Photocontrol of flowering and stem extension of the intermediate-day plantEchinacea purpurea

Intermediate-day plants (IDP) flower most rapidly and completely under inte rmediate photoperiods (e.g., 12-14 h of light), but few species have been i dentified and their flowering responses are not well understood. We identif ied Echinacea purpurea Moench as an IDP and, based on our results, propose a novel mechanism for flowering of IDP, Two genotypes of E. purpurea ('Brav ado' and 'Magnus') flowered most completely (greater than or equal to 79%) and rapidly and at the youngest physiological age under intermediate photop eriods of 13-15 h, Few (less than or equal to 14%) plants flowered under 10 - or 24-h photoperiods, indicating E. purpurea is a strongly quantitative I DP, Plants were also induced to flower when 15-h dark periods were interrup ted with as few as 7.5 min of low-intensity lighting (night interruption, N I), Flowering was progressively earlier as the NI increased to 1 h, but was delayed when the NI was extended to 4 h, Stem length increased by greater than or equal to 230% as the photoperiod or NI duration increased, until pl ants received a saturating duration (at 14 or 1 h, respectively). Flowering was inhibited when 16-h photoperiods mere deficient in red (R, 600-700 nm) light, and was promoted when photoperiods were deficient in far-red (FR, 7 00-800 nm) light, Because of our results, we propose the flowering behavior of IDP such as E. purpurea is composed of two mechanisms: a light-dependen t response operating through light-labile (type I) phytochrome in which flo wering is inhibited by an LD, and a light-stable (type II) phytochrome (i.e ., phyB, D and E) response in which flowering is promoted by a short-night.