INTRODUCTION Onion plants (Allium cepa L.) are one the utmost substantial harvests grown-up in Egypt and might be considered the principal cultured crops in the world owing to their extended storing time and convenience. Production of good quality onion is an important goal by the growers for local market and exports. Many biological activities of Alliums were valued to possess like phenolic and potent sulfur compounds, which provoke unlimited interest (Benkeblia, 2004, 2005; Packia et al., 2015; Aslam et al., 2017). From inception, onion has been used as a food as well as medicine (Martinez et al., 2007). Traditionally, onions were used to prevent infections as they contain many valuable compounds exerted anti- inflammatory, antihistamine effects, anti-parasitic, antifungal, antibacterial (including H. pylori) (Wani and Nisa, 2011; Kirilov et al., 2014; Lekshmi et al., 2015). According to FAO, tomato followed by onion in terms of yearly world production, improved varieties and crop production techniques project to raise economic yields of this crop. Application of bioregulators for onion cultivation is an easy and popular means for improving the yield, oil yield quality and quantity (Shafeek et al., 2015). Current horticulture pursues substitutes to increase the plant quality palatability like biotechnology, crop controlling systems and the biological regulators. Growth regulators are considered the fundamental elements in growth, flowering, fruit setting and seed production in vegetative plants as well as onion and are well examined. Lokhande and Gaikwad (2014) stated that the foliar applications of GA, SA, 6BA, CCC, methionine and cysteine on the two onion varieties significantly increased the chlorophyll content, slightly altered the chlorophyll a/b ratio, decreased carotenoid and total carbohydrate contents, while total sugars were slightly elevated. Numerous classes of plant growth regulators disturb physiological activities of onion plants. Ebrahim and Abu- Grab (1997) found that CCC application increased potassium (K), nitrogen (N) and phosphorus (P) concentrations in onion Pak. J. Agri. Sci., Vol. 56(4), 828-838; 2019 ISSN (Print) 0552-9034, ISSN (Online) 2076-0906 DOI: 10.21162/PAKJAS/19.8782 http://www.pakjas.com.pk THE IMPACT OF SOME BIOREGULATORS ON GROWTH, CHEMICAL CHARACTERS AND RADICAL SCAVENGING PROPERTIES OF ONION VOLATILE OIL Ahmed H. El-Ghorab 1,2 , Khaled F. El-Massry 1,2,* , Ibrahim Hotan Alsohaimi 1 , M. Hamza 3,4 , M.S. Shaheen 2 , M.A. Bekheta 5 , Anees Ahmed Khalil 6 , Muhammad Imran 6 and Mervat A. El-sherif 1 1 Chemistry Department, College of Science, Jouf University, Sakaka, Saudi Arabia; 2 Flavour and Aroma Department, National Research Center, Giza, Egypt; 3 Agronomy Department, Faculty of Agriculture, Cairo University, Egypt; 4 Biology Department, Faculty of Sciences, Jouf University, Sakaka, Saudi Arabia; 5 Botany Department, National Research Center, Giza, Egypt; 6 University Institute of Diet and Nutritional Sciences, Faculty of Allied Health Sciences, The University of Lahore, Pakistan. * Corresponding author’s e-mail: kfaroak@ju.edu.sa The influence of different concentrations of some bioregulators by foliar application {methyl jasmonate (MJ) and Prohexadione- Ca (Pro-Ca)} on growth, pigment content and volatile oil composition and radical scavenging activity of onion plant were accomplished during two consecutive seasons. The obtained results indicated that foliar application of MJ (at 150mg/l) or Pro-Ca at (30 ppm), significantly promoted onion growth and quality as height, branches number and leaves weights for new and dry onion leaves, crisp and dry loads of branches and yield (g/plant). On the other hand, MJ application led to noteworthy decrease in the plant height at all the used treatments. A significant increase in mineral ions content of the produced onion seeds by foliar application of MJ and Pro-Ca at all the used levels. All the usages of bioregulators produced positive modifications in the photosynthetic pigments as well as yield and oil content of onion plants. Based on the finding, it is recommended the use of MJ and Pro-Ca twice, at the rate of 100 and 20 ppm, respectively for higher onion yield /acre (AC). Moreover, GC/MS analysis shown that the chief identified constituents of volatile oil of onion treated with MJ and Pro-Ca were 4-Mercaptononan-2-ol, 3-Mercaptononyl acetate, Diallyl thiosulfinate, 4-Mercaptononan-2-one, Diallyl trisulphide, 2- Heptyl thiophene, Methyl propyl trisulphide, 1,3-Propanedithiol, 3,5-Diethyl-1,2,4 trithiolane and 3,5-Methyl ethyl- 1,2,4,trithiolane. MJ and Pro-Ca usages caused qualitative alterations in these components of onion volatile oil. In addition, the bioregulators used caused significant variation in the free radical scavenging effect measured by DPPH and B- carotene assays of onion volatile oil compared to TBHQ. Keywords: Bioregulators, methyl jasmonate, prohexadione-Ca, Allium cepa L., DPPH