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

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

To determine 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 etc, until the last group is exposed to 24 hours. It is observed afterwards that beyond a specific duration of light exposure plants present or do not present the flowering and the remaining submitted to a shorter photoperiod present opposite behaviour. Duration of the light exposure that separates these two groups is critical photoperiod.