Humic Substances and LED Lighting Improved Tomato Transplant Growth, with Limited Carryover Effects on Fruit Yield and Quality

Humic substances (HS) and light-emitting diode (LED) light applications have shown beneficial effects on plant growth, nutrition, and yield in various vegetable crops. However, their carryover effects on fruit yield and quality, when applied to the growing media before transplanting, have not been widely explored in tomatoes. This study evaluated tomato transplant growth in response to the application of solid HS (1% v/v, control) and varying LED light qualities [10 blue (B):90 red (R), 50B:50R, 100B, and fluorescent] in growth chamber conditions (25/19 °C, 16-hour days/8-hour nights, 100 µmol·m –2 ·s –1 photosynthetic photon flux density, 60% relative humidity), followed by a post-transplant study assessing fruit yield and quality. Round-type ‘Celebrity’ and cherry-type ‘Chadwick’ tomato seedlings were transplanted into 16-L pots 5 weeks after sowing (WAS), and fruit were harvested for 10 weeks. Transplant growth components measured at 5 WAS were significantly greater in seedlings treated with HS compared with control plants, including improved shoot and root dry weight, stem diameter, leaf area, and total root length and surface area. Seedlings grown under 50B:50R light showed a greater increase in shoot and root growth when treated with HS compared with other light treatment groups, whereas plants grown under 10B:90R light exhibited the greatest growth without HS. However, the enhanced seedling growth under HS and specific LED light treatments did not translate fully into increased fruit yield and quality after transplanting. There were no significant increases in fruit yield in response to HS application, except for ‘Chadwick’ under 50B:50R and 100B light. No clear correlation was observed between high-quality transplants and tomato quality traits, such as total soluble solids and secondary metabolites, although the 100B light treatment increased lycopene and total phenolic content. In conclusion, the application of HS to the transplant growing media, combined with specific LED wavelengths, particularly 50B:50R, enhanced shoot and root growth significantly in young tomato seedlings. However, these benefits largely diminished after transplanting and did not result consistently in increased fruit yield or quality.