How plants know when to flower…???

Determining the proper time to flower, important if a plant is to reproduce successfully, involves a sequence of molecular events, a plant's circadian clock and sunlight.

At specific times of year, flowering plants produce a protein known as FLOWERING LOCUS T in their leaves that induces flowering. Once this protein is made, it travels from the leaves to the shoot apex, a part of the plant where cells are undifferentiated, meaning they can either become leaves or flowers.

 At the shoot apex, this protein starts the molecular changes that send cells on the path to becoming flowers.

Changes in day length tell many organisms that the seasons are changing. It has long been known that plants use an internal time-keeping mechanism known as the circadian clock to measure changes in day length. 

Circadian clocks synchronize biological processes during 24-hour periods in people, animals, insects, plants and other organisms.

FKF1 protein is key player in the mechanism by which plants recognize seasonal change and know when to flower. FKF1 protein is a photoreceptor, meaning it is activated by sunlight.

Imaizumi says The FKF1 photoreceptor protein been working on is expressed in the late afternoon every day, and is very tightly regulated by the plant's circadian clock. 

When this protein is expressed during days that are short, this protein cannot be activated, as there is no daylight in the late afternoon. 

When this protein is expressed during a longer day, this photoreceptor makes use of the light and activates the flowering mechanisms involving FLOWERING LOCUS T. The circadian clock regulates the timing of the specific photoreceptor for flowering. That is how plants sense differences in day length.

This system keeps plants from flowering when it's a poor time to reproduce, such as the dead of winter when days are short and nights are long.

**credit to  scienceDaily