Light Quality Impacts on Growth, Morphology, and Flowering of Grafted Tomato Seedlings

Greenhouse tomato growers in northern climates often start seedling propagation during the winter months to initiate production in early spring and take advantage of increasing natural light. Grafting is growing in popularity because of improved disease resistance, stress tolerance, and increased yields. Since natural winter light integrals are low and supplemental lighting is needed, this study was aimed at determining the impact of light quality on grafted seedlings for grafting success, plant growth, morphology, and timing of flower development. Seedlings of ‘Komeett’ and ‘Amsterdam’ were grafted onto the rootstock ‘Maxifort’, 20 d from seeding using the splice/tube grafting technique. Grafted seedlings were placed into the light quality treatments of red LEDs (peak emission at 630 and 665 nm), blue LEDs (peak at 455 nm), red/blue LEDs (80:20, peak at 665 and 455 nm), or white LEDs (3700 K). In addition, T5 fluorescent tubes (4100 K) and natural greenhouse light supplemented with high-pressure sodium (HPS) irradiance were included as treatments. The study was completed in a greenhouse covered with the acrylic material DEGLAS® and conducted from Dec. 24 to Mar. 3. The growing areas of LEDs and fluorescent tubes were separated and shielded from natural and greenhouse light using an opaque blackout material. Plants were grown at a constant 19 ± 2 °C (66 °F) with a 16-h photoperiod at a photosynthetic photon flux of approximately 150 µmol·m^-2s^-1. Success of grafting was 100 percent in all treatments. First open flower occurred on average 62 d from seeding with no differences among treatments or cultivars. The tallest seedlings (media to apical point) after 30 d were those grown under red LEDs at 27.8 ± 1.3 cm (Komeett) and 33.6 ± 1.2 cm (Amsterdam). The shortest heights were recorded in the red/blue LED environment (15.2 ± 1.1 cm for Komeett, 16.8 ± 1.4 cm for Amsterdam). Both cultivars under red LEDs had one additional leaf (larger than 3 cm) compared to fluorescent or white LEDs, and two additional leaves compared to HPS, blue or red/blue LEDs. The caliper of the scion was larger than the rootstock independent of light quality. The smallest stem diameters were recorded for red LED grown Amsterdam seedlings at 5.0 ± 0.3 mm scion caliper and 4.6 ± 0.3 mm rootstock caliper. Although all growing environments supported development of quality transplants, the more compact plants under red/blue LEDs are expected to be preferred for greenhouse crop applications.