A method for simultaneously monitoring phloem and xylem reconnections in grafted watermelon seedlings

Abstract

Grafting is an effective way to increase watermelon's tolerances to biotic and abiotic stresses. However, the survival of grafted seedlings largely depends on successful graft formation. Therefore, understanding the graft formation process, particularly vascular reconnection, is of critical importance. This study found that lignin in watermelon stem showed strong auto-fluorescence under blue-light excitation. Consequently, blue-light excited fluorescent tracers (FTs) such as 5(6)-carboxy fluorescein diacetate (CFDA) were unsuitable for assaying vascular connectivity in watermelon. In contrast, UV-light excited esculin and red-light excited acid fuchsin proved to be efficient FTs for monitoring phloem and xylem connectivity, respectively, in self-grafted watermelon. Furthermore, combining esculin application to the scion cotyledon and acid fuchsin to the rootstock root enabled simultaneous monitoring of both phloem and xylem connectivity in individual self-grafted watermelon seedlings. Based on this assay, we found that phloem and xylem reconnections occurred simutaneously in self-grafted watermelon. Moreover, low temperature and removal of the rootstock cotyledons both delayed vascular reconnection in the self-grafted watermelon. In conclusion, this new method provided a convenient, accurate and rapid way to analyze vascular connectivity in watermelon.