The Biochemistry of Flowering
The Biochemistry of Flowering
The Plants which are sensitive to photoperiods, a definite chemical stimulus is produced in the leaves, when the plants, are exposed to the desired photoperiod. The stimulus is then translocated to the shoot apex where it evokes flowering. Thus, as far as the biochemistry of flowering is concerned, it can be divided into two steps:
(1) The production of floral stimulus and (2) Evocation of flowering.
Grafting experiments suggest that a chemical is produced when plants are placed on a photoinductive cycle (a cycle which induces flowering). A plant kept in a photoinductive cycle flowers, because this chemical is present in that plant. One kept in a non-inductive cycle does not flower. However, if a leaf from a photoinduced plant is removed and then grafted on a non-induced plant, then this plant also flowers. Apparently a chemical susbtance is produced during photoinductive cycle, which is transmitted during grafting of leaf from plants kept in such a cycle.
Several attempts have been made to extract and characterise the floral stimulus from flowering plants or photoinduced plants. However, attempts have met with limited success only. K.C. Hamner and J. Bonner (1938) tested some 246 different kinds of extracts from short-day, long day and day neutral plants, but none had any flower evoking effect in non-inductive plants. Numerous known and unknown substances have also been tried by various investigators since then; but there is no uniformity in either response or chemical nature of the substance. Nevertheless, florigenic acid, gibberellins, sterols, phenolic acids and several other compounds have been shown to act as a flower stimulus in many species. Some of these are described in the following paragraphs.
The name ‘florigen’ (Greek meaning flower generating) was proposed by the Russian plant physiologist, M. Chailakhyan in 1936, for the unknown chemical stimulus which could act as floral inducer. The most significant evidence for the existence of such a substance comes from interspecific grafting experiments. Flowering in a species can be brought about in a non-inductive cycle, if a leaf or twig from a photoinduced species is grafted on that. Such results clearly indicate that floral stimulus produced by plants of one photoperiodic response can be transmitted by grafting to other plants which may have different photoperiodic requirements. But florigen as proposed by Chailakhyan, has never been isolated any fully characterized and remains hypothetical. Either it does not exist in plants or it is too unstable to be isolated. Therefore, we now talk of ‘florigen concept’ instead of florigen. The concept relates to the existence of a chemical substance or substances involved in inducting flowering.
It would be rather useful to give a historical perspective of the florigen concept, to understand its involvement in flowering. After Chailakhyan advanced the idea of existence of florigen, several plant physiologists tried to isolate and characterise the chemical. In 1951, R.G. Lincon, D.L. Mayfield and A. Cunningham obtained a chemical in crystalline form from flowering Xanthium and sunflower plants. The substance did not exist in vegetative plants and it was found to be a water soluble organic acid. It induced flowering only when applied to the leaves and was ineffective when applied to the buds. It was thought to be a precursor of florigen or a substance necessary for florigen synthesis. In 1970, H.K. Hodson and K.C. Hamner tested the efficacy of florigen preparation in Xanthium as well as Lemna. In experiments, the chemical induced flowering in about 75% of the Xanthium plants, to which it was applied, and it induced flowering in about 50% of the Lemna plants. However, in Xanthium, the chemical was effective only when gibberellin was also added, although the gibberellins alone could not induce flowering. In Lemna, the substance induced flowering only when it was applied without gibberellins, added gibberellins made the extract totally ineffective.
The florigen was believed to be synthesis in the photoinduced leaves and from there moved on to the shoot apices for floral evocation, through phloem. S. Imamura and A. Takimoto (1955) studied the translocation rate of this stimulus. In japanese morning glory, the rate was observed to be slow, 2.5 to 3.0 nm per hour as compared to more than 200 nm per hour for most sugars. Thus, the florigen was not just a sugar synthesized in photoinduced leaves. In other studies also, exogenous application of sugars or amino acids did not induce flowering in non-induced plants. This strengthened the belief that the florigen was not just a general type of synthetic output of the photoinduced leaves.
In search for the chemical identity of the florigen, plant physiologists focused their attention to the effect of plant hormones on flowering. Several hormones have flower promoting effects, although none can qualify as a florigen because of its limited effect. Gibberellins have the most spectacular effect, as they induce flowering in most long day plants, but not in short day plants. Following the discovery of effect of gibberellins on flowering, Chailakhyan modified his florigen concept. He proposed that florigen is two hormones rather than one, a gibberellin and a hypothetical hormone he called anthesin. He suggested that long day plants could produce anthesin under any dry length but gibberellins only under long days, that short day plants could produce gibberellins under any dry length but anthesin only under short days, and that day neutra plants could produce both under any day length.A plant could flower only when both gibberellin and anthesin were present. But again anthesin is just as hypothetical and elusive as florigen has been.
Gibberellins and the Florigen concept
Exogenous application of gibberellins (GA) is known to induce flowering in many long day plants, under short day condition. Natural GAS from induced long day plants also cause flowering in non-induced plants. In some plants increase in GA following photoinductive cycle has also been observed.
Although, GA can induce flowering, they cannot be considered to be the primary floral hormone or florigen, mainly because of the following reasons;
- In long day plants, induction of flowering by long days and GA appear to be different. When these plants are kept in long days, differentiation of floral primordia occurs simultaneously with stem elongation. However, in GA induced flowering first stem elongation takes place and then floral primordia are formed. Thus, the effect of Ga appears to be indirect; through stimulated growth and differentiation.
- Gibberellins have been unable to induce flowering in short day plants under non-inductive cycle.
Nevertheless, GAs seem to play an important role in flowering. It may be one of the several known and unknown compounds involved in flowering. It may be directly involved in the formation of florigen or any such floral stimulus. A model based on the role of phytochrome, GA and florigen in the photoperiodic induction of flowering can be proposed. According to the model, photoperiodic reactions involving assimilation of CO, causes the build up of a precursor of GA or GA like hormone. This precursor may be converted to GA. Red light promotes this conversion and far-red or dark causes reconversion of the hormone back to the precursor. This reversible conversion is mediated via phytochrome. The phytochrome may also be considered to be either an integral part or intimately linked to the precursor and hormone. This GA or GA like hormone then causes the formation of florigen either directly or indirectly. It is assumed that a high level of GA like hormone must be maintained in long day plants for the production of florigen. In short day plants, the situation is reverse, a low level of GA is required for florigen. However, once enough florigen is produced, flowering will occur in both long day and short day plants.
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