long-day plant

(LDP) A plant that appears to require long days (i.e. days with more than a certain minimum length of daylight) before it will flower. In actual fact, it requires a daily cycle with no long dark periods. For example, henbane (Hyoscyamus niger) does not flower when given cycles of 12 hours light followed by 12 hours of darkness, but will flower if given cycles of 6 hours light followed by 6 hours darkness. Compare day-neutral plant , short-day plant. See photoperiodism.