plants Article Antioxidant Enzymatic Activities and Growth Response of Quinoa (Chenopodium quinoa Willd) to Exogenous Selenium Application Ahlam Khalofah 1,2 , Hussein Migdadi 3,4, * and Ehab El-Harty 3   Citation: Khalofah, A.; Migdadi, H.; El-Harty, E. Antioxidant Enzymatic Activities and Growth Response of Quinoa (Chenopodium quinoa Willd) to Exogenous Selenium Application. Plants 2021, 10, 719. https://doi.org/ 10.3390/plants10040719 Academic Editors: Brigitta Tóth and Makoena Joyce Moloi Received: 26 February 2021 Accepted: 4 April 2021 Published: 7 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Biology Department, Faculty of Science, King Khalid University, Abha 61413, Saudi Arabia; aalshayeed@kku.edu.sa 2 Research Center for Advanced Materials Science (RCAMS), King Khalid University, Abha 61413, Saudi Arabia 3 Department of Plant Production, King Saud University, College of Food and Agriculture Sciences, Riyadh 11461, Saudi Arabia; ehabelharty@gmail.com 4 National Agricultural Research Center, Baqa 19381, Jordan * Correspondence: Hmigdadi@ksu.edu.sa; Tel.: +96-65-3587-1345 Abstract: Selenium is a trace element essential to many organisms, including higher plants. At low concentrations, it enhances growth and development; however, it is toxic at high concentrations. The development of crops with proper levels of selenium will be worth for both nutrition and Se-based therapeutics. This study aimed to investigate the morphological, physiological, and biochemical responses of the quinoa plant to 0, 2.5, 5, 10, and 20 mg/L of Na 2 SeO 3 ·5H 2 O. Selenium at low concentrations (2.5 and 5 mg/L), quinoa plant showed a significant increase of growth parame- ters, relative water content, photosynthetic pigments, proline, total soluble sugars, and antioxidant enzymes activities as (superoxide dismutase (SOD), catalase (CAT), peroxidase (POD, ascorbate peroxidase (APX), and glutathione reductase (GR)), and contents of malondialdehyde (MDA) and H 2 O 2 were reduced. However, high concentrations (10 and 20) mg/L caused a decrease in plant growth parameters, relative water content, and photosynthetic pigments. In contrast, excess selenium increased the oxidative stress monitored by hydrogen peroxide and lipid peroxidation levels. The enzymatic antioxidant system responded to the selenium supply significantly increased. Osmolytes compounds, such as total sugars and proline, increased in selenium-treated plants. The increase in these osmolytes compounds may show a defense mechanism for the osmotic readjustment of quinoa plants to mitigate the toxicity caused by selenium. This study shows the morphological and physio- logical responses that must be considered for success in the sustainable cultivation of quinoa plants in environments containing excess selenium. Keywords: selenium; quinoa; antioxidant; pigments; proline; sugars 1. Introduction Selenium is an essential component of human and animal cells, but it is not regarded as an essential nutrient for higher plants. However, it enhances plant growth and productivity under environmental stresses [1]. It was dealt with a toxic element until 1957 [2]. Thus, selenium’s significant impact was identified in humans, animals, and to a slighter extent in plants as essential for plant growth [3]. Soils are considered the primary source of selenium for plants, and it exists in various forms; elemental selenium, selenates, selenides, and organic selenium compounds. Soil type, climate, organic matter, and rainfall play a significant role in varied selenium content [4]. Forest soils efficiently retain selenium and then incorporate it into low-molecular-weight fractions of humic substance [5]. The availability of selenium to plant is decreased by low pH, high concentrations of sulfur and phosphorus, and soil of the world’s driest regions [6]. Cultivated plants are an essential source of selenium for humans and livestock. Being chemically analogous to sulfur, selenium is absorbed by all plants by sulfate transporters Plants 2021, 10, 719. https://doi.org/10.3390/plants10040719 https://www.mdpi.com/journal/plants