Let us make an in-depth study of the photoperiodism process and effects of light on flowering of plants.
Early twentieth century workers were of the belief that flowering in plants is a phenomenon effected by nutrition.
Kraus and Kraybill (1918) observed in the case of tomato plant, optimum nitrate and carbohydrates supply accelerated vegetative growth, but with a poor nitrate supply both reproduction and vegetative growth declined.
However, in the year 1906, a commercial variety of tobacco, Maryland narrow-leaved, gave rise to a new mutant called Maryland mammoth. This new variety showed vigorous vegetative growth during the summer, but the plants did not set seeds before the cold weather set in. W. W. Garner and H.A. Allard, plant physiologists, came forward to investigate the cause.
Finally they observed that these plants always bloomed during the short days of the winter months. The plants were made to flower even during the months of summer by cutting down the light period to seven hours a day (Fig. 5.1).
Gamer and Allard published their investigations in the year 1920. They, however, concluded that flowering was caused by exposure to days made up of short light and long dark periods. Since then it has been known that an environmental factor of great significance in controlling flowering is day length.
In tropical regions of earth there is very little change in day length throughout the year, and the days and nights are about equal. In temperate regions, day length changes from winter to summer, and the long days coincide with the warmer season. Many tropical species of plants when brought to temperate zone flower only when the days are short, continued long days prevent the formation of flower buds.
Plants native of temperate zone have a variety of flowering habits. Many temperate plants flower during spring when days are moderately short. Others flower during the summer when the days are long. Still others produce flowers during the short days of late summer and early fall. This mechanism that enables plants to respond to day length so that they flower at a specific time of the year is known as photoperiodism. The length of the daily period of light to which a plant is exposed is called photoperiod.
Plants are grouped according to their response to day length into what are called:
(i) Short-Day Plants
(ii) Long-Day Plants and
(iii) Day-Neutral Plants
(i) Short-Day Plants (SDP):
These plants flower when exposed to day lengths shorter than or below a certain critical maximum. The critical photoperiod, however, varies from species to species. If these plants are exposed to day lengths in excess of this critical point, they continue growing vegetatively (Fig. 5.2 A) Common examples of short-day plants are chrysanthemums, cock-lebur (Xanthium strumarium), tobacco (mutant. ‘Maryland mammoth’, Nicotiana, tabaccum), soyabean (Glycine max), and sugarcane (Sacchamm officinarum), etc. They normally flower in the early spring or autumn.
(ii) Long-Day Plants (LDP):
These plants begin flowering when exposed to day lengths longer than or above a certain critical minimum. Below the critical photoperiod, these plants continue their vegetative growth (Fig. 5.2 B). The critical photoperiod, in such plants also, varies from species to species. Some common examples of long day plants (LDP) are barley (Hordeum vulgare), spinach (Spinacea olemcea), radish (Raphanus sativus), henbane (Hyoscyamus niger), onion (Allium cepa) and carrot (Daucus carota), etc. They normally flower in late spring or early summer.
(iii) Day-Neutral Plants (DNP):
These plants flower after a period of vegetative growth, regardless of the photoperiod. In other words, they are unaffected by the day or night lengths, and flower around the year (Fig. 5.2-C). Some common examples of day-neutral plants are cucumber (Cucumis sativus), cotton (Gossypium hirsutum), tomato (Lycopersicum esculentum), sunflower (Helianthus annuus), Maize (Zea mays) and some varieties of pea, etc.
Photoperiodic Responses are Under the Control of Genes:
It is a matter of common observation that the critical day length of both long-day and short- day plants tends to fall within a range of 12—14 hours. Commercial flower growers can induce or delay flowering by controlling the photoperiodic and temperature conditions in glasshouses to meet the demands of the market. The photoperiodic responses of plant are now considered to the under the control of genes.
These can be modified by various methods to yield varieties responding to required day lengths. For instance, scientists at the National Botanical Research Institute, Lucknow have been able to develop varieties of Chrysanthemum which can bloom in different months of the year including summer.
The critical photoperiod for long and short day plants greatly varies from species to species. For instance. Chrysanthemum and Poinseltias are both short-day plants, but Chrysanthemum form flowers when the days are shorter than 14.5 hours, whereas Poinsettias produce flower buds only when the days are less than 12.5 hours.
Spinach and rose mallow are long-day plants, but spinach flowers when the days are longer than 14 hours, the rose mallow flowers when they are longer than 13 hours. In other words, the short-day plants flower only when the days are shorter than a critical photoperiod, and the long-day plants flower only when the days are longer than the critical duration.
Induction period is the minimum period of exposure to a long day or a short day which is required to induce flowering. Induction period differs in different plants. For instance, Xanthium requires only one cycle of day plus night, but most plants require about ten such cycles.
Long-Night and Short-Night Plants:
The Terms Long-Day and Short-Day Plants are actually misnomers. Earlier when the photoperiodism was discovered, the duration of the light period i.e. photoperiod was thought to be critical for flowering. However, later researches, noted that in short-day plants (SOP), when the long night period was interrupted by a brief exposure to light, the plants failed to flower (Fig. 5.3).
From this observation, scientists concluded that what is critical or essential for these plants to flower is long and un-interrupted dark period rather than a short day length. A brief interruption of the dark period with light nullified the effect of long night. So to be more precise and appropriate, short day plants may be regarded as long-night plants.
Similarly long-day plants (LDP) respond to nights shorter than the critical dark period. Curiously long day plants do not need an uninterrupted dark night. Long-day plants are also regarded as short-night plants.
Theory of Photoperiodic Action:
Attempts have been made to understand as to how day (or night) length affects the plant so as to change the normal leaf primordia of the stem apex into flowering primordia? As written above, in short-day plants it is the dark period rather than the light period which affects induction of flowering, but in long-day plants, dark period is not at all important and they can flower even in continuous day light. This clearly indicates that there must be two different systems operating in two groups of plants for the induction of flowering.
Role of Phytochrome, Florigen and Phytohormones in Flowering:
There are experimental evidence (Hendricks and Borthwick) that only red light (wavelength 660 mμ) is effective in inhibiting flower initiation in short-day plants, when the dark period towards midnight is interrupted with this illumination. This wave length of light at the same time accelerates the growth of stem and root and formation of anthocyanin pigment.
It is more interesting to note that this inhibition of flowering in short day plants can be reversed by treating the plants with far-red light (wave length 730 mμ) (Fig. 5.4). This suggests the existence of a single compound, phytochrome, responsible for photoperiodic action. The phytochrome (probably) exists in two inter convertable forms P730 and P660. When P660 is illuminated with red light (660 mμ) it is transformed to the P730 form.
The P730 form can be converted into the P660 form by far red light (730 mμ). During night P730 is converted into P660 form and hence at the end of day period the predominent form of phytochrome is P730 because sun light contains more red light of 660 mμ) wavelength.
In short day plants flowering is initiated when there is sufficient accumulation of P660 and that is the reason why flowering is inhibited in these plants when dark period is interrupted by red light of 660 mμ wavelength which converts P660 form into P730 form (Fig. 5.4). But the manner in which the flowering is initiated by the phytochrome is not yet well understood.
The only facts known about this flowering substance are that it is proteinaceous in nature and most likely acts as an enzyme which initiates the formation of certain hormone or hormones which ultimately bring about the conversion of vegetative primordia into flowering primordia.
Florigen—A Hypothetical Flowering Hormone:
Evidence that a flowering hormone “florigen” exists in plants comes from the work of Naylor (1952), who states that a plant can be made to bloom by grafting on it a leaf from another variety, species, genus, or even from another family. A certain parasitic plant which grows on the roots of red clover is probably never exposed to light and yet it blooms. It is assumed that this parasite obtains its stimulus for flowering from its host.
1. The metabolism of florigen is believed to be phytochrome-mediated.
2. Florigen has never been isolated. It is a hypothetical hormone.
3. The florigen is translocated to the vegetative bud through phloem, where it transforms vegetative but into flower bud.
4. Florigen is a sort of stimulus. Unlike other phytohomones, it is neither growth promoter nor a growth inhibitor.
5. The seat of synthesis and the seat of action of florigen are leaf and shoot tip respectively.
Chailakhyan (1968) demonstrated that the site of perception of light for photoperiodic inductions (stimulus) are the green leaves. This is evident from the fact that a plant from which all leaves have been removed fails to flower even under the inductive light conditions. Further confirmation was obtained from experiments with Xanthium, a short-day plant, in which flowering occurred even when one-eighth of a leaf was exposed to short days.
Photoperiodic induction received by a single leaf or its part in a plant is considered enough to induce flowering. Further, a floral stimulus from an induced leaf in a long or short-day plant can be transmitted or trans-located to another non-induced plant by grafting. Besides, the floral stimulus is not species-specific because grafting an induced twig of Xanthium on to a vegetative soya bean plant can induce the latter to flower.
The nature of the flower-producing stimulus has been widely debated. Some plant physiologists have proposed the existence of a flower-inducing growth hormone, the florigen (Naylor 1952 and Chailakhyan 1968). The metabolism of florigen is believed to be phytochrome-mediated. Unfortunately, the florigen has never been isolated.
The florigen is trans-located via phloem to the vegetative bud primordia which undergo transformation (morphological changes) leading to the production of floral buds.
Four steps are involved in this transformation:
(i) Perception of the stimulus by phytochrome in the leaves (induction);
(ii) Change to new pattern of metabolism in the leaves leading to the production of flowering hormone, the florigen;
(iii) Translocation of florigen (the stimulus) to the bud primordia; and
(iv) Transformation of vegetative bud primordia into floral buds (the response).
Role of Phytohormones in Flowering:
Researchers have indicated that flowering is also regulated by the interplay of the phytohormones, the auxins, gbberellins, cytokinins and ethylene. Application of hormones can substitute for the necessary photoperiod and can initiate floral development in certain plants. It is interesting to note that IAA (auxin) inhibits flowering in most of the plants. An exception, however, is pineapple (Ananas). Gibbereuic acid (GA) can substitute photoperiodic induction in many long day plants. It, however, is almost ineffective in short-day plants except a few such as Impatiens balsamia (Balsam plant).
Significance/Practical Importance of Photoperiodism:
1. Photoperiodism determines the season in which a particular plant shall come to flower. For example, short-day plants develop flowers in autumn-spring period (e.g., Dahlia, Xanthium) while long-day plants produce flowers in summer (e.g., Amaranthus).
2. Knowledge of photoperiodic effect is useful in keeping some plants in vegetative phase (e.g., many vegetables) to obtain higher yield of tubers, rhizomes etc., or keep the plant in reproductive phase to yield more flowers and fruits.
3. A plant can be made to flower throughout the year by providing favourable photoperiod.
4. Helps the plant breeders in effective cross-breeding in plants.
5. Enable a plant to flower in different seasons thus fruits can be produced during their offseason by controlling photoperiod.