*2.2. Growth Relationship between Above-Ground and Underground Biomass of Glycyrrhiza uralensis under Water Stress*

Under different water stress treatments, there was an extremely significant correlation between the below-ground biomass (BGB) and the above-ground biomass (AGB) of *Glycyrrhiza uralensis* (*p* < 0.01), and the allometric growth relationship was biased toward under-ground biomass accumulation (Figure 2). Each treatment had a correlation growth index α greater than one. T2 had the highest α index (α = 1.9), the CK had the lowest, and T3 had the best fitting effect (R<sup>2</sup> = 0.8592).

0.8 1.2 1.6 2.0 2.4 2.8 3.2

15d

0.8 1.2 1.6 2.0 2.4 2.8

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0.8 1.2 1.6 2.0 2.4 2.8 3.2

3.2 Leaf

30d Root

0.8 1.2 1.6 2.0 2.4 2.8 3.2

45d

Stem

60d

**Figure 1.** The biomass distribution in organs of *Glycyrrhiza uralensis* under different water stress treatments. Different lowercase letters indicate significant differences among treatments at 5% level. **Figure 1.** The biomass distribution in organs of *Glycyrrhiza uralensis* under different water stress treatments. Different lowercase letters indicate significant differences among treatments at 5% level. index α greater than one. T2 had the highest α index (α = 1.9), the CK had the lowest, and T3 had the best fitting effect (R<sup>2</sup> = 0.8592).

0.6 0.4 0.4 0.4 0.2 0.2 0.2 0.0 0.0 0.6 0.6 LogBGB LogBGB y = 1.90x + 0.46 y = 1.80x + 0.45 R² = 0.86 y = 1.78x + 0.62 R² = 0.62 -0.6 -0.4 -0.2 0.0 0.2 -0.6 -0.4 -0.2 0.0 0.2 0.4 -0.6 -0.4 -0.2 0.0 0.2 -0.6 -0.4 -0.2 0.0 0.2 0.4 -0.6 -0.4 -0.2 0.0 -0.6 -0.4 -0.2 0.0 0.2 0.4 -0.2 0.0 0.2 0.4 0.6 -0.2 0.0 0.2 0.4 0.6 LogAGB LogAGB LogBGB LogAGB y = 1.64x + 0.17 R² = 0.85 CK LogBGB LogBGB LogBGB LogBGB y = 1.78x + 0.28 R² = 0.78 T1 y = 1.90x + 0.46 R² = 0.70 T2 B y = 1.80x + 0.45 R² = 0.86 B y = 1.78x + 0.62 R² = 0.62 **Figure 2.** Growth relationship between above-ground and underground biomass of *Glycyrrhiza uralensis* in different water stress treatments. AGB and BGB represent the above-ground biomass and below-ground biomass. The biomass data are logarithmic transformed, its power function is converted to the form of log = logβ + αlogX, where α is the slope of linear regression, and logβ is the intercept of linear regression.

0.0


T3




0.0



T4

0.6


T1

B

LogAGB LogAGB



y = 1.80x + 0.45 R² = 0.86

T2

LogAGB LogAGB LogAGB


LogAGB

T3

y = 1.90x + 0.46 R² = 0.70


LogAGB

T3

0.0

T4

0.6



A



 LogBGB


> y = 1.78x + 0.28 R² = 0.78

> > LogBGB


CK

T4

LogBGB

0.0


y = 1.78x + 0.62 R² = 0.62

y = 1.64x + 0.17 R² = 0.85

0.6

LogAGB

LogBGB

A



B



T2

LogBGB


R² = 0.70


LogBGB



#### *2.3. Effects of Water Stress on Water Potential of Glycyrrhiza uralensis Leaves* The leaf water potential (WP) of *Glycyrrhiza uralensis* decreased to varying degrees

*2.3. Effects of Water Stress on Water Potential of Glycyrrhiza uralensis Leaves*

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intercept of linear regression.

The leaf water potential (WP) of *Glycyrrhiza uralensis* decreased to varying degrees compared to the CK under water stress (Figure 3). With the increase in the water stress degree, WP remained unchanged at the early stages (15 d), decreased first, increased at the middle stages (30 and 45 d), and gradually increased at the late stages (60 d). Except for 15 d, the CK and T1 treatments had a significantly higher WP than T2, T3, and T4 treatments (*p* < 0.05). compared to the CK under water stress (Figure 3). With the increase in the water stress degree, WP remained unchanged at the early stages (15 d), decreased first, increased at the middle stages (30 and 45 d), and gradually increased at the late stages (60 d). Except for 15 d, the CK and T1 treatments had a significantly higher WP than T2, T3, and T4 treatments (*p* < 0.05).

**Figure 2.** Growth relationship between above-ground and underground biomass of *Glycyrrhiza uralensis* in different water stress treatments. AGB and BGB represent the above-ground biomass and below-ground biomass. The biomass data are logarithmic transformed, its power function is converted to the form of log = logβ + αlogX, where α is the slope of linear regression, and logβ is the

**Figure 3.** Changes of water potential in leaves of *Glycyrrhiza uralensis* under different water stress treatments. Different lowercase letters indicate a significant difference among treatments at a 5% level. Different capital letters indicate a significant difference at a 5% level between different times. **Figure 3.** Changes of water potential in leaves of *Glycyrrhiza uralensis* under different water stress treatments. Different lowercase letters indicate a significant difference among treatments at a 5% level. Different capital letters indicate a significant difference at a 5% level between different times.

The CK and T1 treatments fluctuated over time. The WP of the T2 and T3 treatment decreased rapidly at 30 d and then increased rapidly, the lowest at 30 d and the highest at 15 d. The WP at 15 d, 45 d, and 60 d was significantly higher than that at 30d, and the WP at 15 d was significantly higher than that at 45 d and 60 d (*p* < 0.05). The WP of the T4 treatment decreased rapidly and increased slowly, with the lowest at 45 d and the highest at 15 d. The WP at 15 d was significantly higher than that at 30 d, 45 d, and 60 d, and the The CK and T1 treatments fluctuated over time. The WP of the T2 and T3 treatment decreased rapidly at 30 d and then increased rapidly, the lowest at 30 d and the highest at 15 d. The WP at 15 d, 45 d, and 60 d was significantly higher than that at 30d, and the WP at 15 d was significantly higher than that at 45 d and 60 d (*p* < 0.05). The WP of the T4 treatment decreased rapidly and increased slowly, with the lowest at 45 d and the highest at 15 d. The WP at 15 d was significantly higher than that at 30 d, 45 d, and 60 d, and the WP at 30 d and 65 d was significantly higher than that at 45 d (*p* < 0.05).

#### WP at 30 d and 65 d was significantly higher than that at 45 d (*p* < 0.05). *2.4. Effects of Water Stress on Relative Water Content and Proline Content of Glycyrrhiza uralensis Leaves*

*2.4. Effects of Water Stress on Relative Water Content and Proline Content of Glycyrrhiza uralensis Leaves* The relative water content (RWC) of *Glycyrrhiza uralensis* leaves was significantly affected by both the degree of water stress and time passage (Table 1). The RWC continued to decrease slowly as the degree of water stress increased (*p* < 0.05), while proline content The relative water content (RWC) of *Glycyrrhiza uralensis* leaves was significantly affected by both the degree of water stress and time passage (Table 1). The RWC continued to decrease slowly as the degree of water stress increased (*p* < 0.05), while proline content (PR) continued to increase significantly (*p* < 0.05). Compared with the CK, the RWC of T1, T2, T3, and T4 treatments decreased by 8.21%, 11.79%, 21.26%, 21.99%, and the PR of T1, T2, T3, and T4 treatments increased by 44.60%, 111.07%, 174.30%, and 246.91%, respectively.

(PR) continued to increase significantly (*p* < 0.05). Compared with the CK, the RWC of T1, T2, T3, and T4 treatments decreased by 8.21%, 11.79%, 21.26%, 21.99%, and the PR of T1, T2, T3, and T4 treatments increased by 44.60%, 111.07%, 174.30%, and 246.91%, respec-

tively.


**Table 1.** The effects of different water stress treatments on RWC and proline content of *Glycyrrhiza uralensis*. Note: Different lowercase letters indicate a significant difference among treatments or times at a 5% level. **Table 1.** The effects of different water stress treatments on RWC and proline content of *Glycyrrhiza uralensis*. Note: Different lowercase letters indicate a significant difference among treatments or

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The RWC at 30 and 45 d was significantly higher than that at 15 and 60 d (*p* < 0.05) as the stress time increased. The PR of *Glycyrrhiza uralensis* leaves increased at first, then decreased. At 30 d, the PR was significantly higher than at 15, 45, and 60 d; at 15 d, PR was significantly higher than at 45, and 60 d (*p* < 0.05). The RWC at 30 and 45 d was significantly higher than that at 15 and 60 d (*p* < 0.05) as the stress time increased. The PR of *Glycyrrhiza uralensis* leaves increased at first, then decreased. At 30 d, the PR was significantly higher than at 15, 45, and 60 d; at 15 d, PR was

60 d 71.36 ± 1.54b 110.14 ± 14.60c

#### *2.5. Effects of Water Stress on δ <sup>13</sup>C Values in Different Organs of Glycyrrhiza uralensis* significantly higher than at 45, and 60 d (*p* < 0.05).

In different organs of *Glycyrrhiza uralensis*, the δ <sup>13</sup>C values were as follows: δ <sup>13</sup>Croot > δ <sup>13</sup>Cstem > δ <sup>13</sup>Cleave, the δ <sup>13</sup>C values of different organs were significantly different (*p* < 0.05). *2.5. Effects of Water Stress on δ13C Values in Different Organs of Glycyrrhiza uralensis* In different organs of *Glycyrrhiza uralensis*, the δ13C values were as follows: δ13Croot > δ13Cstem > δ13Cleave, the δ13C values of different organs were significantly different (*p* < 0.05).

The δ <sup>13</sup>C values of all organs showed an upward trend as the degree of water stress increased. The δ <sup>13</sup>Croot of T2, T3, and T4 treatments was significantly higher than the CK and the T1 treatment. The δ <sup>13</sup>Cstem of the T4 treatment was significantly higher than the CK, T1, and T2 treatments, and the δ <sup>13</sup>Cleave was significantly higher than the CK, T1, T2, and T3 treatments (*p* < 0.05) (Figure 4). The δ13C values of all organs showed an upward trend as the degree of water stress increased. The δ13Croot of T2, T3, and T4 treatments was significantly higher than the CK and the T1 treatment. The δ13Cstem of the T4 treatment was significantly higher than the CK, T1, and T2 treatments, and the δ13Cleave was significantly higher than the CK, T1, T2, and T3 treatments (*p* < 0.05) (Figure 4).

**Figure 4.** The composition of δ13C value in organs of *Glycyrrhiza uralensis* under different water stress conditions. Different lowercase letters indicate a significant difference among treatments at the 5% level. Different capital letters indicate a significant difference at a 5% level between different organs. **Figure 4.** The composition of δ <sup>13</sup>C value in organs of *Glycyrrhiza uralensis* under different water stress conditions. Different lowercase letters indicate a significant difference among treatments at the 5% level. Different capital letters indicate a significant difference at a 5% level between different organs.

#### *2.6. Effects of Water Stress on the Gas Exchange Parameters of Glycyrrhiza uralensis 2.6. Effects of Water Stress on the Gas Exchange Parameters of Glycyrrhiza uralensis*

As demonstrated in Table 2, the Pn, Tr, Ci, and Gs of *Glycyrrhiza uralensis* leaves decreased with the increasing degree of water stress, the CK had the highest, and the CK and the T1 treatment were significantly higher than the T3 and T4 treatments (*p* < 0.05). The WUEi and Ls increased continuously at first, and then decreased significantly in the As demonstrated in Table 2, the Pn, Tr, Ci, and Gs of *Glycyrrhiza uralensis* leaves decreased with the increasing degree of water stress, the CK had the highest, and the CK and the T1 treatment were significantly higher than the T3 and T4 treatments (*p* < 0.05). The WUEi and Ls increased continuously at first, and then decreased significantly in the

T4 treatment, with the WUEi of T1, T2, and T3 treatments being significantly higher than

the CK and T4 treatment (*p* < 0.05).

T4 treatment, with the WUEi of T1, T2, and T3 treatments being significantly higher than the CK and T4 treatment (*p* < 0.05). *Plants* **2022**, *11*, x FOR PEER REVIEW 6 of 16

> **Table 2.** Effects of water stress on gas exchange parameters of *Glycyrrhiza uralensis*. Different lowercase letters indicate a significant difference at a 5% level between different times.


#### *2.7. Effects of Water Stress on Chloroplast and Stomatal Ultrastructure 2.7. Effects of Water Stress on Chloroplast and Stomatal Ultrastructure*

Under normal water conditions, the chloroplasts of *G. uralensis* mesophyll cells were long and semi-circular, distributed close to the cell edge, with a complete structure and a clear membrane structure. Among them, the crenellations of thylakoids were tight and smooth, and the stromal lamellae were more evenly distributed and arranged, and there were a small number of mesophyll granules. The ultrastructure of chloroplasts changed to varying degrees as the degree of water stress increased (Figure 5a). The T4 treatment caused the chloroplast to become shorter and swollen to varying degrees, and the membrane structure of the chloroplasts gradually blurred. The number of osmiophilic granules increased and accumulated. Starch grains appeared, and their volume gradually increased. The number of granalamellae decreased, and the structure was fuzzy in the T4 treatment. Under normal water conditions, the chloroplasts of *G. uralensis* mesophyll cells were long and semi-circular, distributed close to the cell edge, with a complete structure and a clear membrane structure. Among them, the crenellations of thylakoids were tight and smooth, and the stromal lamellae were more evenly distributed and arranged, and there were a small number of mesophyll granules. The ultrastructure of chloroplasts changed to varying degrees as the degree of water stress increased (Figure 5a). The T4 treatment caused the chloroplast to become shorter and swollen to varying degrees, and the membrane structure of the chloroplasts gradually blurred. The number of osmiophilic granules increased and accumulated. Starch grains appeared, and their volume gradually increased. The number of granalamellae decreased, and the structure was fuzzy in the T4 treatment.

**Figure 5.** *Cont*.

**Figure 5.** (**a**,**b**): Effects of water stress on chloroplast and stomatal ultrastructures of *Glycyrrhiza uralensis*. Ch, Mi, G, S, OG, P, CW, and N represent the chloroplast, mitochondrion, granalamellae, starch grain, osmiophilic granule, plastoglobulis, cell wall, and nucleus, respectively. **Figure 5.** (**a**,**b**): Effects of water stress on chloroplast and stomatal ultrastructures of *Glycyrrhiza uralensis*. Ch, Mi, G, S, OG, P, CW, and N represent the chloroplast, mitochondrion, granalamellae, starch grain, osmiophilic granule, plastoglobulis, cell wall, and nucleus, respectively.

The guard cells in the leaves of *Glycyrrhiza uralensis* were found to have a typical reniform equithick wall, and the two guard cells were arranged symmetrically. Under normal water conditions, the thickness of the upper cell wall of guard cells was larger than that of the lower cell wall. The chloroplast, mitochondria, nuclear stomatal cavity, and other structures are clear and distinguishable, with normal morphology and more starch The guard cells in the leaves of *Glycyrrhiza uralensis* were found to have a typical reniform equithick wall, and the two guard cells were arranged symmetrically. Under normal water conditions, the thickness of the upper cell wall of guard cells was larger than that of the lower cell wall. The chloroplast, mitochondria, nuclear stomatal cavity, and other structures are clear and distinguishable, with normal morphology and more starch grains.

grains. The guard cells became smaller as the degree of water stress increased, with uneven cell wall thickening (Figure 5b—T3, T4), protoplast shrinking volume (Figure 5b—T2, T3, T4), and starch grains gradually disintegrating. The stomatal cavity was shaped like a slender wine cup and severely deformed (Figure 5b–T4). The guard cells became smaller as the degree of water stress increased, with uneven cell wall thickening (Figure 5b—T3, T4), protoplast shrinking volume (Figure 5b—T2, T3, T4), and starch grains gradually disintegrating. The stomatal cavity was shaped like a slender wine cup and severely deformed (Figure 5b–T4).

#### *2.8. Analysis of Different Organ Biomass and Related Physiological Indexes of Glycyrrhiza uralensis*

*2.8. Analysis of Different Organ Biomass and Related Physiological Indexes of Glycyrrhiza uralensis* The biomasses of *Glycyrrhiza uralensis* organs were negatively correlated with R/S, δ13Croot, and PR, and positively correlated with Tr and Gs (*p* < 0.05) (Figure 6). The δ13C values of each organ were positively correlated with the R/S and PR and negatively correlated with the leaf biomass, Pn, Tr, and Gs (*p* < 0.05). The R/S and PR were negatively The biomasses of *Glycyrrhiza uralensis* organs were negatively correlated with R/S, δ <sup>13</sup>Croot, and PR, and positively correlated with Tr and Gs (*p* < 0.05) (Figure 6). The δ <sup>13</sup>C values of each organ were positively correlated with the R/S and PR and negatively correlated with the leaf biomass, Pn, Tr, and Gs (*p* < 0.05). The R/S and PR were negatively correlated with the Pn, Ti, Ci, and Gs (*p* < 0.05). The RWC and WP were positively correlated with the Tr, Ci, and Gs and negatively correlated with the Ls (*p* < 0.05).

correlated with the Pn, Ti, Ci, and Gs (*p* < 0.05). The RWC and WP were positively corre-

lated with the Tr, Ci, and Gs and negatively correlated with the Ls (*p* < 0.05).

**Figure 6.** Correlation coefficient between different organ biomasses and related physiological indexes of *Glycyrrhiza uralensis*. δ <sup>13</sup>Cleaf, δ13Cstem, and δ13Croot represent the δ13C values of leaf, stem, and root; LB, SB, and RB represent the leaf biomass, stem biomass, and root biomass; R/S, PR, RWC, WP, Pn, Tr, Ci, Gs, WUEi, and Ls represent the root to shoot ratio, proline, relative water content, water potential, net photosynthetic rate, transpiration rate, intercellular CO<sup>2</sup> concentration, stomatal conductance, instantaneous water use efficiency, and limiting value of stomata, respectively. \* and \*\* **Figure 6.** Correlation coefficient between different organ biomasses and related physiological indexes of *Glycyrrhiza uralensis*. δ <sup>13</sup>Cleaf, δ <sup>13</sup>Cstem, and δ <sup>13</sup>Croot represent the δ <sup>13</sup>C values of leaf, stem, and root; LB, SB, and RB represent the leaf biomass, stem biomass, and root biomass; R/S, PR, RWC, WP, Pn, Tr, Ci, Gs, WUEi, and Ls represent the root to shoot ratio, proline, relative water content, water potential, net photosynthetic rate, transpiration rate, intercellular CO<sup>2</sup> concentration, stomatal conductance, instantaneous water use efficiency, and limiting value of stomata, respectively. \* and \*\* indicate significant correlation at 0.05 and 0.01 levels, respectively.

#### indicate significant correlation at 0.05 and 0.01 levels, respectively. *2.9. The Relationship between the Biomass of Different G.uralensis Organs and the Photosynthetic Physiological Indexes under Water Stress*

*2.9. The Relationship between the Biomass of Different G.uralensis Organs and the Photosynthetic Physiological Indexes under Water Stress* Redundancy analysis revealed that RDA1 axis could explain 85.82% of the changes in organ biomass, which mainly reflected the changes in physiological regulatory factors. The proline content had the greatest influence on the changes of organ biomass, amount-Redundancy analysis revealed that RDA1 axis could explain 85.82% of the changes in organ biomass, which mainly reflected the changes in physiological regulatory factors. The proline content had the greatest influence on the changes of organ biomass, amounting to 54.3% of the explanations and 55.6% of the contribution rate (*p* < 0.01) (Figure 7). In addition, the Pn and δ <sup>13</sup>C roots were important factors affecting the changes in organ biomass, they accounted for 9.9% of the explanation, contributed 10.1%, and the δ <sup>13</sup>Croot had a significant influence (*p* < 0.05) (Table 3).

**Name Explains % Contribution % Pseudo-F** *p* Proline (PR) 54.3 55.6 15.4 0.002

Net photosynthetic rate (Pn) 9.9 10.1 3.3 0.072 δ13C values of root (δ13Croot) 9.9 10.1 4.2 0.022 Stomatal conductance (Gs) 6.2 6.4 3.2 0.062 Water potential (WP) 2.8 2.8 1.5 0.25

Transpiration rate (Tr) 0.8 0.8 0.4 0.72 δ13C values of stem (δ13Cstem) 1.5 1.6 0.7 0.546

Instantaneous water use efficiency (WUEi) 1.5 1.5 0.8 0.506

ing to 54.3% of the explanations and 55.6% of the contribution rate (*p* < 0.01) (Figure 7). In addition, the Pn and δ13C roots were important factors affecting the changes in organ biomass, they accounted for 9.9% of the explanation, contributed 10.1%, and the δ13Croot had

**Table 3.** Results by redundancy analysis ordination with the first two axes and Monte Carlo per-

a significant influence (*p* < 0.05) (Table 3).

mutation test.

δ13C values of leaf (δ13Cleaf) 5 5.1 3.1 0.084 Limiting value of stomata (Ls) 1.7 1.7 1.1 0.386 Intercellular CO<sup>2</sup> concentration (Ci) 0.9 0.9 0.5 0.648 Relative water content (RWC) 3.2 3.3 2.7 0.14

Explained variation (cumulative) 85.82 93.36 97.62 97.65 Pseudo-canonical correlation 0.9959 0.9214 0.9671 0.9563

Statistic Axis 1 Axis 2 Axis 3 Axis 4 Eigenvalues 0.8582 0.0754 0.0426 0.0003

> **Figure 7.** Results by redundancy analysis between biomass and physiological indexes of *Glycyrrhiza uralensis*. δ <sup>13</sup>Cleaf, δ13Cstem, and δ13Croot represent the δ13C values of leaf, stem, and root; LB, SB, and RB represent the leaf biomass, stem biomass, and root biomass; R/S, PR, RWC, WP, Pn, Tr, Ci, Gs, WUEi and Ls represent the root to shoot ratio, proline, relative water content, water potential, net photosynthetic rate, transpiration rate, intercellular CO<sup>2</sup> concentration, stomatal conductance, instantaneous water use efficiency, and limiting value of stomata, respectively. **3. Discussion Figure 7.** Results by redundancy analysis between biomass and physiological indexes of *Glycyrrhiza uralensis*. δ <sup>13</sup>Cleaf, δ <sup>13</sup>Cstem, and δ <sup>13</sup>Croot represent the δ <sup>13</sup>C values of leaf, stem, and root; LB, SB, and RB represent the leaf biomass, stem biomass, and root biomass; R/S, PR, RWC, WP, Pn, Tr, Ci, Gs, WUEi and Ls represent the root to shoot ratio, proline, relative water content, water potential, net photosynthetic rate, transpiration rate, intercellular CO<sup>2</sup> concentration, stomatal conductance, instantaneous water use efficiency, and limiting value of stomata, respectively.


*3.1. Effects of Water Stress on Biomass Allocation of Glycyrrhiza uralensis* In response to water stress, changes in plant biomass and its distribution ratios to **Table 3.** Results by redundancy analysis ordination with the first two axes and Monte Carlo permutation test.

#### **3. Discussion**

*3.1. Effects of Water Stress on Biomass Allocation of Glycyrrhiza uralensis*

In response to water stress, changes in plant biomass and its distribution ratios to different organs reflect its adaptation methods and abilities to the ecological environment [16]. Meanwhile, there are stable allometric relationships between plant organ biomass and plant metabolism, this change in growth relationships may be the mechanism of drought tolerance in plants [17]. In the present study, the above-ground biomass and total biomass of *Glycyrrhiza uralensis* decrease to varying degrees, and the R/S significantly increased under severe water stress. Thus, the proportion of underground biomass was increased by slowing down the growth of the above-ground part, falling leaves, and other adaptive characteristics. This may be because plants often allocate more biomass to the ground to cope with extreme drought [18]. Under water deficit conditions, *Glycyrrhiza uralensis* increases the available soil water by increasing the root–shoot ratio and thereby boosting

dance of fine roots, thick roots, and thick branches was significantly higher than the leaves

drought tolerance [19]. We also found a correlation between underground biomass and above-ground biomass under different water stresses (*p* < 0.05). The allometric growth content was greater than one, which further indicates that *Glycyrrhiza uralensis* allocates more resources to enhance the ability of water acquisition by the roots.

#### *3.2. δ <sup>13</sup>C Value Composition of Glycyrrhiza uralensis Organs under Water Stress*

In this study, the δ <sup>13</sup>C values of the roots of *Glycyrrhiza uralensis* were significantly higher than the leaves and stems (*p* < 0.01), and the δ <sup>13</sup>C values of different organs were as follows: δ <sup>13</sup>Croots > δ <sup>13</sup>C stems > δ <sup>13</sup>C leaves. Similarly, Gao also confirmed that the <sup>13</sup>C abundance of fine roots, thick roots, and thick branches was significantly higher than the leaves [20]. This may be because roots accumulate <sup>13</sup>C more easily than leaves and stems. Photosynthetic organs usually contain a low δ <sup>13</sup>C value, and most of the stems of herbaceous plants are also green photosynthetic organs. Therefore, the isotopic fractionation metabolism of the stems of herbaceous plants is closer to the leaves than that of wood and roots [21,22]. This indicates that the <sup>13</sup>C fractionation produced by the leaves and stems was smaller than the roots. The main reason for plants <sup>13</sup>C fractionation may be the heterogeneity of the chemical composition of their organs, and organs with high cellulose content are more likely to enrich <sup>13</sup>C [23].

With water stress intensifies, plant species always have improved their δ <sup>13</sup>C values, and WUE will increase with increasing δ <sup>13</sup>C values [11,12]. The *Glycyrrhiza uralensis* in this study also conforms to this feature. However, we found that the leaf δ <sup>13</sup>C value of seedling was significantly lower than that of uncultivated *Glycyrrhiza uralensis* [14,15], indicating that wild plants or cultivated perennials have higher leaf WUE than young plants. This indicates that δ <sup>13</sup>C values of plants have organ and growth stages specificity under water stress [13].
