1. Introduction
The worsening of soil salinization is one of the main factors that restrict the growth and development of plants, endangers the ecological environment, and affects the sustainable development of the global agricultural and forestry economy and ecosystems. Soil salinization is a global environmental problem. The land worldwide that contains saline-alkali soils exceeds 8.3 × 10
8 hm
2, including 53% alkaline soils and 47% saline soils [
1]. The problem is particularly severe in semiarid and arid areas and is predicted to be more drastic in the future [
2]. It is estimated that the area of saline-alkali soil is growing at a rate of 1 × 10
7 to 1.5 × 10
7 hm
2 annually worldwide, resulting in serious salinization hazards and water and soil loss [
3]. Countries seriously affected by saline soil mainly include Australia, Thailand, Syria, the United States, and China [
4]. According to data from previous work, the area of saline soil in China is about 3.6 × 10
7 hm
2, accounting for 4.88% of the available land area in the country [
5]. ** material, with good stress resistance to salt alkali, cold, drought, wind, and sand. It is widely planted in the saline-alkali areas of Northwest China [
31]. Our research team has made some progress in unraveling salt stress physiology and in undertaking salt tolerance-related gene mining in
P. talassica ×
P. euphratica [
29], but the salt tolerance mechanism of
P. talassica ×
P. euphratica is very complex and needs further research. In order to study and improve the salt tolerance of
P. talassica ×
P. euphratica and promote large-scale planting and promotion, the mitigating effects of ABA, PP
333, and SA on different plants under salt stress have been used as a reference in previous studies. In this study, the optimal concentrations of these exogenous plant growth regulators for
P. talassica ×
P. euphratica seedlings under salt stress were investigated to clarify the regulatory mechanism of salt tolerance and provide a reference and theoretical basis for the use of plant growth regulators in alleviating growth inhibition in seedlings under salt stress.
4. Discussion
Proper soil salinity can promote the growth of many halophytes [
40]. However, excessive soil salinity will cause salt damage to plants and inhibit their growth. Therefore, plants must establish a variety of regulatory mechanisms to respond to salt stress and maintain growth [
41]. The rational use of plant growth regulators in the form of foliar sprays can mitigate the adverse effects of salt stress [
42]. Research shows that ABA can improve photosynthetic capacity and biomass accumulation and alleviate the physiological damage caused by stress to plants [
43]. Paclobutrazol can improve the stress resistance of
Sequoia sempervirens seedlings by improving their photosynthetic characteristics [
44]. SA can improve the antioxidant capacity and salt tolerance of potato [
45]. This study clarified the physiological response mechanism of spraying plant growth regulators onto
P. talassica × P. euphratica under soil salt stress, which is necessary to alleviate soil salt stress and improve the utilization of saline soils in forestry.
In this study, we evaluated the effects of soil salinity and plant growth regulators on the growth indicators of
P. talassica × P. euphratica. The whole plant biomass and root parameters of
P. talassica × P. euphratica were significantly reduced at 2% SSc treatment. The osmotic stress and ion toxicity of soil salinity were the main reasons for the growth decline. Roots provide fixation and support for plants while absorbing nutrients and water from the soil and transmitting it to supply its growth and development needs [
46]. Its morphological structure reflects the degree of development of the plant root system and also the growth state of the whole plant. Therefore, when the salt content of soil increases, the root architecture is inhibited by the stress, which restricts the growth and development of plants and even leads to their death [
47]. However, when foliar spraying ABA, PP
333, and SA onto
P. talassica × P. euphratica, its biomass, total root length, root surface area, and root volume showed a trend of first increasing and then decreasing with increasing regulator concentrations. Both 900 mg·L
−1 PP
333 and 120 mg·L
−1 SA had the most obvious positive regulatory influence on the root parameters of the
P. talassica × P. euphratica seedlings and the best mitigating effects. Plant growth regulators played a great role in restoring the salt resistance of the
P. talassica × P. euphratica roots, making them stronger, enhancing their ability to absorb nutrients and water, and alleviating the stressful effects of the salt environment on
P. talassica × P. euphratica to some extent.
Chl enables plant cells to absorb and convert light energy and is the main pigment involved in photosynthesis. Its content can be used as a parameter for physiological metabolism, nutritional status, and aging in the leaves [
48]. Researchers have discovered that SA stimulates the synthesis of Chl and slows the rate of Chl reduction, thus delaying a decline in Chl content [
49,
50]. Our investigation demonstrated a reduction in total Chl content in the leaves of
P. talassica × P. euphratica under salt stress and a decline in its ability to assimilate light energy, which may be related to the enhancement of Chl enzyme activity under salt stress to promote Chl decomposition. The electron transport chain and energy transport in the photosynthetic system may also be impacted [
51], causing the inhibition of photosynthesis. In addition, salt stress can inhibit Pn, Tr, and Gs in the leaves of
P. talassica × P. euphratica, thereby reducing photosynthetic efficiency, which is similar to the results for
Ulmus pumila [
52] and
Populus cathayana [
53]. This may occur because salt stress leads to a decrease in Gs in the leaves, making it difficult to supply the CO
2 concentration required for photosynthesis and leading to a decrease in the rate of photosynthesis. After spraying exogenous ABA, PP
333, and SA on the leaves of
P. talassica × P. euphratica, the Pn, Tr, and Gs increased first and then decreased with increasing hormone concentration, but all increased to varying degrees. Foliar sprays of 15 mg·L
−1 ABA, 900 mg·L
−1 PP
333, and 120 mg·L
−1 SA had the most obvious positive regulatory influence on the photosynthetic parameters of
P. talassica ×
P. euphratica seedlings and the best mitigating effects. Spraying exogenous hormones on the leaves can adjust the Gs, accelerate carbon carboxylation, slow stomatal restriction, and enhance the photosynthetic capacity of plants [
54,
55]. ABA, PP
333, and SA have been shown to alleviate the adverse effects of abiotic stress on plants and improve photosynthetic performance [
56]. The results found in this study were similar to those reported for
Populus [
57],
Curcuma longa [
58], and
Perennial Ryegrass [
59].
MDA is one of the main products of membrane lipid peroxidation. It is combined with H
2O
2 and O
2−, which are a measure of oxidative damage to plants. Excessive reactive oxygen species and MDA lead to membrane peroxidation, increased membrane permeability, and damage to the plant’s defense systems, affecting physiological and biochemical metabolism [
60]. In this study, the contents of MDA, H
2O
2, and O
2− in
P. talassica × P. euphratica seedlings under salt stress increased significantly, which damaged the cell membrane system and led to the continuous accumulation of membrane lipid peroxides. The spraying of ABA, PP
333, and SA can significantly reduce the content of MDA, H
2O
2, and O
2−, and alleviate the damage of membrane lipid peroxide, which is induced by salt injuries to plants. Pro can regulate the osmotic potential of plant cells. The accumulation of its content can enhance osmotic adjustment ability, reduce the inhibition of antioxidant enzyme activity under salt stress, and, thus, improve the salt tolerance of plants [
61]. In this study, Pro content tended to increase under salt stress, indicating that salt stress promotes the accumulation of proline to mitigate salt damage. ABA, PP
333, and SA treatments increased the content of proline, indicating that they had the function of regulating proline metabolism and could alleviate the damage of the
P. talassica × P. euphratica seedlings under salt stress. The results found in this study were similar to those reported for rice seedlings [
62] and
Mentha simplex [
63].
SOD and POD are two important antioxidant enzymes for scavenging active oxygen from plants. Improving the activity of these enzymes can alleviate growth inhibition under salt stress and improve the resistance of the plant seedlings. SOD can catalyze the conversion of O
2− to H
2O
2 [
64], and POD can convert H
2O
2 to H
2O. Exogenous ABA, PP
333, and SA enhanced the activity of SOD and POD in the leaves of
P. talassica × P. euphratica and reduced the increase in MDA, H
2O
2, and O
2− contents under salt stress, indicating that these three exogenous plant growth regulators can alleviate the oxidative stress caused by abiotic stress. ABA is involved in the restoration of osmotic and antioxidant mechanisms of rice under salt stress [
65]. PP
333 can improve the antioxidant capacity of sweet sorghum under salt stress [
66]. This is similar to the results obtained in
Phoenix dactylifera [
67],
Punica granatum [
68], and
Platycladus orientalis [
69].
In conclusion, our results showed that the foliar spraying of exogenous ABA, PP333, and SA at a certain concentration could adjust the osmotic capacity of P. talassica × P. euphratica seedlings, reduce the degree of membrane lipid peroxidation, enhance cell membrane stability and the antioxidant defense system, promote root growth and development and biomass accumulation, reduce chlorophyll decomposition, and improve photosynthetic performance, thereby reducing the adverse effects of salt stress on P. talassica × P. euphratica seedlings. However, how exogenous ABA, PP333, and SA affect the molecular mechanisms of salt resistance and endogenous hormone interactions on P. talassica × P. euphratica seedlings remains to be further researched in the future.
5. Conclusions
Our results revealed that 2% SSc stress significantly inhibited the growth, root structure, photosynthetic characteristics, and physiological and biochemical characteristics of P. talassica × P. euphratica. Foliar spraying with ABA, PP333, and SA in appropriate concentrations significantly promoted root morphological development, which was conducive to the accumulation of dry matter in the plants, alleviating the damage caused by salt stress and promoting the growth of leaves and, thus, increasing chlorophyll content and photosynthetic capacity. Meanwhile, ABA, PP333, and SA could inhibit increases in MDA, H2O2, O2−, and proline contents, protecting the stability of the cell membrane structure and improving the activity of SOD and POD antioxidant enzymes, which could increase cold resistance and reduce the production of reactive oxygen species. The optimal concentrations of ABA, PP333, and SA for alleviating salt stress were 15 mg·L−1, 900 mg·L−1, and 120 mg·L−1, respectively. Among them, 120 mg·L−1 SA was the most effective for protecting P. talassica × P. euphratica against salt stress.