Effect of low night temperature on graft union formation in watermelon grafted onto bottle gourd rootstock

- Concrescence of graft-union occurs fastest at the night temperature of 18 °C than 15 °C and 12 °C.
- Vascular bridges were connected to vascular bundles at 5 days post-grafting at the night temperature of 18 °C.
- Healing rate slowed as night temperature decreased.
- A minimum temperature of 18 °C is indicated during the graft union formation stage.

To explore the effects of low night temperatures on graft union formation in watermelon grafted onto bottle gourd rootstock, an anatomical study during the healing stage was performed. In the present study, we compared the morphological anatomy structures during the healing process in grafted watermelon seedlings under different night temperature treatments. The results showed that concrescence occurs fastest at the night temperature of 18 °C, at which vascular bridges were connected to vascular bundles at 5 days post-grafting (dpg). For night temperatures of 15 °C and 12 °C, vascular bridges were connected to vascular bundles at 7 dpg and 10 dpg, respectively. In general, healing rate slowed as night temperature decreased. Low temperatures delayed the differentiation of vascular tissue, which was not conducive to the connection of vascular bundles. The wider gully between the rootstock and scion cut surface caused the callus formation on both sides to fail to link, which turns to the formation of graft union unsuccessful. These results suggest that night temperatures are implicated in graft development in watermelon plants. A minimum temperature of 18 °C is indicated during the graft union formation stage.