antagonistic to flowering. Practices like pruning, girdling that reduce vegetative growth often promote flowering.
On the other hand, maintenance of a high nutritional status especially as nitrogen supply will favour vegetative growth and incidentally the reproductive development will be delayed. From these observations. Klebs in 1913 concluded that flowering is controlled by the nutritional status of the plant.
He enunciated the carbohydrate/nitrogen (the C/N ratio) relationship theory which indicates that a high endogenous C/N ratio is essential for flowering and vice versa. However, the later workers believed that Klebs theory was too simple to explain the complexity of the flowering process and many subsequent observations have failed to confirm Klebs theory which was soon discarded.
Theory # 2.
Trace Element Nutrition Theory:
Trace elements, particularly copper and iron are critically involved in photoperiodic induction in duckweeds and other plants. Hillman, studied the flowering behaviour of Lemna species and proved that the plants behave as SDP when copper is eliminated from the growing medium. Likewise, addition of copper to the medium leads to the loss of SD requirement and the plants behave as DNP.
Hillman postulated that Cu acting as a SH-inhibitor interferes with phytochrome action, possibly by influencing some metal-sensitive membrane system.
Iron is also involved in photoperiodic induction. Hillman pointed out that flowering in Lemna is inhibited by reducing the iron supply. The role of iron deficiency in the inhibition of flowering process may be related to the role of iron in general metabolism.
Theory # 3.
Water Stress Theory:
In an experiment on Geophila renaris, a perennial herbaceous plant of the tropical rain forest, Brenchart demonstrated that a period of water shortage is absolutely required for flower initiation. This observation suggests that limitation of water supply during certain developmental period may have a direct action on flower formation.
This phenomenon of ‘xeroinduction’ has also been shown in Cichorium intybus (chicory) and Chenopodium polyspermism in which application of excess water promotes regeneration of vegetative buds but proves to be inhibitory to flowering.
Quite an opposite effect of water stress has been observed in the SDP Pharbitis and Xanthium and the LDP Lolium. In these plants, water shortage prevents flower formation which is due to a stress-induced inhibition of translocation of floral stimulus from the induced leaves.
Theory # 4.
Florigen Theory:
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 1 964 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.
Theory # 5.
Floral Inhibitor Theory:
In the years 1949-50, Lona, Von Denffer and others postulated that plants grown in conditions un-favourable for flowering produce floral inhibitors. The conditions that induced the plants to flower either prevent the production of these inhibitory compounds or lower the concentration of the inhibitors below threshold value. It is nothing but the counter-theory to that of the floral hormone.
The inhibitor concept is based on an early observation reported by Lang in 1952 that LDP Hyoscyamusand SDP Chenopodium can be induced to flower in non-inductive photoperiodic conditions by removal of some leaves. In successful grafting experiments, the vegetative receptor plant is usually defoliated before being grafted to the reproductive partner.
Since the inhibitory substances are thought to be generated in leaves, it is likely that the leaves inhibit floral induction. Later in 1977. Lang and associates observed that a non-induced scion of the LDP Nicotiana sylvestris delays or suppresses flowering of a day-neutral tobacco cultivar used as stock.
This result indicates that non-induced. N. sylvestris produces a transmissible flower-inhibitory material. Similar floral inhibitor has also been obtained using non-induced Hyoscyamus (LDP) as scion showing that the Hyoscyamus inhibitor is active in a tobacco receptor.
With a single plant as the study material, Evans (1960) detected a floral inhibitor in LDP Lolium temulentum by keeping some leaves above in LD and some leaves below in SD conditions, it was observed that the induced LD leaves can cause floral initiation in the plant in the absence of SD leaves, but fail to do so when these leaves are kept in SD, i.e.. non-induced condition.
This shows that a transmissible floral inhibitor is produced by the non-induced leaves in Lolium. Like florigen, the chemical nature of these hypothetical inhibitors is unknown. Although it has been suggested that either gallic acid or abscisic acid (ABA) is the inhibitory compound produced in leaves kept in non-inductive condition.
Such a claim has not been confirmed in subsequent study. It is possible that floral evocation at the meristem is controlled by a balance between two kinds of compounds, viz., inhibitors and promoters of flower initiation.
Theory # 6.
Plant Age Theory:
For most plants, vegetative growth proceeds for some time. This is the juvenile or maturation phase through which the plant attains a minimal size till the onset of reproductive development. In the juvenile phase, the plants are not able to show photoperiodic response, whereas at the end of the juvenile phase, they become adult and sensitive to conditions that promote floral induction.
The juvenile phase is obviously the result of several physiological systems and there are many reasons to account for the inability of the young plant to flower even when subjected to favourable conditions. First, insufficient leaf area has been found to delay or reduce flower initiation which may be related to the supply of photosynthetic products to the shoot apex.
Also, the rate of thermo-induction increases with increase in leaf number and area. Immature leaves may act by producing floral inhibitors while mature leaves produce floral stimuli.
Obviously, floral initiation should depend on an optimum ratio of immature (young) leaves to mature leaves. A second reason for juvenility is the relative insensitivity of the young leaves to favourable day length conditions. The third is the inhibitory influence of the root system on flower initiation in the aerial shoots.
A close proximity of the lateral buds to the root system prevents floral initiation and prolongs the juvenile phase. The fourth reason supports the idea that juvenility is probably located in the meristems and not in the leaves. It has been shown that the meristems of juvenile scion when grafted on mature plants bearing flowering shoots remain in the juvenile phase as they are unable to respond to stimuli from mature leaves.