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MECHANISM OF PHOTOPERIODIC RESPONSES
  
LIGHT PERCEPTION, TIMING AND FLORAL INDUCTION IN SHORT-DAY PLANTS
All the critical events happen at night, therefore plants are not photoperiodic but rather are nyctiperiodic.  Short-day plants really are long-night plants.

 

RESPONSE OF PHOTOPERIODIC PLANTS TO DIFFERENT PHOTOPERIODS
It is the trend in response to photoperiod that is important, not the absolute day length.  In the table below, the SDPs flower at all photoperiods below their particular critical photoperiod, and the LDPs flower at all photoperiods above their particular critical photoperiod.
Plant Type*
Photoperiod
  SDP ( ex. Chrysanthemum)
14.5 hr critical photoperiod
 LDP ( ex. Henbane)
11 hr critical photoperiod
4 hour
flowers
no flowers
8 hour
flowers
no flowers
12 hour
flowers
flowers
16 hour
no flowers
flowers
20 hour
no flowers
flowers
24 hour
no flowers
flowers
*day-neutral plants would flower at all day lengths
 
MANIPULATING FLOWERING IN PHOTOPERIODIC PLANTS
Horticulturist manipulate the light and dark periods to which plants are exposed in order to trigger photoperiodic plants to flower during any season of the year.  That is why you can purchase a chrysanthemum year round.