End-of-day Far-red Lighting to Mitigate Intumescences on Tomato Seedlings Grown under LEDs

Light emitting diodes (LEDs) have attracted commercial interest as a light source for transplant production under closed-type conditions. However, intumescences, a physiological disorder characterized by tumors on leaves and stems, are observed in some cultivars of tomatoes grown under LEDs. When damage is severe, leaf necrosis and leaf abscission occur, resulting in growth reduction. Exposing plants to ultraviolet (UV) light (especially UV-B) can inhibit intumescences, however availability of UV-B LEDs is still limited. Morrow and Tibbitts (1988) demonstrated that far-red lighting following red lighting inhibited intumescences in tomato leaf disks. End-of-day far-red (EOD-FR) lighting using LEDs could be a cost effective alternative to inhibit intumescences. The objective of this study was to examine the effect of EOD-FR lighting on intumescences on tomato seedlings grown under LEDs. Tomato rootstock seedlings (cv. ‘Beaufort’) were grown in a growth chamber under 100 μmol·m^-2·s^-1 photosynthetic photon flux (PPF) using blue (B) and red (R) LEDs with 10B-90R% or 75B-25R% photon flux, 18-h photoperiod, 25 °C air temperature, and ambient CO₂. Varied doses (1, 2, 4, 9, or 74 mmol·m^-2·d^-1) of EOD-FR lighting were applied to the seedlings grown under the LED treatments and intumescence development was compared to a control treatment (no EOD-FR lighting). The seedlings were evaluated for incidence of intumescence 16 days after seeding. In the seedlings grown under the 10B-90R% treatment, the 1 mmol·m^-2·d^-1 EOD-FR lighting (5.2 µmol·m^-2·s^-1 for 3.3 minutes) reduced the incidence of stem intumescence and leaf abscission from 62.5% to 0.0% and from 54.2% to 0.0%, respectively compared to the control (no EOD-FR lighting). The percentage of leaves that exhibited intumescences was reduced by the 1 mmol·m^-2·d^-1 EOD-FR lighting from 62.0% to 42.3% compared to the control (no EOD-FR lighting). The efficacy of EOD-FR lighting was the same regardless of doses (1–74 mmol·m^-2·d^-1). The seedlings grown under the 75B-25R% treatment with EOD-FR lighting had 5.0 % of leaves exhibiting intumescences, compared to 38.6% in the control (75B-25R% treatment without EOD-FR lighting) or 39.4% observed in the 10B-90R% treatment with the same dose of EOD-FR lighting. This study showed that intumescences in tomato seedlings grown under LEDs can be mitigated by combining relatively high blue photon flux ratio during the photoperiod and EOD-FR lighting at 1 mmol·m^-2·d^-1.