Growth Responses of Greenhouse Tomato Seedlings to Different Spectra of Supplemental Lighting Are Season-specific in a Northern Climate

Arrays of red (R) and blue (B) light-emitting diodes (LEDs) were compared to a high-pressure sodium (HPS) lamp as supplemental lighting (SL) sources to propagate greenhouse-grown tomato seedlings in a temperate climate (lat. 40°N, long. 86°W). Six tomato cultivars were grown monthly throughout a year in a glass-glazed greenhouse. Five lighting treatments were evaluated: natural solar light only (control), natural + SL from a 100-W HPS lamp, or natural + SL from LEDs using either 80% R + 20% B, 95% R + 5% B, or 100% R. A variant solar daily light integral (DLI) occurred naturally for all treatments while a constant DLI of 5.1 mol·m^-2·d^-1 was provided to all SL treatments. Supplemental lighting generally increased hypocotyl diameter, epicotyl length, shoot dry weight, leaf number, and leaf expansion relative to control across seasons, whereas hypocotyl elongation generally decreased when SL was applied. Overall, growth benefits of SL were greatest during low-ambient DLI. These results suggest that light-quality effects on tomato seedling growth from different SL treatments are season-specific and affected by ambient solar DLI.  In general, a combination of R and B in SL stimulated growth and productivity for tomato seedlings during winter and summer (lowest- and highest-solar DLI, respectively), whereas fewer treatment differences were observed during milder-solar DLI spring. The addition of B light has potential to stimulate overall seedling growth compared to B-deficient SL treatments in northern climates. However, the effectiveness of growth enhancement seems to depend on the SL spectrum used.