309 Peroxidase, Guaiacol Peroxidase and Ascorbate Peroxidase Activity Accumulation in Leaves and Roots of Walnut Trees in Response to Drought Stress N. Lotfi and K. Vahdati a Department of Horticulture College of Abouraihan University of Tehran Pakdasht, Tehran P.C. 3391653755 Iran B. Kholdebarin Department of Biology College of Sciences University of Shiraz Shiraz Iran D. Hassani Department of Horticulture Seed and Plant Improvement Institute Karaj Iran R. Amiri Department of Agronomy and Crops Breeding College of Abouraihan University of Tehran Pakdasht, Tehran Iran Keywords: walnut, antioxidant enzymes, drought tolerance Abbreviations: POX- Peroxidase; ROS- reactive oxygen species; SOD- superoxide dismutase; POD- Guaiacol peroxidase; APX- Ascorbate peroxidase; CP- Control plant; SP- Stressed plant Abstract Peroxidase isozymes (POX, POD and APX) activity were determined in 1-year-old walnut plants grown in environmental conditions characterized by high temperatures and high photosynthetic photon flux density levels and subjected to withholding irrigation for a 20-day period. These mechanisms have been investigated over both irrigated and drought-stressed plants in order to investigate the plant response under stressed conditions and its ability to recover. Before and during the experimental period, leaf and root samples were collected and their POD was analyzed. Significant increases of POD especially APX isozyme activity were also observed during the progressive increment of drought stress in both leaf and root tissues of tolerant genotypes at the seventh day after the beginning of the drought period. The differences in drought tolerance may be closely related with efficient photo protective systems as well as the increased capacity of the anti oxidative system to scavenge reactive oxygen species. INTRODUCTION Drought stress often leads to the accumulation of reactive oxygen species (ROS). ROS can act as a second messenger involved in the stress signal transduction pathway, but excessive ROS production can cause oxidative stress to the photosynthetic apparatus and seriously impair the normal function of cells (Foyer et al., 1994; Smirnoff, 1998; Niyogi, 1999). In addition to proteolysis, ROS can damage lipids, terpenoids, carbohydrates and nucleic acids (Foyer and Noctor, 2005; Moller et al., 2007). To keep the levels of active oxygen species under control, plants have evolved a series of anti- oxidative systems which are composed of metabolites such as ascorbate, glutathione, tocopherol and enzymatic scavengers such as superoxide dismutase (SOD), peroxidase and catalase (Asada, 1999). There are many observations that plants growing in hostile environments exhibit increased antioxidant enzyme activities to combat the deleterious a kvahdati@ut.ac.ir Proc. VI th Intl. Walnut Symposium Ed.: D.L. McNeil Acta Hort. 861, ISHS 2010