Supplemental lighting enhances quality of grafted watermelon seedlings

Research on the use of supplemental lighting has intensified over the last decade in the production process of grafted vegetable seedlings which could be distinguished in three stages: A) growing of seedlings used for scion and rootstock in greenhouses, B) healing of grafted seedlings in a growth chamber, and C) growing grafted seedlings in greenhouses. The aim of the present study was to study the necessity of supplemental lighting during the production process of grafted watermelon seedlings in greenhouse. Watermelon seedlings used as the scion and the rootstock as well as grafted and healed seedlings were grown in a greenhouse with or without supplemental lighting using high pressure sodium lamps while fluorescent lamps were used for seedlings during the graft healing for 7 days in a growth chamber. The evaluated parameters were: shoot length and diameter, shoot and root fresh and dry weights, leaf thickness, leaf and cotyledon areas, relative chlorophyll content, and leaf color. Seedlings used as the scion developed thicker stems (+23%) and leaves (+50%), larger leaves (+220%) and cotyledons (+18%), and greater shoot (+82%) and root (+89%) when grown under supplemental lighting. Grafted seedlings exposed to supplemental lighting after the graft healing had thicker (+18%) and greener (+42% chlorophyll) leaves as well as greater fresh (+14%) and dry (+18%) root weight as compared to seedlings grown under natural light. During healing in the growth chamber only 14% 'optimum' seedlings were produced. The relatively low percentage of `optimum' seedlings may typically lead to a lower quality of grafted seedlings transplanted in the field. It is concluded that supplemental lighting is necessary in the greenhouse for growing of seedlings used as the scion and grafted seedlings, while FL lamps during the healing stage are not as efficient as needed.