What is the critical photoperiod? How can the critical photoperiod relate to flowering be experimentally determined?

The critical photoperiod is the limit of the photoperiod duration for the occurrence of some biological response. This limit can be a maximum or a minimum, according to the characteristics of the biological response and to the studied plant.

To determine the critical photoperiod relating to flowering, 24 groups of plants of the same species can be taken and the following experiment can be done: Each group is submitted to a different photoperiod, the first group to 1 hour of daily exposure to light, the second to 2 hours, the third to 3 hours, and so on, untill the last group is exposed to 24 hours. It is observed later that beyond a specific duration of light exposure plants present or do not present flowering and the remaining submitted to a shorter photoperiod present opposite behaviour. The duration of the light exposure that divides these two groups is the critical photoperiod.