Effects of Far-Red LED Light on the Growth and Development of Tomato Seedlings in a Closed Seedling Production System

Abstract

Year-round tomato cultivation needs high-quality seedlings with stem length and node position of the first flower truss controlled at suitable values. Generally, shade avoidance syndrome (SAS) promotes stem elongation and alters flowering time under natural sunlight. The objective of this study was to develop a method for controlling the stem elon-gation and flowering of tomato seedlings in a closed seedling production system (CSPS) with light emitting diodes (LEDs). We conducted two experiments to investigate how the artificial light environment induces SAS to control stem elongation and flowering. In Experiment 1, which examined the effect of end-of-day light (EOD-light), the phytochrome photostationary states (PSS) of the treatments were 0.56 for EOD-blue treatment, 0.89 for EOD-red (R) treatment, and 0.13 for EOD-far red (FR) treatment. Stem elongation was promoted more with EOD-FR treatment compared to the control. No significant differences among the treatments were observed for the node position of the first flower truss. Therefore, although the EOD-FR treatment did not promote flowering, it did induce SAS. In Experiment 2, which examined the effect of FR light quantity in the CSPS, we reproduced light environment conditions within the canopy at the time of the second nursery in a greenhouse and investigated how much the R/FR ratio induced SAS by adjusting the FR light quantity. The light environment conditions were tested at 150 nmol m-2 s-1 of total photosynthetic photon flux density with four FR light quantities (0, 50, 100, or 150 nmol m-2 s-1). There was no significant differences among the treatments for the node position of the first flower truss, and the stem elongation with FR50, FR100, and FR150 treatments was greater than that at FR0. These results suggested that the PSS, which was less than or equal to 0.72 during the light period (16 h), induced SAS. Thus, light environmental control with LEDs with EOD-FR or a low R/FR ratio is not suitable for the CSPS because it does not promote flowering of tomato seedlings.