Light-emitting Diodes As an Alternative to Cool-white Fluorescents for Healing of Grafted Tomato Transplants

Grafted tomato seedlings propagation must be optimized to increase production efficiencies. Grafting has several steps that include growing, grafting, healing, and acclimation. Healing is considered the most critical step. Healing is the joining of cambium between rootstock and scion to form vascular tissue and is commonly done under modified greenhouse conditions or inside healing chambers. Previous work determined that light during the healing period influences morphology and quality of grafted tomatoes. Cool white fluorescent (CWF) at a photosynthetic photon flux of 100 µmol·m^-2·s^-1 is recommended for healing chambers. Light emitting diodes (LED) make possible optimizing the spectral requirements for healing. Tomato seedlings rootstock (cv. Maxifort) and scion (cv. Trust) were grown in the greenhouse at 13.3 ± 3.8 mol m^-2 d^-1 daily light integral, 23.6/18.67 °C ± 2°C (day/night) temperatures (T), and 38.6 ± 9.6 % relative humidity (RH). After 19 days, tomatoes were grafted and placed inside a healing chamber under 27.7°C ± 0.9°C T and 98%+ RH on day one and decreasing RH over seven days. During healing, the plants were exposed to six different light treatments. Four LED spectra with varied blue (B), green (G) and red (R) percent photon flux (PF) of 4B:5G:89R, 30B:2G:67R, 50B:50R and 76B:24R at 75 µmol·m^-2·s^-1 PF. CWF was used as a control at 75 µmol·m^-2·s^-1 (CWF-75), and 100 µmol·m^-2·s^-1 (CW-100) PF. Plant height and leaf number were measured pre and post healing on each grafted plant. In addition, dry mass, stem diameter, fresh mass, and leaf area were measured on a sub-sample pre grafting and on each plant post grafting. The %-increase from pre to post healing was calculated for all the plant parameters. Plants in CWF-100 had 61% greater height %-increase than in 50B:50R and 76B:24R. No differences were observed between plants in 4B:5G:89R, 30B:2G:67R, CWF75 and CWF-100 in height %-increase. CWF-100 had 73% greater dry mass %-increase than CWF-75 and 30B:2G:67R. 50B:50R, 76B:24R, 4B:5G:89R at 75 µmol·m^-2·s^-1 were not different from CWF-100 for dry mass %-increase. Plants in 4B:5G:89R had 94% greater plant compactness than in CWF-75 and 30B:2G:67R. Plants in 50B:50R, 76B:24R and 4B:5G:89R at 75 µmol·m^-2·s^-1 were not different than in CWF-100 for plant compactness. This preliminary experiment shows that tomato plant quality during healing is comparable under LED (4B:5G:89R, 50B:50R , 76B:24R) at 75 µmol·m^-2·s^-1 to CWF at 100 µmol·m^-2·s^-1.