Development of union abnormalities in grafts between lemon (Citrus limon) and Poncirus trifoliata

Abstract

A complete ring of bark was exchanged between seedlings of P. trifoliata and those of seven lemon cultivars, thereby producing P. trifoliata seedlings with a ring of lemon bark and lemon seedlings with a ring of P. trifoliata bark. Self-grafted controls were also propagated. The incompatibility of Eureka lemon–P. trifoliata combinations was clearly manifested at the graft union 7–9 months after grafting. Union abnormalities were detected only in lemon seedlings with a ring of P. trifoliata bark. They were not detected in P. trifoliata seedlings with a ring of lemon bark, nor in self-grafted controls. Anatomical studies revealed that the initial disturbance occurred in the outer phloem and spread along the rays towards the cambium. After the cambial zone was affected, the excessive proliferation of ray parenchyma disrupted vascular continuity. Extensive necrosis and gum deposition were evident in severely affected unions. The investigation showed that an early diagnosis of incompatibility is possible by examining the structure of the union.