2017 ASHS Annual Conference

In northern latitudes, it is not uncommon for the photosynthetic daily light integral (DLI) to be ≤5 mol·m^-2·d^-1 necessitating the use of supplemental lighting (SL) to reduce plug production time and increase quality. However, some growers are using low-intensity photoperiodic lighting (PL) from light-emitting diodes (LEDs) during plug production. Therefore, our objectives were to: 1) quantify production time and quality between seedlings produced under ambient light, PL, and SL and 2) quantify the effects of continuous 16-h SL to SL with instantaneous thresholds on plug quality. Seeds of begonia, petunia, gerbera, and impatiens were sown in 128-cell trays. Seedlings with cotyledons were grown at 22 °C under four SL and three PL treatments all for 16 h·d^-1 or 24 h·d^-1 or under ambient light (control). Supplemental lighting treatments consisted of high-intensity LEDs providing a light ratio (%) of 9:6:85 blue:green:red (B:G:R) or high-pressure sodium (HPS) lamps providing a PPFD of 70 µmol·m^-2·s^-1 (on continuously for 16-h) or HPS lamps or R:G:B LEDs providing a PPFD of 90 µmol·m^-2·s^-1 based on an instantaneous threshold (on when outdoor PPFD was ≤300 µmol·m^-2·s^-1). Photoperiodic lighting treatments consisted of low intensity LEDs providing 2 µmol·m^-2·s^-1 of a light ratio (%) of 6:15:77:2 B:G:R:far-red (FR) for 16-h, 7:12:35:46 B:G:R:FR for 16-h, or 3:17:48:32 B:G:R:FR for 24-h. Seedlings from each treatment were transplanted and finished under SL from B:G:R LEDs as previously mentioned. The most compact plugs were those grown under continuous 16-h of SL from LEDs or HPS lamps or LED SL with instantaneous thresholds and the most elongated were those under PL. Generally, the RDM of seedlings under SL was significantly higher compared to those under ambient or PL. For example, RDM of petunia under SL was 145% higher than seedlings under 24-h of PL and 220% higher than seedlings under ambient light or 16-h of PL. Increased leaf area and internode elongation under B:G:R:FR LEDs can give the perception that seedlings production time is reduced. However, time to produce a pullable plug was delayed by one or two weeks, respectively, under PL or ambient light compared to seedlings grown under SL. Petunia seedlings propagated under PL providing FR and HPS lamps flowered 9 days earlier than those under PL or SL without FR while height and branch number at first flower were not significantly different.