In closed-type plant production systems using LED lighting for tomato seedling cultivation, the physiological disorder known as intumescence has emerged as a significant issue, particularly in susceptible cultivars. The objective of this study was to investigate the effects of varying durations of red and blue LED light on the occurrence of intumescence and seedling growth in tomato. Using the intumescence-susceptible rootstock cultivar ‘Arnold’, we conducted a factorial experiment with red and blue light exposure durations set at 0, 4, 8, 12, 16, 20, and 24 hours, excluding the 0 × 0 combination, resulting in 48 treatment groups. Evaluations were conducted 15 days after sowing, including assessments of intumescence severity, shoot dry weight, and root ball formation rate. The results indicated the duration of red light irradiation exhibited a strong and significant positive effect on intumescence severity, whereas the effect of blue light irradiation duration was not significant. However, the interaction term between red and blue light showed a significantly negative coefficient, indicating that the promotive effect of red light on intumescence was partially mitigated when combined with blue light. Shoot dry weight and root ball formation rate, which indicate seedling growth, tended to increase with longer durations of red and blue light exposure. A second experiment involving 22 tomato cultivars under three red light durations (0, 8, and 12 hours) under continuous blue light exposure revealed that in intumescence-susceptible cultivars, symptoms were mitigated at 0, 8, and 12 hours of red light. In cultivars not prone to intumescence, growth performance was unaffected by transitioning from conventional lighting to blue light dominant conditions. These findings suggest that reducing the duration of red light to prevent intumescence, while using prolonged blue light exposure to maintain the daily light integral (DLI), may enable both healthy growth and suppression of physiological disorders in tomato seedlings.
Yoshimizu et al. (Thu,) studied this question.