S03 - Session O2 - Exploring the role of light-emitting diodes during the healing of grafted vegetable seedlings

Authors: Athanasios Koukounaras *, Filippos Bantis, Christodoulos Daggitsis

Vegetable grafting is a means to overcome modern cultivation constraints such as abiotic and biotic stress factors. Tomato ( Solanum lycopersicum ) and watermelon ( Citrullus lanatus ) are two economically important vegetables highly established through grafted seedlings. Especially the healing stage is crucial for the development of vegetable seedlings with high quality and vigorous development after transplantation both for the growers and the seedling industry. Nowadays, healing of grafted seedlings usually takes place in healing chambers with controlled environments including artificial lighting. Light-emitting diodes (LEDs) nowadays tend to replace traditional fluorescent (FL) light sources due to several benefits in controlled environment agriculture. In particular, among other parameters of plant production, light quality affects stem elongation, leaf expansion, root development, biomass accumulation, and phytohormonal activity. In our investigation involving the healing of grafted tomato and watermelon seedlings, the impact of FL was compared to LEDs with varying wavelength distribution including narrow-band red (R) and blue (B), as well as an 88/12% RB combination, and light intensity of 85 μmol m -2 s -1 . After exiting the healing chamber, in both species, shoots were longer under R and 12B, while FL induced the lowest values. Moreover, both tomato and watermelon had larger leaves under the effect of 12B. Watermelon shoot and root dry weight, and tomato shoot dry weight were enhanced under 12B and R. Regarding root architecture, watermelon seedlings exhibited higher total root length along with greater root surface area under the effect of 12B. Grafted tomato seedlings did not show significant differences in root architecture. Overall, a RB combination including 12% blue wavelength during healing proved beneficial for the development of high quality grafted watermelon and tomato seedlings. On the contrary, FL (mainly) and B (secondarily) led to inferior seedling quality in both vegetables.