Bichromatic red and blue LEDs during healing enhance the vegetative growth and quality of grafted watermelon seedlings

Watermelon is an economically important crop that is exclusively established through the use of grafted seedlings. Grafting procedure and the period of healing are critical for the production of high quality seedlings. During healing, grafted seedlings are grown in a chamber with controlled conditions, including supplementary lighting. The aim of the present study was to evaluate the impact of commonly used fluorescent (FL, control) lamps or light-emitting diodes (LEDs) with varying red (R) and blue (B) proportions [100 % R (R), 12 % B and 88 % R (12B), 24 % B and 76 % R (24B), 36 % B and 64 % R (36B) or 100 % B (8)] employed in the healing chamber on the quality of grafted watermelon seedlings. Seedlings under FL and B performed poorly in almost all tested parameters compared to the other LEDs. However, monochromatic B led to the greatest rates of photosynthesis (P-max), stomatal conductance (g) and transpiration (E). On the contrary, R promoted the seedling length, fresh and dry weight (FW and DW) of shoots and roots, and root-to-shoot ratio (R/S) but led to inferior gas exchange and photosynthetic efficiency (F-v/F-m). B addition to R light (12B and 24B) enhanced the morphological and growth characteristics tested at least as much as R light or even more (leaf area, shoot DW). It should be noted that the entire root system of the seedlings was cut off during the grafting procedure and its re-growth was highly promoted by dichromatic LEDs (R plus B). Altogether, the above results demonstrate that R light with addition of low proportion of B light (12-24%) enhances the production of high quality grafted watermelon seedlings.