Abstract: [Objective] The mechanism of histological changes between rootstock and scion during callus formation, proliferation and connection of transfusion tissue at different grafting periods was studied through histological observation of the healing process of walnut(Juglans regia) bud grafting, in order to provide theoretical guidance for the practice of grafting walnut and other woody plants.[Method] The buds of 12-year-old semi-lignified new shoots of J. regia ‘Xiangling’ were used as scions and 2-year-old walnut seedlings were used as rootstocks, bud grafting experiments were carried out on 40-47 days and 100-107 days after bud germination respectively by square bud grafting. Histological observation of healing process was carried out with the method of heterogeneous tissue of woody plants section established by our research group.[Result] The mode of healing between rootstock and scion 40-47 days after bud germination of walnut was consistent with previous reports. The cambium cells played a dominant role and the main histological changes during healing included:isolation layer formation (3 days)→cambium cell differentiation to produce callus (4 days)→callus connection (6 days)→original cambium repair and connection between scion and rootstock (12 days). It took 12-15 days to complete the healing, and the survival rate of grafting was 95.6%; In the process of healing 100-107 days after bud germination, a new way of grafting healing was found different from the conventional healing. The xylem ray cells played a leading role in this process, and the healing process went through isolation layer generation (3 days)→xylem ray cell differentiation to produce callus (4 days)→callus connection (9 days)→vessel differentiation (15 days)→new cambium formation and connection of xylem rays between scion and rootstock (25 days), it took 25-30 days to complete the healing process, and the survival rate of grafting was 61.7%; During the healing process, the isolation layer on the side of rootstock dissolves before the scion and disappears when the callus was connected, while the isolation layer on the surface of scion still existed when the callus was differentiated, until the new cambium was formed.[Conclusion] Besides the common mode of healing dominated by cambium cells, there was a new mode of healing dominated by xylem ray cells in the healing process of bud grafting in walnut, xylem ray cells dedifferentiated to produce callus, and further differentiated into vessel, cambium and xylem ray to complete the final connection. During this process, the callus mainly came from the xylem ray cells of the rootstock, and the isolation layer of the rootstock dissolved before the scion.

Key words: Juglans regia, bud grafting, xylem ray, healing mode, histological mechanism