ISSN 1021-4437, Russian Journal of Plant Physiology, 2015, Vol. 62, No. 4, pp. 480–486. © Pleiades Publishing, Ltd., 2015. 480 1 INTRODUCTION Drought is considered as one of the most important environmental stresses limiting plant growth and crop productivity. Water stress can be defined as the absence of adequate soil moisture necessary for a plant to grow normally and complete its life cycle. Common plant symptoms to water deficit are stunt growth, limit CO 2 diffusion to chloroplasts because of stomata closure, reduce photosynthesis rate and accelerate leaf senes- cence [1]. One of the biochemical changes occurring in plants when subjected to water stress is the accumulation of ROS (reactive oxygen species) [2]. Chloroplasts, mitochondria, and peroxisomes in plant cells are important intracellular generators of ROS, which can be responsible for the occurrence of oxidative damages under abiotic stress [2]. Increasing evidences suggest that water stress induces oxidative stress in various 1 The article is published in the original. plants [1], including both free radical (superoxide rad- icals, hydroxyl radical, perhydroxy radical and alkoxy radicals) and non-radical (molecular) forms (hydro- gen peroxide and singlet oxygen), which can destroy proteins, lipids, carbohydrates and nucleic acids [3]. Plants can respond to osmotic stress at morphological, anatomical and cellular levels with modifications that allow the plant to avoid the stress or to increase its tol- erance [4]. The induction of ROS-scavenging enzymes, such as superoxide dismutase (SOD), per- oxidase (POD), ascorbate peroxidase (APX), catalase (CAT) and other compounds such as carotenoids, ascorbic acid, thiols, soluble protein and glutathione are the most common mechanism for detoxifying ROS synthesized during stress [5]. Plants under water stress also produce large amounts of amino acids, such as proline for improving drought resistance. Proline prevents oxidation inside the cells under drought stress [6]. Damage to fatty acids of mem- brane could produce small hydrocarbon fragments including MDA. MDA is the final product of plant cell membrane lipid peroxidation and is an important sign of membrane system injury [7]. Biochemical and Physiological Responses of Brassica napus Plants to Humic Acid Under Water Stress 1 R. Lotfi a , P. Gharavi-Kouchebagh b , and H. Khoshvaghti a a Department of Agriculture, Payame Noor University (PNU), Iran b Department of Plant Eco-physiology, Faculty of Agriculture, University of Tabriz, Tabriz, Iran e-mail: r.lotfi1988@gmail.com Received October 22, 2014 Abstract—This study examines the effects of humic acid (HA, control, 3 and 6 mg/L) on some biochemical and physiological parameters of rapeseed (Brassica napus L.) plants under different water supply conditions (60, 100, and 140 mm evaporation from class A pan). Water stress decreased chlorophyll a (Chl a) and total chlorophyll (Chl T ) content in plants but proline content partly increased with increasing water stress severity. Plants treated by HA had more Chl a and Chl T content under both well and limited water conditions. Appli- cation of HA improved the PSII and peroxidase activity of rapeseed plants under all irrigation treatments. Ascorbate peroxidase activity under severe water stress condition increased by 70 and 95%, compared with that under moderate and well watering conditions, respectively. Catalase activity was 51 and 69% less under well watering than that of moderate and severe water stress conditions, respectively. The highest activity of ascorbate peroxidase was recorded in plants treated by 6 mg/L HA. HA-treated plants had 42, 8.5, and 15% more soluble protein content under well watering, moderate and severe water stress conditions, respectively, compared with control plants. Malondialdehyde increased with increasing the severity of water stress, in con- trast, application of HA significantly reduced the amount of this trait under water stress conditions. It was shown that application of HA increased the activity of antioxidant enzymes, improved PSII activity and con- sequently decreased lipid peroxidation in rapeseed plants. Keywords: Brassica napus, antioxidant enzymes, chlorophyll content, humic acid, lipid peroxidation, pro- line, PSII activity, water stress DOI: 10.1134/S1021443715040123 Abbreviations: APX—ascorbate peroxidase; CAT—catalase; Chl a—chlorophyll a; Chl T —total chlorophyll; HA—humic acid; POD—peroxidase; MWS—moderate water stress; SWS—severe water stress; WWC—well watering conditions. RESEARCH PAPERS