Duration of light-emitting diode (LED) supplemental lighting providing far-red radiation during seedling production influences subsequent time to flower of long-day annuals

Abstract

A low red (R) to far-red (FR) radiation ratio can hasten or promote a flowering response in many long-day plants (LDPs). However, the majority of light-emitting diode (LED) supplemental lighting (SL) fixtures on the market only contain blue (B) and R LEDs and thus, flowering of some LDPs may be delayed if FR radiation is not included in the spectrum. The objectives of this study were to determine 1) the minimum duration of FR radiation needed at the seedling stage to promote subsequent flowering of LDPs and 2) to quantify the response of day-neutral plants to the addition of FR radiation. Upon germination, seedlings of calibrachoa ‘Kabloom Light Pink Blast’ or ‘Kabloom Pink Blast’ (Calibrachoa ×hybrida), impatiens ‘Accent Premium Red’ (Impatiens walleriana), petunia ‘Wave Carmine Velour’ (Petunia ×hybrida), and snapdragon ‘Liberty Classic Yellow’ (Antirrhinum majus), were placed under lighting treatments that included a 9-h truncated short day (SD) or five SL treatments delivering a total photon flux density (TPFD) of 70 μmol m−2 s−1 for up to 16 h day–1 based on an instantaneous threshold. The LED SL treatments defined by their wavebands (photon flux density in μmol m–2 s–1) of B (400–500 nm) and R (600–700 nm) radiation with or without FR (700–800 nm) radiation were B15R40FR15 or B20R50. After 14 or 21 days under B20R50 LED SL, randomly selected seedlings were transferred to B15R40FR15 LED SL where they stayed for the remainder of the experiment for a total of 28 days. Thus, the treatments were 9-h SD, HPS SL, B20R50, B20R50 (21 days) + B15R40FR15 (7 days), B20R50 (14 days) + B15R40FR15 (14 days), and B15R40FR15 (28 days) LED SL. Petunia stem length was not influenced by lighting treatment whereas snapdragon was 17% taller under B20R50 (21 or 14 days) + B15R40FR15 SL (7 or 14 days) and HPS SL compared to B15R40FR15 SL. Additionally, the root dry mass for snapdragon was up to 33% and 22% lower under B20R50 (14 days) + B15R40FR15 SL (14 days) or B15R40FR15 SL, respectively, compared to all other treatments. Time to first visible bud and flower was hastened by B15R40FR15 SL (7–28 days) up to 3 and 8 days for calibrachoa and snapdragon, respectively, compared to HPS or B20R50 SL. Our results indicate that the inclusion of FR radiation via LED SL provided for ≥14 days at the end of the seedling stage can hasten time to flower of LDPs without excessive stem elongation at flower.