Light quality of continuous illuminating at night to induce floral initiation of Fragaria chiloensis L. CHI-24-1

A wild strawberry strain, Fragaria chiloensis CHI-24-1, produced inflorescences from both parent and asexually propagated daughter plants linked with runners when grown at 23 °C/20 °C (day/night) under a 24 h day-length (DL) of daylight plus nightly lighting by an incandescent lamp, but not under 8 or 16 h DLs. In the present study, the effect of light quality for continuous illuminating at night on floral initiation of CHI-24-1 plants grown under a 24 h DL was examined. The CHI-24-1 plants were grown under a 24 h DL consisting of natural daylight and continuous lighting at night by an incandescent, a blue fluorescent, a red fluorescent or a far-red fluorescent lamp for 40 days in summer and autumn. Also, the CHI-24-1 plants were grown for 40 days in a growth chamber at 25 °C/20 °C (day/night) with natural daylight and continuous lighting at night by red- and four types of far-red light-emitting-diodes (LEDs with peak wavelengths of 660, 700, 735, 780 and 830 nm). In both experiments, floral initiation of the parent and daughter plants was observed under a stereomicroscope. Although more than 50% of the parent and daughter plants initiated flower buds under the incandescent and far-red fluorescent lamps, about 15% and 0% of those initiated flower buds under blue and red fluorescent lamps, respectively. Floral initiation of the parent and daughter plants occurred under the far-red LED light source whose peak wavelength was 735 nm, but not under the red or the other far-red LEDs. From these results, it can be concluded that the effective light wavelength range of nightly continuous illuminating for floral induction in the CHI-24-1 plants is 735 nm in the far-red light region. Hence, the induction of floral initiation by nightly continuous far-red light (735 nm) appeared to be a response mediated by phytochrome.