**1. Introduction**

Buckwheat (*Fagopyrum esculentum* Moench, 2n = 8) is an annual or perennial grain crop used as food and forage [1]. In China, buckwheat is divided into three main cultivars: Tartary buckwheat (*F. tataricum*), sweet buckwheat (*F. esculentum*), and golden buckwheat (*F. cymosum*), in addition to wild species [2]. Tartary buckwheat is a dicotyledonous plant in the family Polygonaceae [3]. China is not only the largest producer of Tartary buckwheat, but also the global center of buckwheat diversity. Tartary buckwheat is suitable for growing in high-altitude southwestern regions of China, such as Yunnan, Guizhou, and Sichuan Provinces, which have a cold climate and short frost-free period [4]. The southwest region of China is internationally recognized as the center of origin for buckwheat [5]. China ranks among the highest in the world in terms of planting area and output, with a current output second only to Russia. Tartary buckwheat has been promoted, grown, and eaten in many countries in the world [6]. Tartary buckwheat is rich in flavonoids and other biologically active substances, including rutin, quercetin, kaempferol, morin, and other natural compounds, and the rutin is the main essential ingredient in Tartary buckwheat. The content of rutin in the shoots is significantly higher than that in the roots [7]. Tartary buckwheat also contains a variety of nutrients, such as protein, starch, and vitamins, which can relieve cardiovascular sclerosis, diabetes, and other diseases [4,8]. Therefore, with

**Citation:** Yao, X.; Guo, H.; Zhou, M.; Ruan, J.; Peng, Y.; Ma, C.; Wu, W.; Gao, A.; Weng, W.; Cheng, J. Physiological and Biochemical Regulation Mechanism of Exogenous Hydrogen Peroxide in Alleviating NaCl Stress Toxicity in Tartary Buckwheat (*Fagopyrum tataricum* (L.) Gaertn). *Int. J. Mol. Sci.* **2022**, *23*, 10698. https://doi.org/10.3390/ ijms231810698

Academic Editors: Zhiyong Li and Jian Zhang

Received: 23 August 2022 Accepted: 8 September 2022 Published: 14 September 2022

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the rising demand for Tartary buckwheat products, its development and utilization have gained increasing attention.

In natural biological populations, interactions between genotype and environment affect the phenotype of plants. In plants with the same genotype, different environments can lead to various phenotypic differences [9,10]. With increasingly serious global warming and rising land evaporation, the problem of soil salinization has become prominent in countries at middle and low latitudes [11]. Salt stress refers to the adverse effect of excess soil salt on plants [12], and is a major abiotic stress that seriously affects the growth and development of plants, inhibits their physiological and biochemical metabolic processes, even causing plant death, greatly reduces agricultural yields, and restricts agricultural development potential [13,14]. When plants are subjected to salt stress, with increasing salt concentration, the growth trend of plants decreases [15], the content of reactive oxygen species (ROS) increases [16], the activity of antioxidant enzymes reduces [17], photosynthesis weakens [18], and the homeostasis of sodium (Na<sup>+</sup> ) and potassium (K<sup>+</sup> ) ions is unbalanced [19]. Therefore, the improvement, development, and utilization of soil salinization is an effective way to improve the growth environment of plants, and improving plant growth and yield under salt stress has become a hot research topic globally.

Reactive oxygen species (ROS) are biochemical substances that play a vital role in seed dormancy and germination. In particular, the intracellular homeostasis of hydrogen peroxide (H2O2), superoxide anion (O<sup>2</sup> −), and hydroxyl radical (OH−) is involved in signaling cascades, and this determines the growth and development process and stress response [20]. H2O<sup>2</sup> is an important regulatory component of plant signal transduction. It is not only a free radical produced by oxidative stress products, but also helps to maintain plant cell homeostasis [21]. Concurrently, H2O<sup>2</sup> can regulate the expression of various genes in physiological metabolism, including the genes encoding antioxidant enzymes, regulating biotic and abiotic stress response proteins [22,23]. H2O<sup>2</sup> has a concentrationdependent effect on physiological and biochemical processes. That is, high concentrations of H2O<sup>2</sup> produce an oxidative stress reaction in plants, leading to cell damage [24]; whereas at low concentrations, H2O<sup>2</sup> acts as a signal molecule participating in tolerance to various abiotic [25] and biological stresses [26], and also plays a regulatory role. A number of studies have reported that H2O<sup>2</sup> is involved in the regulation of physiological activities such as seed germination [27] and photosynthesis [28].

As a multifunctional signaling molecule, H2O<sup>2</sup> plays an important role in a series of physiological and biochemical processes. However, the involvement of H2O<sup>2</sup> in the antistress metabolism of Tartary buckwheat, and particularly the mechanism of salt tolerance, is rarely studied. In previous research by the research group, it was found that 1–5 mmol·L −1 H2O<sup>2</sup> (without NaCl treatment) could significantly increase the root growth of Tartary buckwheat at the germination stage [29], and at the germination stage 5–10 mmol·L −1 , H2O<sup>2</sup> can significantly promote the germination of Tartary buckwheat under 50 mmol·L −1 NaCl treatment [1]. Therefore, in this study, different concentrations of exogenous H2O<sup>2</sup> were applied to the leaves of Tartary buckwheat to explore the growth, photosynthesis, antioxidant enzyme activity, membrane lipid peroxidation, and expression of related genes of Tartary buckwheat under NaCl stress. The physiological metabolic mechanism by which H2O<sup>2</sup> regulates the salt tolerance of Tartary buckwheat was evaluated through correlation and principal component analyses to provide a scientific theoretical basis for exogenous H2O<sup>2</sup> treatment to improve the salt tolerance of Tartary buckwheat.

#### **2. Results**
