Nocturnal LED Supplemental Lighting Improves Quality of Tomato Seedlings by Increasing Biomass Accumulation in a Controlled Environment

Tomato (<i>Solanum lycopersicum</i> L. cv. Zhongza NO. 9) was used as the experimental material to investigate the effects of nocturnal LED supplemental light with the photosynthetic photon flux density (PPFD) of 100, 200, 300 μmol·m⁻²·s⁻¹, and the light time of 1, 2 h on the seedling quality in a controlled environment, with seedlings without nocturnal supplemental lighting serving as the control. The results demonstrate that an increase in PPFD at night progressively enhances the plant height and leaf number of tomato seedlings, while stem diameter and leaf area initially increase and subsequently decrease. Although light time and light period-of-time at night did not significantly affect seedling morphology, PPFD and light time notably influenced chlorophyll content and net photosynthetic rate. An optimal lighting energy amount at night augmented photosynthetic capacity. However, excessive PPFD induced photoinhibition in the leaves. Additionally, appropriate nocturnal LED supplemental lighting significantly improved the antioxidant capacity of the seedlings, increased proline content, reduced malondialdehyde content, and bolstered the self-protection mechanisms of the seedlings against nocturnal light stress. Both the PPFD and light time at night promoted biomass accumulation in tomato seedlings. Specifically, when supplemental lighting was applied for 2 h at an intensity of 200 μmol·m⁻²·s⁻¹, both the fresh and dry weights of the shoot and root significantly increased, and the seedling health index was highest. Therefore, appropriate nocturnal LED supplemental lighting positively impacts the health index and photosynthate accumulation of tomato seedlings, but controlling PPFD is essential to avoid photoinhibition.