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 IS 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. © Annals of Botany CompanyKey words: Callus, cambium, Cydonia oblonga, differentiation, graft, histology, incompatibility, pear, Pyrus communis, quince. INTRODUCTIONQuince is widely used as a dwarfing rootstock for pear, although graft incompatibility frequently occurs between quince rootstocks and many pear cultivars . The main features shown by incompatible graft combinations are cell necrosis and vascular discontinuity at the graft interface, which lead to the breakage of the union or to the slow decline of the tree . It has been demonstrated that the occurrence of these typical symptoms requires actual contact between the tissues of the two graft partners, since incompatibility can be overcome by insertion of a mutually compatible interstock . These characteristics led to class incompatible pear / quince combinations into the localized type of incompatibility, which also includes apricot/plum and apricot/peach combinations . In pear/quince grafts, a possible mechanism for this so-called localized incompatibility has been proposed, involving the potential toxic effects of prunasin, a cyanogenic glycoside . According to this hypothesis prunasin, contained in the bark tissues of quince rootstocks, would ascend into the pear scion where it would be hydrolysed by ,8-glucosidases, liberating cyanide which would be toxic to the cambial cells, and therefore responsible for the cell necrosis at the graft interface. However, the details of tissue and cellular mechanisms of cyanogenesis at the graft interface have not yet been elucidated. In the same way, little is known about the em...

Evaluation of Graft Compatibility Between Quince Rootstocks and Pear Scions

“…Gur et al (1968) postulate that in the pear and quince grafting a possible mechanism for this so-called localized incompatibility has been proposed, involving the potential toxic effects of prunasin, a cyanogenic glycoside. According to this hypothesis prunasin that is present in the bark tissues of quince rootstocks, would ascend into the pear scion where it is hydrolyzed by βglicosidases, releasing cyanide which is toxic to cambial cells causing therefore necrosis of cell tissue on the scar (Ermel et al, 1997). The necrosis reduces the transport of water and nutrients through the graft union.…”

Histological Evaluation of Early Graft Compatibility in Uapaca kirkiana Müell Arg. Scion/Stock Combinations

“…Errea et al (2001) used contingency tables, but no statistical differences were found and there was reduction in validity of the test. Correspondence analysis seems to offer a better procedure to discriminate compatible combinations from incompatible combinations (Ermel et al, 1997). It is based on c 2 transformation and produces dimensions that represent the c 2 distances (Lebart et al, 1984).…”

“…5C shows that prolific callus cells make the union line narrow. According to Ermel et al (1997), cell necrosis and discontinuity of vascular connections at the union are the main indicators of incompatibility. Gebhardt and Feucht (1982) reported a high quantity of phenols above the union as a cause of graft incompatibility in Prunus species.…”

Analysis of phenolic compounds for determination of cambium differentiation and tracheal elements in olive graft combinations

“…Likewise Ermel et al (1997) showed that cell necrosis and lack of vascular connection continuity in graft union was the manifest symptoms of graft incompatibility. Errea et al (1994a) reported that insufficient differentiation of callus cells in some areas of graft union affected the newly formed xylem and phloem activity.…”