Abstract: Watermelon seedlings of different ages were grafted onto cucurbit rootstocks. The graft union formation was observed using paraffin section and a transmission electron microscope（TEM），and the survival rate was statistically analyzed. In the early days after grafting，a necrotic layer was induced between scion and rootstock. The concrescence rate of the graft combinations with 5 d，7 d and 10 d scions were higher，the necrotic layer disappeared on the 8th day after grafting，and abundant organelles occurred in the cells adjacent to the necrotic layer. Vascular bundles began to regrow after 6 days of grafting，and vascular bridges were formed after 8 days of grafting. The survival rates of these three combinations reached more than 90%. By contrast，in the graft combinations with 13 d- and 18 d-old scions，the necrotic layer could still be observed on the 8th day after grafting，and few organelles and vascular bridges were formed in the cells adjacent to the necrotic layer. The survival rates of seedlings with 13 d- and 18 d-old scions were 83.3% and 72%，respectively. Thus，the connection of vascular bridges between rootstock and scion，and abundant organelles in the cell adjacent to the necrotic layer could be considered as morphological signs of grafted seedling survival. The scion age below 10 d was appropriate for grafting.

Key words: watermelon, Citrullus lanatus, graft union, scion age, vascular bundle, ultrastructure