**1. Introduction**

In the desert steppe, water is the most important factor limiting plant growth. Different plants adopt water use strategies to adapt to changes of water conditions [1]. A plant's drought adaptability is closely related to their water use strategies [2,3]. Zhang et al. believes that drought resistance and the ability to use water effectively are essential mechanisms for revealing a plant's drought resistance under long-term water stress conditions [4]. Therefore, the adaptation of plants to arid habitats requires the evolution of their water use strategies.

A previous study reported that plants could adjust the distribution of biomass among different organs to adapt to water stress, showing that the biomass of various organs of plants decreases due to water stress, while the root to shoot ratio (R/S) increases [5]. The biomass allocation strategy is also directly influenced by the ability of plants to adapt to their environment. Plants allocate resources to the most needed organs after environmental changes to effectively obtain scarce resources [6,7]. Plants obtain water by increasing root biomass when water is deficient. When soil moisture is sufficient, plants promote photosynthetic capacity by increasing the distribution ratio of above-ground biomass. However, above-ground biomass and underground biomass accumulation are not synchronized [8]. Therefore, plant organ biomass allocation results from a balance between reproduction and survival and a trade-off strategy for plants to adapt to their environment.

**Citation:** Song, K.; Hu, H.; Xie, Y.; Fu, L. The Effect of Soil Water Deficiency on Water Use Strategies and Response Mechanisms of *Glycyrrhiza uralensis* Fisch. *Plants* **2022**, *11*, 1464. https://doi.org/10.3390/ plants11111464

Academic Editor: James A. Bunce

Received: 19 April 2022 Accepted: 25 May 2022 Published: 30 May 2022

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**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/).

Precipitation can change Ci/Ca (intercellular CO<sup>2</sup> concentration/atmospheric CO<sup>2</sup> concentration) by changing the stomatal conductance of leaves, which then affects the δ <sup>13</sup>C value of plants [9]. The δ <sup>13</sup>C values of C<sup>3</sup> plants in arid and desert areas ranged from <sup>−</sup>20‰ to −35‰ with decreasing precipitation. In extreme arid areas, the δ <sup>13</sup>C values ranged from −20‰ to −26‰, and precipitation has a significant negative correlation with the δ <sup>13</sup>C values [10]. It has been suggested that C<sup>3</sup> plant tissues under water stress generally have higher δ <sup>13</sup>C values, while those under non-water stress have lower δ <sup>13</sup>C values [11], but most of the previous results were from field experiments. Liu Ying et al. studied the effects of different drought stress conditions on δ <sup>13</sup>C values of the C<sup>4</sup> plant Leymus chinensis and found that the δ <sup>13</sup>C values were significantly positively correlated with water use efficiency (WUE) [12]. It is feasible to determine water use efficiency of *L. chinensis* by the δ <sup>13</sup>C value. However, some earlier studies suggested that δ <sup>13</sup>C values have growth stages and organ specificity when used to study WUE and biomass in plants [13].

*Glycyrrhiza uralensis* is the dominant species in the natural restoration of the desert steppe. It is not only a high-quality pasture and a Chinese herbal medicine, but it also has a higher ability to survive and compete, making it important in the study of grassland restoration. Hu found that *Glycyrrhiza uralensis* has the highest δ <sup>13</sup>C value and photosynthetic rate among four dominant species of the desert steppe, including *Glycyrrhiza uralensis*, *Lespedeza potaninii*, *Stipa breviflora*, and *Agropyron mongolicum*, under various precipitation conditions [14,15]. This research focuses on *Glycyrrhiza uralensis*. It uses a water control experiment to investigate the responses of biomass allocation, water use efficiency, and physiological and morphological characteristics to water stress to uncover the water use strategies and drought resistance mechanisms in *Glycyrrhiza uralensis*. It gives a scientific basis for choosing species to restore the grassland to its natural state.

#### **2. Results**

#### *2.1. Biomass Allocation and the Root–Shoot Ratio of Glycyrrhiza uralensis under Water Stress*

Under the same level of water stress, the total biomass of *Glycyrrhiza uralensis* increased first, then decreased; the highest biomass was obtained after 45 d of treatment. The root and leaf biomass accumulated quickly within 30–45 d, with root biomass at 45 d and 60 d significantly higher than at 15 and 30 d (*p* < 0.01). The root–shoot ratio (R/S) of *Glycyrrhiza uralensis* gradually increased over time, the R/S values under different water stress all showed 60 d > 45 d > 30 d > 15 d.

Under the same time, the total biomass of *Glycyrrhiza uralensis* decreased gradually as the degree of water stress increased, but the stem biomass remained unchanged. The R/S increased significantly (*p* < 0.05). The total biomass of the T4 treatment was the lowest, and the R/S of the T4 treatment was the highest at 15, 30, 45 and 60 d. These results indicated that drought stress resulted in more allocation of biomass to the roots and less allocation to the stems and leaves (Figure 1).
