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We have tested whether the promotion of flowering by long days (LD) in Arabidopsis thaliana is a consequence of photoperiodic induction. To achieve this, the flowering responses of Arabidopsis thaliana (L.) Heynh. Landsberg erecta (Ler) and the long-hypocotyl mutants hy2, hy3 and hy4 were determined with respect to age, daylength and light quality. Ler was capable of distinguishing between short days (SD) and long days (LD) from about 4 d after sowing at 20 °C, the time at which cotyledons were expanding and greening. At this stage, the critical daylength was between 8 h and 10 h. At 7 d, seedlings required five LD for induction and, as the seedlings aged, they became more sensitive so that by day 20, one LD was fully inductive. The response to SD in newly germinated seedlings was to delay flowering without altering leaf number, but after about 10 d, delay of flowering by SD was accompanied by extra leaves. In light quality experiments, blue light (B) was inductive for 5-d-old plants and in all subsequent treatments, far-red (FR) caused induction in treatments at 12 d and 18 d and low pressure sodium, equivalent to red, was not inductive at 5 d and 12 d, but partially inductive at day 18. Hence, both a specific blue-light photoreceptor and phytochrome A in High Irradiance Response mode promote floral induction. In daylength transfer experiments all three hy mutants responded to LD by earlier flowering. Both hy2 and hy3 produced substantially fewer leaves than Ler in SD and hy3 flowered slightly earlier than Ler. The hy4 mutants flowered later than Ler in SD and had a higher leaf number. A scheme is proposed in which photoperiodic induction depends on the ability of the plant to sense photoperiod, the stage of development and the photobiological input. We also propose that phytochrome A and the blue photoreceptor promote flowering whereas phytochrome B promotes vegetative development.
Mozley et al. (Sun,) studied this question.