http://aev.bcnf.ir Page 1 Biochemical and physiological evaluation of At.TC rapeseed ( Brassica napus L.) under drought stress Atefeh Majidi 1,* - Alireza Abbasi 2 - Manijeh Sabokdast 3 1,2,3 . Agronomy and Plant Breeding Dept., University College of Agriculture and Natural Resources, University of Tehran, Iran. ABSTRACT Drought stress is one of the major problems in crop production like rapeseed for two reasons. The first reason is that it reduces the plant growth rate through the alternation in physiological, biochemical and molecular processes. The second reason is that it disrupts the balance between the production of reactive oxygen species and the plant's antioxidant defense activities which leads to oxidative stress. One of the plant defense mechanisms is the use of non-enzymatic antioxidants such as vitamin E. This study aims to investigate several biochemical and physiological parameters in some rapeseed genotypes and their transgenic lines. Nima, Hyola4815, RGS003, Dalgan and Zafar genotypes besides transgenic rapeseed plants including Hyola4815 (Line5 and Line6) and RGS003 (Line3), are studied at four levels of 30%, 50%, 70%, and 90% field capacity (FC) using a factorial experiment in the frame of Completely Randomized Design (CRD). It is observed that by decreasing FC, the relative water content (RWC) reduces. On the contrary, malondialdehyde (MDA), polyphenol oxidase (PPO) and catalase (CAT) increase. Keywords: Drought stress, At.TC gene, Rapeseed, Transgenic plants 1. INTRODUCTION Drought stress is one of the most destructive stresses among environmental stresses. This stress inhibits crop production due to its negative impacts on physiological changes and biochemical processes in plants [1]. Defense mechanisms against stresses are often composed of enzymatic and non-enzymatic antioxidants. Vitamin E (tocopherol) is a non-enzymatic antioxidant. Tocopherol protects the photosynthetic apparatus from oxidative damages. In addition, it preserves unsaturated fatty acids from lipid peroxidation in chloroplast membranes [2]. Rapeseed with the scientific name of Brassica napus L. is the second important source for edible oil production in the world after soybean [3]. Water shortage can have detrimental effects on rapeseed yield which depends on the genotype, growth stage and adaptation of the plant to drought conditions. For instance, before the reproductive period, water stress affects the green growth of the plant. While, after this period, it has a direct effect on flower pollination, seed formation and yield [4]. Relative water content (RWC) is a useful index for measuring water status in plant tissues [5]. Drought resistance depends on plant ability to maintain high RWC in the leaves [6]. Lipid peroxidation represents oxidative stress in plants that leads to the destruction of biological membranes. Consequently, malondialdehyde (MDA) is produced, which is used as a biomarker to detect the degree of oxidative damages to lipids [7, 8]. Polyphenol oxidases (PPOs) are enzymes that have almost all peroxidase activities, such as protection against disease, stress, and pests [9]. For many plant species, it has been reported that the activity of the enzyme catalase (CAT) increases due to environmental stresses. Thus, this makes the plant resistant to adverse environmental conditions [10].