Effects of Monochromatic Blue Light on Intumescence Injury for Tomato Seedlings Grown Under LEDs.

Physiological disorder called intumescence has been observed in tomato seedlings of sensitive cultivars grown under LED light, presumably due to the lack of UV-B light. A previous study showed that phytochrome is involved in regulating intumescence and high phytochrome photostationary state (Pfr/Ptotal, such as 0.9 for red light) under UV-B deficit light induces intumescence in tomato. In our previous research the intumescence symptoms were mitigated by end-of-day (EOD) far red (FR) lighting (Pfr/Ptotal = 0.1). However, seedlings grown with EOD FR light become extensively stretched. Monochromatic blue (B) light has relatively low Pfr/Ptotal of 0.5. Further, B light may increase stomatal conductance through the action of blue light receptors, and thereby potentially mitigates the turgor in cells that may induce the problematic hypertrophy when applied during the night time. Therefore, this study focused on application of B light as a possible means to mitigate intumescences. B light was applied for ‘Beaufort’ tomato seedlings in two ways (with the same B light dose of 0.17 mol m^–2 d^–1): 1) EOD light (20 µmol m^–2 s^–1 for 2.5 hours, followed by 3.5-hour darkness and 18-hour photoperiod) and 2) overnight lighting (8 µmol m^–2 s^–1 for 6 hours, followed by 18-hour photoperiod). Light during the photoperiod was provided by B and R LEDs at 200 ± 5 µmol m^–2 s^–1 (10% B, Pfr/Ptotal = 0.9). In addition, no lighting (6 hours, followed by 18-hour photoperiod) or overnight R or FR lighting (8 µmol m^–2 s^–1, Pfr/Ptotal=0.9 or 0.1, respectively) was examined for comparison. We repeated the experiment twice. For both repetitions, consistently, the control or R light treatment developed severe intumescence injury and FR treatment mitigated the symptom. For example, leaf area exhibiting intumescence injury in second true leaf was 55-63% smaller and fresh weight was 28-78% greater under FR than under the control or R light treatment. In contrast, B light effect was somewhat inconsistent. The first experiment showed significant but mild mitigation of intumescence under both B light treatments, resulting in 31-47% greater fresh weight than under the control or R light treatment. However, intumescence inhibitory effects were not significant when the experiment was repeated. These results have shown that monochromatic blue light has no or little effect on intumescence compared with FR light.