Elementary idea on Photoperiodism and Vernalization
Photoperiodism is the response of the plants to the relative lengths of day and night for their flowering. Garner and Allard (1920) showed that flowering of many plants could be induced or prevented simply by controlling the length of the daylight period. Based on flowering, in response to different length of photoperiod, they divided flowering plants into three groups (i) short day plants (ii) Long day plants (iii) day neutral plants.
(i) Short day plants –
The flowering is induced by exposure to short days or day lengths less than certain critical length. Some common short day plants, are – Nicotiana, Chrysanthemum.
(ii) Long day plants –
Flowering is induced here if a plant is subjected to light periods of 12 hours or more. Most of the agricultural plants of the temperate regions are long day plants. Example – wheat, potato.
(iii) Day-neutral plants –
The flowering is independent of the photoperiod. Some common examples are cucumber, balsam, tomato, maize.
Importance of dark period the photoperiod -equired by a plant to induce flowering is called critical photopcriod or critical day length. C -tical day length is species specific. It usually varies between 11-14 hours of light period in the daily 24 hour cycle. The remaining time from the 24 hours of the day is night or dark period. Actually, dark period is critical to flowering.
For example, flowering is inhibited in short day plants if the dark period is interrupted by a brief exposure to light. Interruption of light period by short day plants are more appropriately long night plants, because their flowering is actually promoted by long continuous dark period.
Similarly, long day plants are actually short night plants. Long continuous dark period inhibits their flowering. They flower if the dark period is interrupted and some even flower under continuous light.