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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.