Mina farsi - Department of Horticulture Science, College of Agriculture & Natural Resources, University of Tehran, Karaj, Iran
Mohammadreza Fatahi Moghadam - Department of Horticulture Science, College of Agriculture & Natural Resources, University of Tehran, Karaj, Iran
Zabihollah Zamani - Department of Horticulture Science, College of Agriculture & Natural Resources, University of Tehran, Karaj, Iran
Darab Hassani - Temperate Fruits Research Center, Horticultural Science Research Institute, Karaj, Iran

Compared to other techniques of propagation, grafting is the most successful and feasible technique for asexual propagation of walnut plants. There is little information about graft union formation in walnut. Therefore, the objective of this study was to evaluate histological events during graft union formation in Juglans regia L. cv. Chandler scions when minigrafted on the one-year-old seedlings. Cross and longitudinal sections of the graft union were taken for examining different stages of grafting process after 1, 14, 30, 60, 120, 180, 420 and 540 days of minigrafting. One day after grafting, brown necrotic layers were observed at the cut edges. The first callus cells were initiated from cambium layer of rootstock 14 days after grafting but there was weak connection between two parts. New vascular connections between rootstock and scion were observed 30 days after grafting . Vascular connections were increased in central parts of the graft union 60 days after grafting. In mid-summer (180 days after grafting), necrotic layer was almost disappeared in the central longitudinal sections but they were increased especially in the bark, callus and top parts of the graft union due to high temperature and low humidity. In 420 days after grafting, two parts were strongly connected by xylem vessels, but necrotic layer was still remained and observed in some parts. In 540 day after grafting which was coincided with the end of second growing season, two parts were tightly connected to each other and necrotic layer disappeared in most of cross and longitudinal sections.

callus, graft union, rootstock, scion, vascular connection, walnut