Early Scion/Stock Junction in Compatible and Incompatible Pear/Pear and Pear/Quince Grafts: a Histo-cytological Study

Abstract

A comparative histo-cytological study of compatible and incompatible pear/pear and pear/quince grafts, grown in a glasshouse, was carried out on unions collected from day 1 to day 22 after budding (DAB). Successive steps leading to the graft junction were studied. Adhesion between the two partners was observed along the inner bark tissues (phloem and cambium) just 24 h after budding, while a necrotic layer appeared at the graft interface. The first cell divisions were observed 3 DAB. In the inner bark interface these led to the formation of a junction callus, inside which new cambial strands (neocambia) appeared between 10 and 15 DAB, starting from the cut edges of the scion and stock cambia, and joining later when they began to produce both phloem and xylem derivatives. Along the outer bark interface, where the cut surfaces of the bark parenchyma were exposed to the air, divisions of the parenchyma cells gave rise to a new phellogen layer (neophellogen) approx. 6 DAB, and suber production was observed 10 DAB. No obvious differences were noticed between compatible and incompatible grafts during the first stages of graft junction. No particular cell necrosis phenomenon was observed at the interface of incompatible grafts. Nevertheless, the neocambial junction appeared to be delayed in pear/quince heterografts, especially in incompatible ones. This could be the first structural event of the incompatibility response in pear/quince grafts.