Photomorphogenesis and Photoperiodism
Apart from the hormonal factors, many external environmental factors, such as, light, temperature, humidity etc., also have tremendous influence on the growth and development of plants. It is well known how environmental factors influence the daily life and activities of plants. One aspect of plant life that has been studied extensively in terms of the effects of environmental factors is flowering.
Angiosperms naturally initiate flowers and blooms at a characteristic and precise time of the year. At least three or four important morphological or environmental conditions control or influence the initiation of flowers. One of these is called ripeness to flower, that is, the appropriate age or stage of morphological development before flower initiation can occur. Another factor is relative lengths of days and nights, a response called photoperiodism. Low temperature is known to accelerate flowering in certain plants, a process referred to as vernalisation. Less important for flower initiation but quite important for other aspects of plant growth is the phenomenon of thermoperiodism, which is the influence of alternation of different day and night temperatures. Some species of plants are influenced by one of these factors and some by many.
The phenomenon of photoperiodism was discovered in 1920 by W.W. Garner and H.A. Allard in U.S.A. These scientists were working with a mutant variety of tobacco plant with large leaves. This variety is noted for its active vegetative growth. This mutant, Maryland Mammoth tobacco, does not flower in field conditions, but when brought into greenhouse conditions, it flowers in the middle of December. The seeds of this mutant were sown with the normal plant and observed under field conditions. It was seen that the mutant variety remained vegetative in field, but flowered in December only when brought to green house conditions.
In another experiment, the mutant was exposed to short-day lengths typical of mid-December; this led to flower induction in the mutant. However, in winter months, if the mutant was exposed to artificial light, it could be retained in vegetative state. Thus it was observed that the Maryland Mammoth tobacco flowered only in short-day conditions. Garner and Allard termed this response, photoperiodism.
Based on the flowering response, three groups can be recognized: Long-day plants (LDP), short-day plants (SDP) and day neutral plants (DNP). LDP initiate flowers only when the day-length is longer than the critical day length, SDP only when the day-length is shorter than a certain critical day length, and DNP can initiate flowering under any natural day-length. Later studies have indicated that some species do not fit into any of these three classes. For example, some sugar cane varieties proved to be intermediate day plants (IDP) and remained vegetative under too long or too short days.
A few species are either short-long-day plants (SLDP) or long-short-day plants (LSDP). In these plants long day-length is responsible for flower initiation and the short for continued flower development. The SLDP, such as some varieties of wheat and rye, are plants that bloom in the spring or early summer when the required sequence of day-length occurs, whereas LSDP, such as night blooming jasmine (Cestrum nocturnum), bloom in late summer or fall.
Various studies have indicated that photoperiodism does not affect flowering alone, but many aspects of vegetative development also. For example, long days promote onion bulb formation, the growth of fibrous roots of dahlia, the growth of strawberry runners, and vigorous vegetative growth in general. Although, Bryophyllum, is a SDP as regards flowering, long days promote development of its leaves. On the other hand, short days, promote leaf senescence, abscission and dormancy of trees and other perennials and development of underground storage organs such as tubers of potatoes and the roots of radishes, yams and dahlias.