Using leds to quantify the effect of the red to far-red ratio of night-interruption lighting on flowering of photoperiodic crops

Many commercial floriculture growers use incandescent (INC) lamps for photoperiodic lighting because they are inexpensive and emit an effective spectrum. However, since INC lamps are energy inefficient and are being phased out of production, we investigated the use of light-emitting diodes (LEDs) for regulating flowering of the long-day plants petunia (Petunia multiflora 'Easy Wave White') and snapdragon (Antirrhinum majus 'Liberty Classic Cherry'), and the short-day plant marigold (Tagetes erecta 'American Antigua Yellow'). In addition to a 9-h short-day (SD) control, 4-h night interruption (NI) treatments delivered during the 15-h night were provided by INC lamps or LEDs with seven different red (R, 600 to 700 nm) to far-red (FR, 700 to 800 nm) ratios (R:FR) ranging from only R to only FR. Under the LED treatments, the estimated phytochrome photoequilibria (P(FR)/P(R+FR)) in plants ranged from 0.16 (under FR light) to 0.89 (under R light). Seedling height and node number at transplant, date of first visible bud, date of first open flower, flower number, plant height, node number below the first flower, and lateral branch number were recorded. The INC NI (R:FR=0.59) and LEDs with an R:(FR)of 0.66 to 2.38 and 0.28 to 1.07 promoted flowering the most in petunia and snapdragon, respectively, and LEDs with an R:(FR) ≥0.66 inhibited flowering the most in marigold. There was little or no effect of NI treatments on inflorescence or flower bud number for marigold and petunia, but the treatments that accelerated flowering of snapdragon the most generally resulted in fewer flower buds. Plant height was greatest under moderate R:(FR) in marigold and petunia, while snapdragon exhibited the opposite trend. We conclude that the LED treatments with a moderate R:(FR) were effective both for promoting flowering in petunia and snapdragon and for inhibiting flowering in marigold.