Soil salinization, alkalization, and drought have been considered as serious environmental hazards to the agricultural, grassland, and urban ecosystems, which also severely affects plant growth and productivity worldwide (Sdouga et al., 2019; Barnard et al., 2021). It is estimated that more than 10% of total global land has suffered from salinization, and the area is still expanding (Farooq et al., 2015). In addition, soil salinization is often accompanied by the occurrence of alkalization in the northeast of China, where alkaline salts (e.g., NaHCO₃) are the main components in the soil (Yin et al., 2019). As another important abiotic stress, it has been reported that drought would affect 50% of the global arable land, and drought stress is often caused indirectly by soil salinization and alkalization, which damage the growth and metabolism process of plants (Igiehon and Babalola, 2021).

Hordeum jubatum is a biennial herb of the Gramineae family. It is native to the cold temperate zone of North America and Eurasia, and is widely distributed in northeastern China. This plant is a salt-tolerant species, which can survive in the soil with high concentration salts (Israelsen et al., 2011). In addition, H. jubatum also has good drought tolerance, its seed can germinate under very low water potential conditions (Zou et al., 2018). Due to the good stress resistance under these adverse environments, this plant plays an important role in soil improvement and ecological restoration in the northeast of China. However, no information is available on the stress tolerance mechanism under saline–alkali and drought stresses during the early seedling stage.

The early seedling stage is an extremely crucial period of plants, and is susceptible to the adverse environments (Wang et al., 2019; Ocvirk et al., 2020; Ren et al., 2020). During this period, water absorption is a determining factor and it regulates various physiological processes in the plant (Klugl and Di Pietro, 2021). In general, both salt-alkali and drought stresses can induce lower water potential, which limits water uptake and inhibits growth of plants. Salt stress disturbs the ion balance and nutrient uptake due to osmotic stress and toxic ions (accumulation of excessive Na⁺), ultimately affecting normal physiological processes (Xu et al., 2018). Previous studies have demonstrated that alkali stress and salt stress are significantly different. Alkali stress has not only the same damaging factors as salt stress but also adds high-pH stress, leading to more destruction to the plant (Lin et al., 2016; Song et al., 2017). Additionally, drought stress directly causes soil water deficit, which leads to a decreased ability of leaf water absorption and excessive transpiration, thereby inhibiting plant growth (Chmielewska et al., 2016). Most studies have mainly comparatively analyzed the effects of salt and drought stresses (Elbasan et al., 2020; He et al., 2021) or salt and alkali stresses (Hu et al., 2019; Jia et al., 2019) on the plant, but little is known on the effects of salt, alkali, and drought stresses simultaneously on plants based on the uniform water potential.

Photosynthesis is one of the most important physiological processes in the carbon assimilation of plants. Photosynthetic capacity can affect the primary productivity and measure the growth level of plants under stress conditions. Generally, salt stress decreases the activity of the PSII reaction center of the leaf. Massive accumulation of surplus electrons in the electron transport chain due to a decrease of electron transfer rates causes electron leakage and then accelerates the degree of breakage of PSII reaction center under salt stress (Zhang et al., 2017; Hou et al., 2018). In addition, Wang et al. (2020) have clarified that compared with salt stress, alkali stress decreased the photosynthetic electron transport rate and stomatal conductance, and also reduced the aerenchyma amount, and then influenced the CO₂ influx into mesophyll cells of Leymus chinensis. Drought stress directly makes the stomata closed, resulting in a reduction of CO₂ absorption and transportation of non-structural carbon (NSC), which is an essential element of the photosynthetic system (Steppe et al., 2015; Hajiboland et al., 2017). The above results suggest that photosynthesis responses of plant to salt, alkali, and drought stresses are markedly different. Therefore, it is of great scientific significance to explore the physiological mechanism of plant photosynthetic systems under such conditions.

Therefore, in the present study, we comparatively investigated seedling growth, photosynthesis, photosynthetic pigments, and chlorophyll fluorescence of H. jubatum under salt, alkali, and drought stresses based on the same water potential. The aims were to explore the response of growth and photosynthetic performance to a wide range of low water potentials caused by the three stresses and clarify their different stress features.

Early seedling stage plays a vital role in the growth and development of plants, which is greatly sensitive to various environmental abiotic stresses, such as salt, alkali, and drought (Li et al., 2005). In the present study, the fresh and dry weights of H. jubatum seedlings under alkali stress were significantly lower than those of salt and drought stresses with the decrease of water potential (Figures 1A,B), indicating that alkali stress had the greatest adverse impacts on the plant morphological characteristics and biomass due to its high pH.

Water is a determining factor for seedling growth of plants. In this study, water potential had significant correlations with many photosynthetic and fluorescent parameters such as Pn, Fv/Fm, and ΦPSII (Tables 2–4). Plants often suffer from osmotic stress under salt-alkali and drought stresses because of the significant decrease in water potential of the surrounding rhizosphere environment (Verslues et al., 2006). The WC and WUE of seedlings are considered as two important indicators to explore the mechanism of physiological response in plants under abiotic stress (Liu and Shi, 2010). Here, the WC decreased and the WUE increased in all the stresses, indicating that plant could respond to salt, alkali, and drought stresses by increasing WUE and decreasing WC (Figures 1C, 2E). Furthermore, the results also showed that the WC declined most under alkali stress, indicating that alkali stress caused the most serious water loss in the seedlings of H. jubatum compared with salt and drought stresses (Figure 1C). This result may be due to that the high pH severely limited water absorption in the roots of seedlings, which broke water balance and then substantially reduced the water transported from roots to the shoots. In addition, the growth parameters of H. jubatum seedlings under salt stress were higher than those under alkali and drought stresses. H. jubatum has good salt tolerance. It can reduce the absorption of Na⁺ and selectively absorb K⁺ in a high-salt environment to maintain normal physiological processes (Garthwaite et al., 2005). A similar result was also found by Hussain et al. (2020) in Panicum antidotale.

Photosynthesis is very sensitive to the environmental stresses and affects seedling growth and stress tolerance of plants (Chaves et al., 2009). In the present study, the photosynthetic gas exchange parameters (Pn, Gs, Ci, and Tr) were all decreased under salt, alkali, and drought stresses with the decrease of water potential, indicating that photosynthesis of H. jubatum seedlings was significantly inhibited by these stresses (Figures 2A–D). Meanwhile, alkali stress also had the lowest photosynthetic capacity at the uniform water potential. This result indicated that the high pH caused by NaHCO₃ induced much more destructive effects to the photosynthesis of H. jubatum seedlings. Similar results have also been reported in the halophyte Chloris virgata by Yang et al. (2008). In addition, the results showed that when the Pn value decreased with the decrease of water potential, the Gs, Tr, and Ci all decreased under salt stress. However, the Ci showed little change when the Pn, Gs, and Tr decreased at different water potentials under drought stress. Additionally, the Pn was slightly increased at the highest alkalinity (Figures 2A–D). Farquhar et al. (1980) have earlier reported that the changing trends of Pn, Gs, and Ci consistently indicated that the decreased photosynthetic capacity of plants was mainly due to stomatal factors. Otherwise, the non-stomatal factors play a vital role. Thus, the present research suggested that stomatal factors were the main reason that photosynthesis of H. jubatum under salt stress was inhibited. When the roots face high sodium ions environment, it is difficult to absorb water from the soil, and the stomata of the leaf will be closed by signal transduction to reduce the transpiration rate and photosynthetic rate. Moreover, the decreased photosynthetic capacity of this species under alkali stress and drought stress was mainly attributed to non-stomatal factors, such as declined CO₂ assimilation capacity in mesophyll cells, changed photosynthetic pigments, reduced PSII reaction center activity, and membrane disruption (Albert et al., 2011; Ashraf and Harris, 2013).

Chlorophyll fluorescence can reflect the activity of the reaction center of the photosynthetic system by analyzing plant responses to light absorption and carbon assimilation processes (Basu et al., 2019; Dos Santos et al., 2019). Fv/Fm, the indicator of maximum quantum efficiency of PSII photochemistry, represents maximum efficiency at which light absorbed by PSII is often used for evaluating reduction of Q_A and the degree of photoinhibition (Murata et al., 2007). In the present study, the Fv/Fm under salt, alkali, and drought stresses slightly reduced with the decrease of water potential, and alkali stress had the lowest Fv/Fm value (Figure 3A). Thus, these stresses all increased the susceptibility of PSII to photoinhibition and damage of PSII reaction centers, which occurred more severely in alkali stress, especially at the lowest water potential (−0.82 Mpa). ΦPSII is the actual quantum yield of PSII photochemical and is used in photochemistry, which reflects the proportion of light absorbed by chlorophyll related to the PSII (Maxwell and Johnson, 2000). In addition, the photochemical quenching (q_p) always estimates the redox status of the primary quinone electron acceptor of PSII (Q_A) and can also indicate the proportion of opened PSII reaction centers (Baker, 2008). In this study, both the ΦPSII and q_p in the three stresses were decreased with the decreasing of water potential, and reached the lowest under alkali stress (Figures 3B,C). This indicated that the reduction of photosynthetic activity caused by decreased light absorption for photochemistry and increased PSII negative reaction centers of H. jubatum seedlings occurred under the three stresses and seemed more easily affected by alkali stress. The possible reason is that the high pH of NaHCO₃ much more harmed photosynthesis by damaging oxygen-evolving complex (OEC) on the donor side of PSII, and hindered the electron transfer on the PSII acceptor side (Zhang et al., 2019). Moreover, the ΦPSII and q_p values of the three stresses much decreased when the water potential was below −0.47 MPa, especially under alkali stress, indicating that the photosynthetic system was more easily influenced by stresses under lower water potential. Although the Fv/Fm, ΦPSII, and q_p values under drought stress were slightly higher than those of salt stress, the differences were not significant, and the Pn value under salt stress was also significantly higher than that of drought stress, indicating that salt tolerance was stronger than drought tolerance of H. jubatum seedlings.

Non-photochemical quenching can afford photoprotection to the photosynthetic apparatus by dissipating excess light energy that cannot be used for photochemistry in the form of heat under stresses (Osmond et al., 2021). In the present study, the NPQ values of both salt and drought stresses increased with the decreasing of water potential and was much higher under salt stress (Figure 3D). This result showed that the seedlings had stronger ability of releasing surplus light energy by heat dissipating to weaken the damage on photosynthetic apparatus under salt stress. However, the NPQ value of alkali stress increased first and then decreased at the lowest water potential, indicating that lower water potential (below −0.47 MPa) caused by high NaHCO₃ concentration disturbed the photoprotection mechanism of H. jubatum seedlings. It is concluded that the xanthophyll cycle depended on the proton gradient across the thylakoid membrane, the xanthophyll Zx and the PsbS protein plays a vital role in photoprotection of NPQ (Jahns and Holzwarth, 2012). Therefore, the reason of NPQ decreased may be that the high pH blocked the xanthophyll photoprotective mechanism, which needs further research.

The photosynthetic pigment content intuitively reflects the photosynthetic performance of plants under stress conditions (Meloni et al., 2003). Chlorophyll generally exists on thylakoid membrane, which has a crucial role for absorption and transmission of light energy in photosynthesis. The previous studies found that plants with less chlorophyll were more susceptible to photoinhibition under various abiotic stresses (Patsikka et al., 2002). Carotenoids play an important role in protecting photosynthetic apparatus against various adverse environmental stresses, such as dissipating excessive light energy, scavenging reactive oxygen species, and enhancing the stability of thylakoid membrane (Strzalka et al., 2003). In the present study, the reduced chlorophyll content values (Chl a, Chl b, and total Chl) were found in the three stresses with the decrease of water potentials, and the most significant groups were under alkali stress among the three stresses (Figure 4). This indicated that the degradation of chlorophyll content in the seedlings of H. jubatum occurred when subjected to these stresses. It hindered the photosynthesis process, especially under alkali stress. The reason may be that the high pH of alkali stress makes Fe, Mg, and other essential elements for chlorophyll synthesis reduced effectiveness (Yang et al., 2007). Moreover, it was noted that the most degradation of Chl a content occurred and the Chl a content value of salt stress was lower than drought stress at the lowest water potential (−0.82 MPa). This indicated that this water potential severely disturbed photosynthesis by degrading chlorophyll, and the high concentration of sodium ions under salt stress had more negative effects on the Chl a content than water deficit of drought stress. In addition, the Car content of the three stresses had little increase with the decrease of water potentials, indicating that these stresses all reduced the ability of H. jubatum seedlings to dissipate excess light, resulting in a decrease of photosynthetic efficiency. Meanwhile, alkali stress also had the highest value in comparison to other stresses at the uniform water potential, indicating that the high pH of NaHCO₃ caused more decline to the photochemical assimilation.

In summary, the result clearly analyzed the comparative effects of salt, drought, and alkali stresses on the growth, photosynthetic, and chlorophyll fluorescence performances in the seedlings of H. jubatum based on the same water potential. According to the photosynthetic gas exchange parameters, the decreased photosynthetic rate of H. jubatum under salt stress was mainly dominated by stomatal factors, but the non-stomatal factors played a major role under drought and alkali stresses. The chlorophyll content reduced under the three stresses reflected that the absorption and transmission of light energy in photosynthesis was degraded, among which was the most significant under alkali stress. Additionally, the decreased Fv/Fm, ΦPSII, and q_P under these stresses indicated that salt, alkali, and drought stresses all increased susceptibility of PSII to photoinhibition, reduced the photosynthetic activity by the declined absorption of light for photochemistry, and increased PSII active reaction centers closed. Moreover, the NPQ was different from the salt and drought stresses under alkali stress, showing that the high pH caused by the NaHCO₃ disturbed the photoprotection mechanism depending on the xanthophyll cycle. Overall, the H. jubatum had stronger tolerance of salt than that of drought and alkali, and the negative effects of alkali stress on growth and photosynthetic performance of this species were most serious.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

CS wrote the manuscript and did the experiment. FY analyzed the data. ZL and XD did a part of the experiment. YL revised the grammar mistakes. JW and JL conceived and directed the manuscript. All authors contributed to manuscript revision and gave final approval for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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