Kiwifruit is a popular type of edible and nutritious fruit of Actinidia chinensis Planchon. Extensive use of rootstocks is needed to control the traits of grafted cultivars, including vigor, early flowering, fruit quality and yield. However, this practice remains limited to a few Actinidia species, mainly due to inconsistent graft success and difficulties in predicting graft compatibility responses of proposed scion-rootstock combinations. Aiming to identify graft (in)compatibility and its potential mechanisms, we grafted 44 kiwifruit (Actinidia chinensis Planchon) cultivars and 18 different Actinidia species onto commercialized kiwifruit (Miliang) rootstocks to study the compatibility behavior of homo- and heterografts in the genus Actinidia. Based on phenotypic and histological observations, the kiwifruit homografts demonstrated high affinity between rootstocks and scions, whereas the heterografts exhibited obviously higher translocated graft incompatibility. In addition, there was no significant difference in graft success rate within the different homograft groups (diploid, tetraploid and hexaploid cultivars), but compared with the other species, the species belonging to Sect. Leiocarpae and used in heterografts presented an obviously lower success rate. When the internal anatomy of the graft unions was observed, the compatible rootstock and scion combinations presented consistent cell and vascular connections at the graft union, whereas the incompatible combinations were separated in many places by clusters of necrotic layers and abnormal vascular complexes. Correlation analysis further confirmed that the phylogenetic relatedness between rootstocks and scions was negatively correlated with grafting survival rate. This work not only indicates the relationship between genetic affinity coefficients and graft incompatibility but also is of practical interest to growers and breeders in terms of their choice of selecting optimal combinations of rootstocks and scions in the genus Actinidia
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