The following points highlight the three main types of plants that respond to photoperiodism. The types are: 1. Short Day Plants 2. Long Day Plants 3. Day Neutral Plants.
Type # 1. Short Day Plants:
For flowering of short day plants, the day length must not exceed a certain critical value, the day length required is less than a certain critical length. Short day plants be more correctly called as long night plants, as a certain minimum of uninterrupted dark period in 24 hours is necessary for their flowering. In the case of the short day plants, the critical thing is the length of dark period.
Even if the dark period is less than a critical length, flowering will not occur. Short day plants will not flower even if a flash of light is provided during the continuous dark period. Flowering is also checked even if very weak light is provided during the dark period. Some examples of short day plants are—soya-bean, potato, sugarcane, cosmos, chrysanthemum, dahlia, tobacco, etc.
Type # 2. Long Day Plants:
Long day plants require a photoperiod of more than a critical length which may vary from 4 to 18 hours. The best flowering of long day plants usually occurs in continuous light. For flowering, they require either no dark period or a very short dark period. Examples of long day plants are—spinach, radish, lettuce, alfalfa, opium poppy, maize, oats, wheat, etc.
Type # 3. Day Neutral Plants:
Tomato, cucumber, cotton, pea, sunflower, etc., are examples of day neutral plants. They can flower even if the light period provided is from few hours to round the clock illumination.
In recent years, efforts have been made to define different states leading to flowering. These states are described as induction, evocation and differentiation. At one time, it was assumed that the plant responses could be qualified as ‘qualitative’ (obligate) or quantitative (facultative).
The distinction between the characteristics has been narrowed down, if not eliminated altogether. Furthermore, it has become clear that the intensity of flowering is a quantitative expression even in qualitative plants.
The graft experiments between ‘induced’ plants and ‘non-induced’ under non-inductive conditions resulted in the development of the concept of a transmissible flowering stimulus. A unified concept of such a stimulus gained prominence because flowering could be induced between SD and ND, LD and ND or SD and LD.
In addition, it was shown through graft experiments that the formation and transmission of an inhibitor could be responsible in controlling flowering.
Either any flowering stimulus or hormone or an inhibitor has not been isolated, but the influence of many chemicals such as gibberellins, cytokinins, ethylene, etc., has been shown. In recent years, several phenolic compounds such as salicylic acid have been found to induce flowering.
It has been hypothesised that there is a hormonal substance called florigen which is responsible for flowering. But there is no experimental proof in support of this hypothesis.
It is believed that this active principle is synthesised in the leaves, when the plants are exposed to the inductive photoperiod, and is translocated to the apical meristem for inducing flowering by conversion of vegetative meristem into reproductive meristem.