289 Drought-Induced Accumulation of Sugars and Proline in Radicle and Plumule of Tolerant Walnut Varieties during Germination Phase N. Lotfi and K. Vahdati a Department of Horticulture College of Abouraihan University of Tehran Pakdasht, Tehran, P.C. 3391653755 Iran R. Amiri Department of Agronomy and Crops Breeding College of Abouraihan University of Tehran Pakdasht, Tehran Iran B. Kholdebarin Department of Biology College of Sciences University of Shiraz Shiraz Iran Keywords: walnut, free proline, osmoregulation, drought tolerance Abstract Drought stress is a major factor in reducing growth, development and produc- tion of walnuts. Stress was applied with polyethylene glycol (PEG) 6000 giving osmotic water potentials of: zero (control) and six drought stress levels of -0.10, -0.50, -0.75, -1.00, -1.50 and -2.00 MPa. Solutes accumulation of walnut (Juglans regia L.) varieties and selections (‘Z 30 ’, ‘Z 53 ’, ‘Z 67 ’, ‘Z 60 ’, ‘Z 63 ’, ‘K 72 ’, ‘B 21 ’, ‘V 30 ’, ‘Panegin 20 ’, ‘Hartley’, ‘Pedro’, ‘Vina’, ‘Lara’, ‘Serr’, ‘Rounde de Montignac’, ‘Chandler’) were determined after drought stress. A higher amount of soluble sugars and a lower amount of starch were found under stress. Soluble sugars concentration increased (from 29.62 to 227.63 mg/L DW) in roots and shoots of tolerant varieties when the studied varieties were subjected to drought stress, but starch content significantly (P≤0.05) decreased (by 16 to 84%) in all varieties. This suggests that sugars play an important role in Osmotic Adjustment (OA) in walnut in germination phases. The free proline level also increased (from 10.23 to 610.4 mg/ml FW) in response to drought stress and the increase in ‘Chandler’ was higher than other varieties. Proline may play a role in minimizing the damage caused by dehydration. Increases of proline content in shoots was higher than in roots, but increases of soluble sugar content and decreases of starch content in roots was higher than shoots. INTRODUCTION Plants have many adaptive strategies in response to abiotic environmental stresses such as dehydration and excessive osmotic pressure. These adaptive mechanisms include changes in physiological and biochemical processes. Among them, the accumulation of compatible solutes as a metabolic response has drawn much attention. The maintenance of plant turgor potential under water deficit is essential for continued growth and can be achieved by osmotic adjustment mechanisms resulting from the accumulation of com- patible solutes (such as proline, glycine betaine, organic acids or sugars such as mannitol and sucrose) in the cytoplasm (Ingram and Bartels, 1996; Hare et al., 1998). Proline acts as a compatible osmolyte since it can accumulate at high concentrations without damaging cellular function and structure. Proline can also serve as a nitrogen and carbon source in the cell (Chiang and Dandekar, 1995; Verbruggen et al., 1996). Moreover, proline has a protective action which prevents membrane damage and protein denatura- tion during severe drought stress (Ain-Lhout et al., 2001), and replenishes the NADP supply in altered redox potentials (Hare and Cress, 1997; Hare et al., 1999). It has also a kvahdati@ut.ac.ir Proc. VI th Intl. Walnut Symposium Ed.: D.L. McNeil Acta Hort. 861, ISHS 2010