Photoperiod: the relative length of day and night required by plants.
The ability of plants to measure the relative length of day and night is termed photoperiodism.
The specific amount of photoperiod required by plants to flower is termed a critical photoperiod.
On the basis of critical photoperiod, the plants are divided into three categories namely, long day plants(LDPs), short day plants(SDPs), and day-neutral plants.
LDPs- are the plants that require 12-14hrs of continuous daylight for flowering to occur.Examples- Spinach, spring barley, and wheat.
SDPs- are the plants that require 12-14hrs of a continuous dark period for flowering to take place.
For example tobacco, and soybean.
Day-neutral plants: they are plants that do not require any specific amount of photoperiod for flowering. For example tomato, sunflower, cotton.
Final Answer:
The minimum amount of photoperiod required for flowering is critical photoperiod.