183 Increasing Drought Tolerance of Tomato Plants by Grafting H. Altunlu Mugla University Ortaca Vocational School Ortaca-Mugla Turkey A. Gul Ege University Faculty of Agriculture Department of Horticulture Izmir Turkey Keywords: drought stress, weighted ranking method Abstract The experiment was conducted in 3 steps in a growth chamber by aerated nutrient solution culture. Drought stress was fixed as Ψ s =-1.0 MPa; and reached gradually with PEG addition. In the first step 10 commercially available rootstock genotypes (‘Beaufort’, ‘Maxifort’, ‘Unifort’, ‘Yedi’, ‘Kemerit’, ‘King Kong’, ‘Spirit’, ‘Resistar’, ‘500292’ and ‘Toro’) and in the second step 12 indeterminate tomato cultivars from different types in respect to their fruit weights (Cherry (10-25 g): ‘Sweet 100’, ‘AG1015’, and ‘M25’; Cocktail (25-65 g): ‘AG1051’, ‘Elettro’, and ‘M28’; Mid-jumbo (100-140 g) ‘Filinta’, ‘Petrus’, and ‘Sırma’; Jumbo (over 180 g) ‘Alyans’, ‘Borneo’, and ‘Ceylin’) were tested. Selection was made by weighted ranking method. Four rootstock genotypes were selected as ‘Beaufort’, ‘Maxifort’, ‘Yedi’ and ‘Resistar’ of which resistance against drought stress decreased in order. Among the tested cultivars; ‘M28’, ‘Petrus’ and ‘Alyans’ were selected according to response to drought stress that ‘M28’ and ‘Alyans’ were determined as resistant and sensitive, respectively. On the other hand ‘Petrus’ gave intermediate values in this respect. In the third step, the selected tomato cultivars were grafted on the selected rootstocks, and themselves (self grafted). It was determined that grafting increased the drought tolerance of tomatoes depending on the genotype of the rootstock. INTRODUCTION Plants are exposed to numerous stress factors during their life. Out of the total arable farming areas on the world 90% are under stress; the main stress factors are drought, salt and cold which affect 26, 20 and 15% of total area, respectively. The rest of the stress factors get a 29% share, whereas only 10% of the farming area is not exposed to any stress (Blum, 1986). Tomato (Lycopersicon esculentum Mill.) is one of the most important horticultural crops in the world; its production is very concentrated in semi-arid regions and limited by abiotic and biotic stress. Vegetable production with grafted seedlings is now rapidly spreading over the world since grafting vegetables onto proper rootstocks offers different advantages such as tolerance to soil-borne diseases, plant vigor promotion, yield increase, and tolerance to abiotic stresses (Lee et al., 2010). Tomato belongs together with watermelon to the major vegetable crops that make use of grafting. The early use of this technique was associated with soil-borne diseases. However, grafting has also gained importance in ameliorating crop losses caused by adverse environmental conditions (Khah, 2005; Lee et al., 2010). Drought stress, one of the most common environmental limitations affecting growth and productivity of plants, causes many metabolic, mechanic and oxidative changes in plants. Drought cause water deficit in plants, water deficit induces changes in some plant growth characteristics and physiological features such as decreased growth and cell water potential, and stomatal closure, which causes lower CO 2 availability, and reduced photosynthetic activity for the plant (Ranjbarfordoei et al., 2000; Zgallai et al., 2005). The search for features related to drought tolerance is an important step in the selection of tomato plants with improved performance under conditions of limited water Proc. V th Balkan Symp. on Vegetables and Potatoes Eds.: A. Balliu and N. Gruda Acta Hort. 960, ISHS 2012