*Article* **Regulatory Control and the Effects of Condensation Water on Water Migration and Reverse Migration of** *Halostachys caspica* **(M.Bieb.) C.A.Mey. in Different Saline Habitats**

**Lu Qin 1,2,†, Xuemin He 3,4,5,†, Guanghui Lv 3,4,5,\* and Jianjun Yang 3,4,5**


**Abstract:** Condensation water has been a recent focus in ecological hydrology research. As one of the main water sources that maintains the food chain in arid regions, condensation water has a significant impact on water balance in arid environments and plays an important role in desert vegetation. This study takes drought desert areas and high-salinity habitats as its focus—selecting *Halostachys caspica* (M.Bieb.) C.A.Mey. and its community in mild, moderate, and severe salinity soil—analyzed the source of condensation water utilized by these plants, and calculated its percentage of contribution. I. Study results revealed: (1) Scale-like leaves can absorb condensation water and the order of condensation water contribution to plant growth in different salinity habitats are severe > mild > moderate, such that the average contribution rates were 11.13%, 7.10%, and 3.79%, respectively; (2) The migration path of water movement in these three communities are formed in two main ways: (a) rain and condensation water recharge the soil to compensate for groundwater, while some groundwater compensates for river water and partially returns to the atmosphere by soil evaporation and plant transpiration; and (b) rain and condensation water directly compensate for river water and plant roots absorb river water, groundwater, and soil water in order to grow; (3) in mild habitats, the water movement path in plants is as follows: shallow root → stem → branches → leaves and shallow root → deep root; (4) in moderate habitats, stems act as the bifurcation point and the path follows as: stem → branches → leaves and stem → shallow root → deep root; and (5) in severe habitats, the path is as follows: deep root → shallow root → stem → branches → leaves, and finally returning to the atmosphere. These results elucidate the contribution of condensation water on *Halostachys caspica* growth and the migration path through the *Halostachys caspica* body. Condensation water obtained by *Halostachys caspica* communities in different salinity habitats provides a theoretical basis and data supporting the need for future research of condensation water on plants at the physiological level in arid regions and provides reference for the protection of saline soil and its ecological environment in arid regions.

**Keywords:** condensation water; isotope; *Halostachys caspica*; salinity; moisture migration

## **1. Introduction**

In arid regions, precipitation is limited and evaporation is intense, and any supplementary water has a positive impact on the ecosystems. Condensation water and precipitation are two water sources in desert regions that play an important role in desert ecosystems [1–3]. In the desert, water is scarce and apart from precipitation, condensation

**Citation:** Qin, L.; He, X.; Lv, G.; Yang, J. Regulatory Control and the Effects of Condensation Water on Water Migration and Reverse Migration of *Halostachys caspica* (M.Bieb.) C.A.Mey. in Different Saline Habitats. *Forests* **2022**, *13*, 1442. https://doi.org/ 10.3390/f13091442

Academic Editor: Giovanna Battipaglia

Received: 23 July 2022 Accepted: 2 September 2022 Published: 8 September 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**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/).

water is an important, vital source of water. In drought conditions, there is less soil water content and fewer perennial plants. Despite the small volume of condensation water, it plays an important role in the local water balance [4–6], and especially in drought years its importance is more obvious [5,7].

In the abundant precipitation region, the amount of condensation is trivial. However, in the arid and semi-arid regions, condensation water plays a supplementary role (e.g., in the desert along the Atlantic coast) [8–10]. Maphangwaa et al. [11] found that atmospheric water vapor is the main water source that lichen absorb, and it is atmospheric water vapor, not precipitation, that determines the amount of lichen richness and overall distribution. Thus, conducting research on condensate water and the water balance in arid regions should be a focus of present and future studies.

In a former study conducted by Tao and Zhang [12], it was demonstrated that the tippy of desert moss crust can significantly reduce and delay evaporation in the crust, which prolongs plant hydration time. The greater the amount of precipitation, the more obvious the slowing effect is observed in the tippy of desert moss crust. This slowing effect is also helpful as it allows for the utilization of condensation and precipitation, enhancing the moss crust's ability to adapt to drought conditions. In a separate study, Temina and Kidron [13] discovered that the duration of condensate water determines the distribution of lichen on rocks in the Negev desert. Cheng et al. [14] found in the Alpine Sandy Desert that biological soil crust is conducive to moisture absorption and condensate water and, with the development of the crust, the content of water vapor increases. As an important source of water, condensation water plays a significant role in the survival of vegetation in arid areas [15,16]. Therefore, when conducting research on water balance in arid regions, condensate water cannot be ignored and the importance of water vapor on desert vegetation has been a recent focus of present-day studies.

There are two aspects of plants' use for condensation water. One is indirect utilization, that is, when the dew congeals on the surface of the plant or when fog is intercepted by the canopy and as a result, small water droplets form on a big plant's leaves and water drips down to the soil surface where it is absorbed by the root system of shallow root plants. This use of condensation water allows scholars to focus on the issues of water sources for root systems, such as groundwater, soil natural water, precipitation, fog, dew, and more. Thus, the contribution of various sources of water supply for a specific plant can be calculated, mainly for water sources that contain different δD and δ <sup>18</sup>O isotopes to distinguish various sources. For example, Goebel and Lascano [17] conducted a quantitative analysis of the water use conditions of cotton, including the utilization of condensation water by measuring the different sources of water containing δD and δ <sup>18</sup>O.

Additionally, when plants absorb water, it affects the direction of moisture migration through the plant body, but research on this characteristic is rare. Previous research focused on the quantity of water in terms of soil condensation, plant canopy condensation, occurrence regularity, and impact factors. There are very few studies on the quantity of condensation water utilized by plants and moisture migration path through the plant body [18], and the signature of <sup>18</sup>O isotope tracer (the phenomenon of the enrichment of <sup>18</sup>O in the photosynthetic organs, secondary branches, and trunk xylem) showed that the photosynthetic organs of desert woody plants are able to absorb canopy dew and transfer it to the trunk xylem. In high humidity conditions, assimilating branches of *Haloxylon ammodendron* (C.A.Mey.) Bunge actively absorbed canopy dew and transferred the canopy dew down to the secondary shoots through the reverse water potential (Ψ) gradient between photosynthetic organs and secondary branches (Ψ Photosynthetic organs > Ψ Secondary branches), and the photosynthetic organs can transport the excess dew to the trunk stem via reverse water potential gradient, which is conducive to the continuous absorption and utilization for canopy dew [18]. Leaves of desert plants usually carnify and degenerate to form spiny or scaly parts, while succulent leaves can effectively store water but reduce water transpiration by reducing the amount of leaf area exposed to the air [19]. *Halostachys caspica* is a model organism that belongs to the saline-xerophytic-juicy subshrub and is an

important windproof, dune-fixing shrub, whose branchlets contain succulent juice and has scale-like leaves. Studies have shown that irregular leaf surface can capture small water droplets, and the leaves of *Halostachys caspica* have an irregular surface; thus, we can infer that *Halostachys caspica* could absorb and utilize condensate water.

In this study, we test the following three hypotheses: (1) The leaf of *Halostachys caspica* has the ability to absorb condensation water; (2) The contribution of condensation water to plant growth in *Halostachys caspica* habitats with salinity differences; and (3) The different ways in which condensation water is utilized by *Halostachys caspica*. This study selected the typical desert shrub species, *Halostachys caspica*, as the research organism, which is found in the Ebinur Lake wetland national nature reserve. The treatments were divided into three different salinity habitats: mild, moderate, and severe salinity soils. We conducted a field investigation and analyzed isotope composition to determine the source of condensation water that plants used, as well as its proportion, and investigated the migratory path of condensation water in the plant body. The findings of this study will help to further our understanding of the contribution of condensation water and the migration path in the *Halostachys caspica* body under different salinity conditions and to probe the migration path of obtained condensation water in the *Halostachys caspica* body. The results help to further understand the contribution and the migration path in the *Halostachys caspica* body to the obtained condensation water by the *Halostachys caspica* communities under a different salinity habitat. It is of great significance to elucidate the positive effects of condensation water on plant growth in desert ecosystems in order to provide a theoretical basis and data support for propelling the future research of condensation water on plant physiological level in arid regions and to provide reference for the protection of saline soil and its ecological environment in arid regions.

#### **2. Materials and Methods**

#### *2.1. The Study Region*

The study region is located on the northwest edge of the Junggar Basin (82◦360–83◦50 E, 44◦300–45◦090 N) of the Xinjiang Uygur Autonomous Region. The climate is very dry, there is scarce rainfall, copious amounts of sunlight, strong winds, dust storms, hot summers, and cold winters, which is typical of climates belonging to temperate continental arid regions [20]. Influenced by landform characteristics and climactic conditions, the distribution of vegetation within the Ebinur Lake basin is primarily dominated by two plant flora native to Asia and Mongolia. These two plants had a clear transition into these areas and represent most desert plant species in Xinjiang. *Halostachys caspica* is one of the dominant species in the study area and it is a saline, xerophytism, succulent subshrub that is mainly distributed in the salt-alkali beach area, river valley, and by the salt lake.

Salinization reverse succession was the main method utilized in this study. In the study area, we selected communities for mild, moderate, and severe salinity (Figure 1), and their soil salt content were 0.500 ± 0.275%, 1.428 ± 0.286%, and 2.022 ± 0.329%, respectively. The mild salinity community is adjacent to the river, located at the north of the river with a vertical distance from the river of 50 m. The moderate salinity community is far away from the river bank, located at the north of the river with a vertical distance from the river of 3700 m. The severe salinity community is far away from the river bank, located south of the river with a vertical distance from the river of 700 m. Three well-growing *Halostachys caspica* were selected for isotope sampling in each habitat.

**Figure 1.** Schemes of study sample regions. **Figure 1.** Schemes of study sample regions.

#### *2.2. Experimental Method and Samples Collection 2.2. Experimental Method and Samples Collection*

This article had set seven water sources, which were: topsoil (0–5 cm), shallow soil (5–20 cm), deep soil (20–30 cm), rain water, river water, ground water, and condensation This article had set seven water sources, which were: topsoil (0–5 cm), shallow soil (5–20 cm), deep soil (20–30 cm), rain water, river water, ground water, and condensation water.

water. Plant samples were collected before sunrise and three species of plants were randomly selected as representatives of either the mild, moderate, or severe salinity habitats; there was a total of nine samples. We used loppers to acquire leaves. Suberification was utilized after gathering mature branches, stems, shallow roots (5–20 cm), and deep roots (20–50 cm) in order to remove the phloem but leave in the xylem. We then quickly put samples into 50 mL isotope sampling bottles and sealed them with parafilm membrane. Samples were transported back to the lab, then placed in the fridge to freeze samples at −20 ℃ until they were analyzed for isotope composition. Plant samples were collected before sunrise and three species of plants were randomly selected as representatives of either the mild, moderate, or severe salinity habitats; there was a total of nine samples. We used loppers to acquire leaves. Suberification was utilized after gathering mature branches, stems, shallow roots (5–20 cm), and deep roots (20–50 cm) in order to remove the phloem but leave in the xylem. We then quickly put samples into 50 mL isotope sampling bottles and sealed them with parafilm membrane. Samples were transported back to the lab, then placed in the fridge to freeze samples at −20 °C until they were analyzed for isotope composition.

Soil samples were collected before sunrise from each of the three salinity habitats. Samples were gathered under the canopy of plant samples, close to the root. Samples were collected from the surface soil (0–5 cm), shallow soil (5–20 cm), and deep soil (20–30 cm). We then placed the samples into 50-mL isotope sampling bottles and sealed them with parafilm membrane. Samples were transported back to the lab, then placed in the fridge to freeze samples at −20 °C until they were analyzed for isotope composition. Soil samples were collected before sunrise from each of the three salinity habitats. Samples were gathered under the canopy of plant samples, close to the root. Samples were collected from the surface soil (0–5 cm), shallow soil (5–20 cm), and deep soil (20–30 cm). We then placed the samples into 50-mL isotope sampling bottles and sealed them with parafilm membrane. Samples were transported back to the lab, then placed in the fridge to freeze samples at −20 ◦C until they were analyzed for isotope composition.

Natural water samples were collected from the underground water source near the study point. To ensure the samples were true representatives, we collected deep water samples from the Aqikesu River. Rain and condensation water samples were collected two days before plant and soil sampling, then placed in a 50 mL isotope sampling bottle and sealed with parafilm membrane. Samples were transported back to the lab, then placed in the fridge to be kept at 4 ℃ until they were analyzed for isotope composition. Natural water samples were collected from the underground water source near the study point. To ensure the samples were true representatives, we collected deep water samples from the Aqikesu River. Rain and condensation water samples were collected two days before plant and soil sampling, then placed in a 50 mL isotope sampling bottle and sealed with parafilm membrane. Samples were transported back to the lab, then placed in the fridge to be kept at 4 °C until they were analyzed for isotope composition.

Using a handheld weather instrument (Kestrel 4500 NV), we were able to determine the air temperature, dew-point temperature, relative humidity of the atmosphere, wind speed, and air pressure at 1.5 m above the surface when all of the samples were collected. Using a handheld weather instrument (Kestrel 4500 NV), we were able to determine the air temperature, dew-point temperature, relative humidity of the atmosphere, wind speed, and air pressure at 1.5 m above the surface when all of the samples were collected.

#### *2.3. Plant Physiological Determination 2.3. Plant Physiological Determination*

2.3.1. Presence and Absence Condensation Water Treatment Setting 2.3.1. Presence and Absence Condensation Water Treatment Setting

In the mild salinity, moderate salinity, and severe salinity communities, some leaves with the same water potential were selected in the observation plot approximately 0.5 h In the mild salinity, moderate salinity, and severe salinity communities, some leaves with the same water potential were selected in the observation plot approximately 0.5 h before sunset, and they were divided into three groups, namely: (1) Before treatment (B), the initial water potential of branches were measured immediately after taking down the

branches; (2) presence of condensation water treatment (W1), that is, branches were not bagged in the natural state; (3) absence of condensation water treatment (W0), sealed with plastic bags (so that the surface of branches was not affected by night condensation water).

#### 2.3.2. Water Potentials

Approximately 0.5 h after sunrise the next day, the branches were removed to check the presence and absence of condensation water, and the water potential of the branches was immediately measured, with 9 repetitions in each group.

#### 2.3.3. Fresh Leaf Water Absorption per Unit Area

In mild salinity, moderate salinity, and severe salinity communities, six leaves with and without condensate treatment were picked, the radius (*r*) and length (*l*) of the leaves were measured, and the weight *m*1 was weighed. Then, the leaves were placed in clean water and left for approximately 24 h. After the leaves fully absorbed water, they were removed from the water. The water on the surface of the leaves was completely absorbed by filter paper, and *m*2 was weighed again. The water absorption per unit area of fresh leaves was calculated by the following formula:

> Fresh leaf water absorption per unit area = (*m*2 − *m*1)/(*πr* 2 *l*) (1)

#### 2.3.4. Transpiration Rate (Tr) and Water Use Efficiency (WUE)

The transpiration rate and the net photosynthetic rate (Pn) of *Halostachys caspica* under the presence and absence condensation water treatment were measured by cluster leaf chamber of Li-6400XT portable photosynthetic apparatus (Li-Cor, Lincoln, Nebraska, USA), and the WUE was calculated by the following formula:

$$\text{WUE} = \text{Pr} / \text{Tr} \tag{2}$$

In the presence and absence condensation water treatment, 30 repetitions are determined for each treatment.

#### *2.4. Water Extraction and Isotope Determination*

We used the Liquid Water Isotope Analyzer (LWIA, DLT–100, Los Gatos Research Inc., Mountain View, CA, USA) to determine the hydrogen and oxygen isotopic composition of the samples. Plant water and soil water were extracted using a cryogenic vacuum distillation line [21] and the extracted water samples were stored in sealed glass vials at 2C. Then, the hydrogen and oxygen isotopic compositions of the samples were determined by an isotope ratio infrared spectroscopy (IRIS) analyzer—the Liquid Water Isotope Analyzer (LWIA, DLT–100, Los Gatos Research Inc., Mountain View, CA, USA), and the determination of each sample was repeated 12 times. Analytical precision of individual measurement were ±0.1‰ for δD and ±0.25‰ for δ <sup>18</sup>O. The isotopic composition can be expressed as:

$$
\delta \mathbf{X} = \left(\frac{R\_{sample}}{R\_{standard}} - 1\right) \times 1000\,\%\,\tag{3}
$$

where X is D or <sup>18</sup>O, *Rsample* and *Rstandard* are the hydrogen or oxygen isotopic composition ( <sup>2</sup>H/1H or <sup>18</sup>O/16O molar ratio) of the sample and the standard water (Standard Mean Ocean Water, SMOW), respectively.

#### *2.5. Data Analysis*

A one-way analysis of variance (ANOVA) was utilized to analyze the data. Data manipulation and visual representations were conducted using Excel 2003 software and Sigmaplot 10.0. All of the statistical analyses were run with a significance level of 0.05 using StatView 5.0 (SAS Institute, Inc., Cary, NC, USA). There were seven sources of water in this study, according to the upper and lower limits method Proposed by Phillips and Gregg [22]. IsoSource 1.3.1 software was used to analyze the water source of plant and soil samples. soil samples. **3. Result and Analysis**

A one-way analysis of variance (ANOVA) was utilized to analyze the data. Data manipulation and visual representations were conducted using Excel 2003 software and Sigmaplot 10.0. All of the statistical analyses were run with a significance level of 0.05 using StatView 5.0 (SAS Institute, Inc., Cary, NC, USA). There were seven sources of water in this study, according to the upper and lower limits method Proposed by Phillips and Gregg [22]. IsoSource 1.3.1 software was used to analyze the water source of plant and

#### **3. Result and Analysis** *3.1. Plant Physiological Characteristics in Different Salinity Habitats*

*2.5. Data Analysis*

#### *3.1. Plant Physiological Characteristics in Different Salinity Habitats* 3.1.1. Water Potentials

*Forests* **2022**, *13*, x FOR PEER REVIEW 6 of 18

3.1.1. Water Potentials As can be seen from the Figure 2, night condensation water can improve the water

As can be seen from the Figure 2, night condensation water can improve the water potential of plant leaves in those three communities, but without condensation water supplement, it will reduce the water potential of plant leaves, indicating that the leaves of *Halostachys caspica* can absorb condensation water and improve the water potential of plant leaves. potential of plant leaves in those three communities, but without condensation water supplement, it will reduce the water potential of plant leaves, indicating that the leaves of *Halostachys caspica* can absorb condensation water and improve the water potential of plant leaves.

**Figure 2.** Comparation of *Halostachys caspica* leaves water potentials under presence and absence of water treatment in different salinity habitats*.* Before treatment (B), presence of condensation water **Figure 2.** Comparation of *Halostachys caspica* leaves water potentials under presence and absence of water treatment in different salinity habitats. Before treatment (B), presence of condensation water treatment (W1), absence of condensation water treatment (W0).

#### treatment (W1), absence of condensation water treatment (W0). 3.1.2. Fresh Leaf Water Absorption per Unit Area

3.1.2. Fresh Leaf Water Absorption per Unit Area Using the results of fresh leaf water absorption per unit area in mild salinity, moderate salinity, and severe salinity communities under the presence and absence condensa-Using the results of fresh leaf water absorption per unit area in mild salinity, moderate salinity, and severe salinity communities under the presence and absence condensation water treatment, the following figure can be drawn.

tion water treatment, the following figure can be drawn. In mild salinity, moderate salinity, and severe salinity communities, the fresh leaf water absorption per unit area of leaves of absence condensation water is higher than that presence condensation water (Figure 3), which indicates that the short-term lack of water on the surface of leaves can stimulate the water absorption per unit area. In addition, under the condition of absence condensation water treatment, the water absorption per unit area of leaves of *Halostachys caspica* in the mild salinity, moderate salinity, and severe salinity communities showed the following order: moderate salinity community > severe salinity community > mild salinity community, which indicated that if the leaves were temporarily short of water, and once there was enough water in the air, and the saliniza-In mild salinity, moderate salinity, and severe salinity communities, the fresh leaf water absorption per unit area of leaves of absence condensation water is higher than that presence condensation water (Figure 3), which indicates that the short-term lack of water on the surface of leaves can stimulate the water absorption per unit area. In addition, under the condition of absence condensation water treatment, the water absorption per unit area of leaves of *Halostachys caspica* in the mild salinity, moderate salinity, and severe salinity communities showed the following order: moderate salinity community > severe salinity community > mild salinity community, which indicated that if the leaves were temporarily short of water, and once there was enough water in the air, and the salinization degree increased, the plant leaves growing on them would have stronger water absorption capacity.

tion degree increased, the plant leaves growing on them would have stronger water absorption capacity. Under the presence and absence condensation water treatment, the transpiration rates of three communities are as follows: severe salinity community > moderate salinity > mild salinity community (Figure 4), which indicated that the ecological effect of condensation water on severe salinity community was higher than that of mild salinity and moderate salinity community. In those three salinity habitats, the transpiration rate of plants treated of presence condensation water was higher than that absence condensation water, which indicated that condensation water can improve the transpiration rate of plants and had certain ecological effects on plants. In addition, it was found that the condensation water formed at night can supplement the evaporated water to a certain extent in the three communities [23].

*Forests* **2022**, *13*, x FOR PEER REVIEW 7 of 18

**Figure 3***.* Fresh leaf water absorption per unit area under presence and absence of condensation water treatment. The same lowercase letter indicates that there is no significant difference among different treatments in the same habitat (*p* > 0.05). The same capital letter indicates that there is no significant difference between different habitat of the same treatment (*p* > 0.05), presence of condensation water treatment (W1), absence of condensation water treatment (W0), the same below. **Figure 3.** Fresh leaf water absorption per unit area under presence and absence of condensation water treatment. The same lowercase letter indicates that there is no significant difference among different treatments in the same habitat (*p* > 0.05). The same capital letter indicates that there is no significant difference between different habitat of the same treatment (*p* > 0.05), presence of condensation water treatment (W1), absence of condensation water treatment (W0), the same below. treated of presence condensation water was higher than that absence condensation water, which indicated that condensation water can improve the transpiration rate of plants and had certain ecological effects on plants. In addition, it was found that the condensation water formed at night can supplement the evaporated water to a certain extent in the three communities [23].

erate salinity community. In those three salinity habitats, the transpiration rate of plants

Aa

Tr

(mmolH2O•m-2•s-1)

Ab

Ab **Figure 4.** Tr of *Halostachys caspica* under the presence and absence condensation water treatment*.* **Figure 4.** Tr of *Halostachys caspica* under the presence and absence condensation water treatment.

The instantaneous water use efficiency (WUE) of plants is a comprehensive physiological index to evaluate the adaptability of plants to the environment. The instantaneous water use efficiency (WUE) of plants is a comprehensive physiological index to evaluate the adaptability of plants to the environment.

**Figure 4.** Tr of *Halostachys caspica* under the presence and absence condensation water treatment*.* The instantaneous water use efficiency (WUE) of plants is a comprehensive physiological index to evaluate the adaptability of plants to the environment. W1 W0 W1 W0 W1 W0 mild salinity moderate salinity severe salinity Daily mean value of the WUE of moderate and mild salinity communities under the presence condensation water treatment higher than the absence condensation water (Figure 5), which may be temporary water balance is out of balance in plants at night without condensation water supplement, which reduces the water use efficiency. In the severe sa-Daily mean value of the WUE of moderate and mild salinity communities under the presence condensation water treatment higher than the absence condensation water (Figure 5), which may be temporary water balance is out of balance in plants at night without condensation water supplement, which reduces the water use efficiency. In the severe salinity, the daily average WUE of *Halostachys caspica* under the presence condensation water treatment is lower than that of absence treatment. This is because the severe salinity community is far from the water source, lives in a low water environment for a long time, has a survival strategy, has a strong self-regulating mechanism for water stress to improve water utilization ability to cope with water deficit [24], which is similar to Yu's research result for maize [25].

Daily mean value of the WUE of moderate and mild salinity communities under the presence condensation water treatment higher than the absence condensation water (Figure 5), which may be temporary water balance is out of balance in plants at night without condensation water supplement, which reduces the water use efficiency. In the severe sa-

linity, the daily average WUE of *Halostachys caspica* under the presence condensation water treatment is lower than that of absence treatment. This is because the severe salinity community is far from the water source, lives in a low water environment for a long time, has a survival strategy, has a strong self-regulating mechanism for water stress to improve water utilization ability to cope with water deficit [24], which is similar to Yu*'*s research

*Forests* **2022**, *13*, x FOR PEER REVIEW 8 of 18

result for maize [25].

**Figure 5.** Comparison of *Halostachys caspica* WUE under presence and absence of condensation wa-**Figure 5.** Comparison of *Halostachys caspica* WUE under presence and absence of condensation water treatment in different salinity habitats.

ter treatment in different salinity habitats*.* 3.1.3. δD and δ <sup>18</sup>O Composition in Plant Structures and the δD-δ <sup>18</sup>O Relationship in Different Salinity Habitats

3.1.3. δD and δ18O Composition in Plant Structures and the δD-δ18O Relationship in Different Salinity Habitats Plant leaves enriched with δD and δ18O isotopes have partially negative δD values ranging from −40.0844 ± 1.6106 to −40.0844 ± 1.6706%; when there are partially positive δ18O values, they range from 4.9758 ± 0.686 to 12.469 ± 1.653% (Figure 6). The max value was observed in the severe salinity community, while the minimum value was observed in the light salinity community, which indicates that *Halostachys caspica* growing in severe salinity soil were better at evapotranspiration. Additionally, compared to aerial parts of the plant, such as branches and stems, the shallow roots from moderate salinity soil had lower δD and δ18O values. This may be because soil evaporation has allowed for the transpiration and evaporation at the roots of the plant. Moreover, transpiration and evaporation in aerial parts of the plant strengthen water delivery from the roots to aerial parts of plant, which suggests that there is a regulatory effect to the plant from soil moisture Plant leaves enriched with δD and δ <sup>18</sup>O isotopes have partially negative δD values ranging from −40.0844 ± 1.6106 to −40.0844 ± 1.6706%; when there are partially positive δ <sup>18</sup>O values, they range from 4.9758 <sup>±</sup> 0.686 to 12.469 <sup>±</sup> 1.653% (Figure 6). The max value was observed in the severe salinity community, while the minimum value was observed in the light salinity community, which indicates that *Halostachys caspica* growing in severe salinity soil were better at evapotranspiration. Additionally, compared to aerial parts of the plant, such as branches and stems, the shallow roots from moderate salinity soil had lower δD and δ <sup>18</sup>O values. This may be because soil evaporation has allowed for the transpiration and evaporation at the roots of the plant. Moreover, transpiration and evaporation in aerial parts of the plant strengthen water delivery from the roots to aerial parts of plant, which suggests that there is a regulatory effect to the plant from soil moisture through a hydraulic redistribution effect [26]. *Forests* **2022**, *13*, x FOR PEER REVIEW 9 of 18

**Figure 6.** Distribution of δD and δ18O composition of structures in different salinity habitats. **Figure 6.** Distribution of δD and δ <sup>18</sup>O composition of structures in different salinity habitats.

The three salinity habitats were ranked as mild > moderate > severe, which indicates that the degree of soil salinity enhances the evapotranspiration ability of *Halostachys caspica* (Figure 7). This in turn causes an isotope fractionation effect. It could also be a survival strategy for plant growth in saline soil is to take excess salt from the plant of body as evapotranspiration increases. The three salinity habitats were ranked as mild > moderate > severe, which indicates that the degree of soil salinity enhances the evapotranspiration ability of *Halostachys caspica* (Figure 7). This in turn causes an isotope fractionation effect. It could also be a survival strategy for plant growth in saline soil is to take excess salt from the plant of body as evapotranspiration increases.

**Figure 7.** δD-δ18O relationship in water samples from plant structures found in different salinity

habitats. Fitting equations for each habitat are noted in the figure.

evapotranspiration increases.

**Figure 6.** Distribution of δD and δ18O composition of structures in different salinity habitats.

The three salinity habitats were ranked as mild > moderate > severe, which indicates that the degree of soil salinity enhances the evapotranspiration ability of *Halostachys caspica* (Figure 7). This in turn causes an isotope fractionation effect. It could also be a survival strategy for plant growth in saline soil is to take excess salt from the plant of body as

*Forests* **2022**, *13*, x FOR PEER REVIEW 9 of 18

**Figure 7.** δD-δ18O relationship in water samples from plant structures found in different salinity **Figure 7.** δD-δ <sup>18</sup>O relationship in water samples from plant structures found in different salinity habitats. Fitting equations for each habitat are noted in the figure. *Forests* **2022**, *13*, x FOR PEER REVIEW 10 of 18

#### habitats. Fitting equations for each habitat are noted in the figure. *3.2. Hydrogen and Oxygen Isotope Composition and Soil Profiles of Halostachys Caspica Communities 3.2. Hydrogen and Oxygen Isotope Composition and Soil Profiles of Halostachys Caspica Communities*

Results revealed that there are significant differences in δD and δ <sup>18</sup>O values based on the salinity habitat (Figure 8). The values were (−70.524 ± 4.326)–(−46.432 ± 3.638)‰ and (−5.197 ± 0.906)–(1.987 ± 0.626)‰, respectively. The higher the composition of δD and δ <sup>18</sup>O isotopes, the more enriched the soil surface layer and with the increasing soil depth, the δD and δ <sup>18</sup>O values decrease gradually. Results revealed that there are significant differences in δD and δ18O values based on the salinity habitat (Figure 8). The values were (−70.524 ± 4.326)–(−46.432 ± 3.638)‰ and (−5.197 ± 0.906)–(1.987 ± 0.626)‰, respectively. The higher the composition of δD and δ18O isotopes, the more enriched the soil surface layer and with the increasing soil depth, the δD and δ18O values decrease gradually.

**Figure 8.** δD and δ18O of soil water in different soil and natural water of salinity habitats. **Figure 8.** δD and δ <sup>18</sup>O of soil water in different soil and natural water of salinity habitats.

All of the soil layers were enriched with δD in the mild salinity habitat. The topsoil in severe salinity habitats had slightly higher δ18O values than the mild salinity, while δ18O values in mild salinity soil water were higher than moderate and severe, which indicates that there is soil evaporation and an isotope fractionation effect in mild salinity, although the volatility of δD and δ18O values are at a minimum. Meanwhile, soil water hydrogen and oxygen stable isotopes are lacking in deep soil of severe salinity habitats. All of the soil layers were enriched with δD in the mild salinity habitat. The topsoil in severe salinity habitats had slightly higher δ <sup>18</sup>O values than the mild salinity, while δ18O values in mild salinity soil water were higher than moderate and severe, which indicates that there is soil evaporation and an isotope fractionation effect in mild salinity, although the volatility of δD and δ <sup>18</sup>O values are at a minimum. Meanwhile, soil water hydrogen and oxygen stable isotopes are lacking in deep soil of severe salinity habitats.

Results revealed that closer to the surface, δ18O values for soil water are partially positive. Additionally, Δ δD values for soil water in the different salinity habitats were between the values for rain and the other three sources of water (river water, condensation water, and groundwater). Lastly, the δ18O values were all partially negative for all water sources. Results revealed that closer to the surface, δ <sup>18</sup>O values for soil water are partially positive. Additionally, ∆ δD values for soil water in the different salinity habitats were between the values for rain and the other three sources of water (river water, condensation water, and groundwater). Lastly, the δ <sup>18</sup>O values were all partially negative for all water sources.

Due to strong fractionation in the leaves, there were no results for leaf water source, so we analyzed the water source for branches, stems, and shallow roots. Water sources were topsoil, shallow soil, deep soil, rain water, river water, groundwater, and condensa-

**Soil Water Rain River Water Ground** 

mild salinity 11.375 ± 2.071 8.525 ± 1.280 27.025 ± 0.330 46.1 ± 4.594 7.05 ± 1.100

salinity 6.775 ± 2.507 4.575 ± 1.661 18.5 ± 5.750 66.4 ± 11.253 3.775 ± 1.375 severe salinity 27.9 ± 9.484 11.925 ± 2.258 20.1 ± 3.955 28.975 ± 10.150 11.125 ± 2.478

**Water**

**Condensation Water**

**Table 1.** Sources of water in different salinity habitats (mean ± SE).

**Habitat Contribution of Water Source (%)**

tion water (Table 1).

moderate

*3.3. The Contribution of Condensation Water to Plant Growth*
