Acta Hortic. 1199. ISHS 2018. DOI 10.17660/ActaHortic.2018.1199.13 Proc. VIII Int. Olive Symposium Eds.: S. Perica et al. 81 Effects of phenolic compounds on differentiation of cambium and tracheal elements in graft unions of olive cultivars (Olea europaea L.) M.T. Özkaya 1,a , M. Azimi 1 , H. Colgecen 2 and H.N. Buyukkartal 3 1 Faculty of Agriculture, Ankara, Turkey; 2 Faculty of Arts and Science, Zonguldak, Turkey; 3 Faculty of Science, Ankara, Turkey. Abstract Olive ‘Gemlik’ has been planted in different ecological zones in Turkey; growers have begun to focus on some problems of yield and quality, and they would like to use grafting (top-working) with local cultivars. Although there is no problem of grafting compatibility between cultivars according to the literature, it has proved very difficult to give advice to growers for top-working because problems have been encountered when grafted local cultivars onto ‘Gemlik’ rootstock. The plant material used in this study consisted of cultivars ‘Ayvalik’, ‘Domat’, ‘Gemlik’, ‘Memecik’, ‘Nizip Yağlik’ and ‘Sari Ulak’. These cultivars were grafted (T-budding) onto 1-year-old ‘Gemlik’ rootstock propagated by cuttings. According to the results of phenolic compound variance analysis, high levels of 4-hydroxyphenylacetic acid, vanillic acid and ferulic acid content were determined in ‘Ayvalik’, ‘Nizip Yağlik’ and ‘Domat’. As a result, we showed that, during recovery of the graft zone, development of new cambium tissues, vascular connections and sclerenchyma tissues occurred imperfectly in ‘Ayvalik’, ‘Domat’ and ‘Nizip Yağlik’ when grafted onto ‘Gemlik’ rootstocks. For this reason, the graft zones of these combinations were weak. Keywords: olive, T-budding, cambial differentiation, ‘Gemlik’ INTRODUCTION Grafting is one of the most common plant propagation methods in fruit trees besides growth control of the scion. This method is of considerable importance in the adaptation of important olive cultivars in different areas. The successful formation of a graft depends on various complex biochemical and structural procedures, including callus formation, establishment of new vascular tissue, and formation of a functional vascular system across the graft interface (Feucht, 1988; Hartmann et al., 1990; Errea et al., 2001; Pina and Errea, 2005). Many researchers have used different methods for determination of graft incompatibility, such as phenol analyses (Mng’omba et al., 2008; Usenik et al., 2006), histological studies (Mng’omba et al., 2007; Errea et al., 1994b), isozyme analyses (Fernandez-Garcia et al., 2004; Gulen et al., 2005) and carbohydrate analyses (Ciobotari et al., 2009). Errea et al. (1994a) showed accumulation of two phenolics in incompatible combinations of apricot. An effect of phenolic compounds in incompatible combinations was reported by Treutter and Feucht (1986) in cherry trees. Prunin and p-coumaryl glucoside were found in the phloem of less-compatible combinations of Prunus avium on Prunus cerasus. Mng’omba et al. (2008) showed that less-compatible combinations achieved high phenolic concentrations compared with compatible combinations. High peaks of r-coumaric acid were obtained above the union. They concluded that phenolics, especially r-coumaric acids and flavonoids, caused poor callus formation at the union, and hence were implicated a E-mail: ozkaya@agri.ankara.edu.tr