International Journal of Plant Research 2016, 6(3): 64-87 DOI: 10.5923/j.plant.20160603.03 Effect of Gibberellic Acid, Paclobutrazol and Zinc on Growth, Physiological Attributes and the Antioxidant Defense System of Soybean (Glycine max) under Salinity Stress Mahmoud R. Sofy Botany and Microbiology Department, Faculty of Science, Al-Azhar University, 11884 Nasr City, Cairo, Egypt Abstract Soil salinity is one of the major abiotic stresses which caused significant reduction in the growth parameters, photosynthetic pigments and yield components of soybean plants. The present study aims to improvement of soybean production under saline conditions and also tries to elucidate the possible mechanisms of plant tolerance by using three different treatments (Gibberellic acid, Paclobutrazol and Zinc sulphate). The magnitude of reduction, increased by increasing salinity level. Application of the above mentioned treatments, in the absence and presence of NaCl, has greater changes in most of the assayed parameters where the adverse effects of salinity as regards the growth characters, photosynthetic pigments and yield components as well as soluble carbohydrates, soluble protein and oil contents in the yielded seeds were significantly mitigated by treatment with either GA 3 , PBZ or Zn. Salinity caused significant decreases in the activities of endogenous gibberellic acid (GA 3 ) and indole acetic acid (IAA) while activities of both Jasmonic acid (JA) and Abscisic acid (ABA) were increased. Significant increases were observed in the activities of superoxide dismutase (SOD), peroxidase (POX), catalase (CAT) and glutathione reductase (GR) in shoots of salt stressed plants. Application of GA 3 , PBZ or Zn caused great variations in the activities of endogenous phytohormones and antioxidant enzymes. Treatment with either GA 3 , PBZ or Zn caused significant reduction in both lipid peroxidation and proline that observed in shoots and root of salinized soybean plants. The electrophoregram of protein pattern of the yielded seeds of the soybean in response to their pre-emergence treatment with different concentrations of NaCl (50 mM & 100 mM) with or without GA 3 , PBZ or Zn appeared variations in the number of electrophoretic protein bands. A total of 23 bands was detected with different molecular weights ranging from 129 kDa to 9 KDa. Two newly formed protein bands have appeared, 1 of them (16 KDa) at 50 mM with Zn or PBZ and the other band (19 KDa) has appeared at 50 mM NaCl with PBZ. Generally, it could be concluded that application of either GA 3 , PBZ or Zn have (to more extent) a beneficial regulatory role in plants grown under salt stress conditions. Keywords Gibberellic acid, Paclobutrazol, Zinc, Antioxidant defense system, Soybean (Glycine max), Salinity stress 1. Introduction Worldwide, 20% of total cultivated and 33% of irrigated agricultural lands are exacerbated by high salinity. Phenomena like low precipitation, high surface evaporation, irrigation with saline water, weathering of native rocks, and poor agricultural practices have increased the rate of soil salinization to 10% per annum. It has been predicted that more than 50% of the arable land would be salinized by the year 2050 [1]. High salt levels generate a two-component stress on plants: * Corresponding author: mahmoud_sofy@yahoo.com (Mahmoud R. Sofy) Published online at http://journal.sapub.org/plant Copyright © 2016 Scientific & Academic Publishing. All Rights Reserved an osmotic stress caused by reducing water availability in soil and an ionic stress due to imbalance of solutes in the cytosol [2]. Recent research has identified various adaptive responses to salinity stress at molecular, cellular, metabolic, and physiological levels, although mechanisms underlying salinity tolerance are far from being completely understood. This paper provides a comprehensive review of major research advances on biochemical, physiological, and molecular mechanisms regulating plant adaptation and tolerance to salinity stress [3]. Soybean (Glycine max L.) a legume species native to East Asia, is now widely grown as the primary oilseed crop in the world including in many regions in the world. It is noteworthy that China is currently the largest importing country for soybean despite being one of its origins. To meet the increasing demand for food, oil and protein resources,