Citation: Didi, D.A.; Su, S.; Sam, F.E.; Tiika, R.J.; Zhang, X. Effect of Plant Growth Regulators on Osmotic Regulatory Substances and Antioxidant Enzyme Activity of Nitraria tangutorum. Plants 2022, 11, 2559. https://doi.org/10.3390/ plants11192559 Academic Editors: Massimiliano Tattini, Antonella Gori and Luana Beatriz dos Santos Nascimento Received: 5 September 2022 Accepted: 24 September 2022 Published: 28 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). plants Article Effect of Plant Growth Regulators on Osmotic Regulatory Substances and Antioxidant Enzyme Activity of Nitraria tangutorum Dom Alizet Didi 1 , Shiping Su 1, *, Faisal Eudes Sam 2 , Richard John Tiika 1 and Xu Zhang 1 1 College of Forestry, Gansu Agricultural University, Lanzhou 730070, China 2 College of Enology, Northwest A&F University, Yangling 712100, China * Correspondence: susp008@163.com Abstract: Plant growth regulators (PGRs) are natural hormones and synthetic hormone analogues. At low concentrations, PGRs have the ability to influence cell division, cell expansion, and cell structure and function, in addition to mediating environmental stress. In this study, experiments were conducted to determine how exogenous PGRs indole acetic acid (IAA), abscisic acid (ABA), and gibberellic acid (GA) influenced osmotic regulatory substances and activity of antioxidant enzymes in Nitraria tangutorum. Using a completely randomized design, IAA, ABA, and GA3 were applied as foliar spray at concentrations of 50 mg/L, 100 mg/L, 150 mg/L, and 200 mg/L to N. tangutorum shrubs. Some selected shrubs did not receive any treatment and served as the control (Ck). The results showed that the foliar spray of IAA, ABA, and GA3 significantly increased the content of osmotic regulatory substances (soluble sugar, soluble protein, and proline) and antioxidant enzymes (SOD and POD) at most concentrations. In addition, the malondialdehyde (MDA) content significantly reduced after treatment, but after regrowth of coppiced shrubs, lipid peroxidation increased and was still lower than Ck. Our study provides evidence that 100 mg/L 150 mg/L, and 200 mg/L concentrations of IAA, ABA, and GA3 treatments are effective for enhancing osmotic regulatory substances and the activity of antioxidant enzymes in N. tangutorum, which offers an effective strategy not only for increasing tolerance to abiotic and biotic stresses, but also improving the adaptability of N. tangutorum shrubs to the environment. Keywords: plant growth regulator; abscisic acid; indole acetic acid; gibberellic acid; osmotic regulatory substances; antioxidant enzymes; Nitraria tangutorum Bobr. 1. Introduction Plant growth regulators (PGRs) were first discovered in plants in the 20th century. PGRs are a variety of diverse molecules known as plant hormones that act as chemical messengers to control plant growth and development [1,2]. PGRs exist in both natural (extracted) and synthetic (manufactured) forms. Although a synthetically produced plant growth regulator is structurally identical to a plant regulator or hormone, it is not consid- ered to be one [2]. The production and growth of roots, shoots, buds, flowers, and fruits can be controlled by the use of synthetic PGRs [3]. PGRs are compounds that promote plant growth and increase their stress tolerance under various stress conditions [4,5]. PGRs affect a variety of plant traits, including plant height, number of leaves, leaf area index, dry matter, chlorophyll concentration, and photosynthetic parameters, among others [6]. PGRs can also improve plant tolerance to various abiotic stimuli, increase antioxidant capacity, and accelerate plant growth [7]. PGRs have been synthesized and can be exogenously applied to plants to control their growth and development. PGRs come in a variety of forms, including auxins such as indoleacetic acid (IAA), abscisic acid (ABA), and gibberellic acid (GA). In plants, IAA is synthesized by tryptophan or tryptophan-independent mecha- nisms [8]. IAA has been found to promote plant growth [9]. It is a tiny molecule with a Plants 2022, 11, 2559. https://doi.org/10.3390/plants11192559 https://www.mdpi.com/journal/plants