Regeneration of the vascular bundle at the graft interface in auto- and heterografts with juvenile nucellar seedlings of satsuma mandarin, yuzu and trifoliate orange

Abstract

The histology of the graft union by splice grafting epicotyls in auto- and heterografts of 4-week-old nucellar seedlings of satsuma mandarin (Citrus unshiu Marc.), yuzu (C. junos Sieb. ex Tanaka), plus common trifoliate orange (Poncirus trifoliata Raf.) and its crooked spine type variety, Hiryu (Flying Dragon) (P. trifoliata var. monstrosa (T.Ito) Swingle) was studied under light microscope. Callus formation at the graft interface that started immediately after grafting was especially active adjacent to the severed vascular bundles. Regeneration of the vascular cambium occurred within 1 week after grafting in all graft combinations that led to the formation of a new vascular bundle. The new vascular bundle was formed with exterior winding by obstruction of the callus mass proliferating at the graft interface; the winding tended to be deeper in heterografts with satsuma than that in autograft of satsuma. However, the winding tended to revert to a straight configuration by the subsequent formation of newer vascular bundles. No obvious morphological differences of cells were observed at the graft interfaces in the auto- and heterografts during the process of callus formation and vascular bundle generation during the 6 weeks after grafting. Callus formation at the graft interface as a response to wounding and vascular bundle regeneration may be spatially competitive processes, and the anatomical and physiological mismatches between scion and stock cause the delay of vascular bundle formation, resulting in the winding.