Seed germination is a fundamental stage in the life cycle of a plant (Bewley, 1997; Nimbalkar et al., 2020). Seed germination is significantly affected by both physical and biological factors such as temperature and species identity (Larsen et al., 2004; Bewley et al., 2013; Zhang et al., 2020). Soil salinization is one of the major drivers of soil degradation (Zhang et al., 2015a; Gorji et al., 2017), and it can significantly affect seed germination and the following stages such as seedling establishment (Khan and Gulzar, 2003; Qu et al., 2008). Over 900 Mha land is impacted by salinity in the whole world (Rengasamy, 2006; Shiade and Boelt, 2020). Climate change such as extreme warming is expected to be more frequent in the future (Khan and Qaiser, 2006; Blackport and Screen, 2020; Bai et al., 2021). Such change could significantly affect seed germination (Walck et al., 2011; Mondoni et al., 2012). Soil salinization could become more serious in the face of climate change because global warming generally increases evaporation, which can promote soil salinization (Utset and Borroto, 2001). Therefore, salinity and temperature would jointly affect seed germination, especially in the arid and semi-arid areas of northeastern China, where the soil salinization area covers over 70% of the total land area (Wang et al., 2011). Moreover, several species are facing population reductions due to human disturbances and climate change (Richmond et al., 2007; Ureta et al., 2012; Gu et al., 2018). Thus, exploring seed germination under the ongoing soil salinization and global warming is important in assessing the stability of plant community.

Theoretically, the seed germination of each species has an optimal temperature, under which seeds could germinate better than under other temperatures. Previous studies found that salinity decreased seed germination of some species compared with non-saline conditions (Khan and Gulzar, 2003; Qu et al., 2008). However, the impact of salinity on seed germination might be modified by temperature, as Gorai and Neffati (2007) found that negative effects of salinity on seed germination were less severe at the optimum temperature, as the additional environmental stress at low or high temperatures would thus be alleviated (Al-Khateeb, 2006). Yet, Khan and Ungar (2001) found that the effect of salinity was stronger at lower temperatures, while Delesalle and Blum (1994) revealed that such effect was stronger at higher temperatures. Finally, Khan and Ungar (1998) showed that the effect of salinity was not affected by temperature in their experiment. Thus, the joint effects of salinity and temperature on seed germination are still unclear (Fernandez et al., 2015; Lin et al., 2018).

In response to local salinity and suboptimal temperatures, plant species developed different strategies, including adjusting germination percentage or germination rate through modifying seed dormancy and/or seed viability (Ungar, 1995; Khan et al., 2001; Khan and Ungar, 2001; Shahba et al., 2008; Guan et al., 2009). Such responses can further alter seedling establishment and seedling growth (Gu et al., 2018; Del Vecchio et al., 2021). Exploring the effects of salinity and temperature on seed germination may shed light on understanding the mechanisms of species coexistence. However, studying such effects under natural conditions is difficult since (1) soil conditions such as temperature and salinity vary spatially and temporally (Hermans et al., 2016), which makes it difficult to keep a constant level of temperature or salinity. (2) Other environmental variables such as radiation and soil moisture hamper separating the roles of temperature and salinity from these factors (Khan and Ungar, 1997; De Boeck et al., 2015; Borja et al., 2016; Bhatt et al., 2020). (3) Some particular species in a community such as halophytes and xerophytes may skew the results, where halophytes can modify their strategies (e.g., reduce seed germination percentage or delay the start of germination under the high level of salinity) to adapt to different salinity levels (Gulzar and Khan, 2001; Khan and Gul, 2006; El-Keblawy et al., 2020), and xerophytes can grow well under conditions with a large variation of temperature (Zhang et al., 2015b).

To explore the joint effects of temperature and salinity on seed germination of grass species with less confounding factors (Figure 1), a controlled experiment was thus conducted. Three typical grass species (Festuca arundinacea, Bromus inermis, and Elymus breviaristatus) widely used as forage species (Lu et al., 2008) that can be potentially grown in saline soils were exposed to three levels of temperature and five levels of salinity. Specifically, (1) we expect seed germination in general to be the highest at the intermediate level of temperature (20°C), which is thought to be closest to the optimal temperature for seed germination for such grasses (Romo and Eddleman, 1995; Lu et al., 2008; Zhang et al., 2013). (2) We assume that seed germination would consistently decrease with increasing salinity (Wu et al., 2015; Zhang and Dai, 2019). (3) We anticipate that the intermediate (and supposed optimum) temperature level would alleviate the negative effects of salinity on seed germination (Gorai and Neffati, 2007).

In the first analysis, GP, AGR, and DGR varied within species, germination date, and species × germination date interaction (Table 2 and Figure 2). On average, the GP of F. arundinacea was higher than that of E. breviaristatus and B. inermis, and the GP of E. breviaristatus was in turn higher than that of B. inermis (Figure 2A). Such a pattern was also found for AGR (Figure 2B) and DGR (Figure 2C). B. inermis germinated faster at the beginning of the experiment, while its germination decreased faster than the other two species during the experiment (Figure 2C). The interaction effect between species and germination date was likely caused by the convergence of the seed germination (Figure 2).

In the second analysis, on investigating the effects of species, temperature, and their interaction on GP, the three target species demonstrated different responses (Table 3 and Figure 3). The GP of F. arundinacea was not sensitive to the relatively high levels of temperature (Figure 3A). The GP of B. inermis was highest at the intermediate temperature level (Figure 3B), and the GP of E. breviaristatus was highest at the highest temperature level in this study (Figure 3C).

In the third analysis, on testing the effects of species, salinity, and their interaction on GP, the three target species likewise showed different patterns (Table 4 and Figure 3). The GP of F. arundinacea was not sensitive to relatively low levels of salinity. However, the other two species showed a different pattern, where the higher salinity levels decreased the GP of B. inermis, while the intermediate level of salinity increased. The GP of E. breviaristatus consistently decreased with increasing salinity levels. Moreover, the intermediate temperature level (i.e., 20°C) × lowest salinity level (i.e., 0 mmol/L) generated the highest GP for F. arundinacea, while the highest temperature level (i.e., 25°C) × lowest salinity level (i.e., 0 mmol/L) generated the highest GP for B. inermis and E. breviaristatus (Figure 4).

Finally, exploring the effects at the last day of the experiment, species, temperature, salinity, species × temperature, species salinity, and species × temperature × salinity significantly affected GP, AGR, and DGR (Table 5 and Figure 4), while there were no significant temperature × salinity effects at this measurement data.

The first hypothesis stated that seed germination would be the highest at the intermediate level of temperature. This was partly supported as such a pattern was found in one of the target plant species (i.e., B. inermis, Figure 3B), where lower germination was found at lower temperatures. This is partly consistent with the finding of Ao et al. (2014), where seed germination of B. inermis was low at lower temperatures. Note that such a pattern was not found in the other two target species. For F. arundinacea, temperature levels in this study may have all been in the optimal temperature range of this species (Lu et al., 2008), while for E. breviaristatus, the optimal temperature of seed germination might have been higher than the temperature levels we set (Figure 3C).

Our second hypothesis aimed to test whether seed germination would be reduced at higher levels of salinity. This was supported as seed germination of the three target species was generally lower at higher salinity levels, even though they responded inconsistently to the salinity gradient (Figure 3). Such results are in line with previous studies on the target species F. arundinacea (Shiade and Boelt, 2020), B. inermis (Yang et al., 2009). and E. breviaristatus (Li et al., 2019), and on other species such as Helianthus annuus (Wu et al., 2015), Oryza sativa (Xu et al., 2011), and Zea mays (Khodarahmpour et al., 2012). Such results could be related to the effects of ion toxicity on seed germination (Panuccio et al., 2014). The different responses of plants to salinity are likely caused by the genetic traits of these species (Vu et al., 2015; Chamorro et al., 2017) and their growing conditions (Mira et al., 2017).

The last hypothesis focused on the joint effects of salinity and temperature on seed germination, and we expected that the negative effect of salinity on seed germination would be alleviated at the intermediate level of temperature. This was supported by our findings in one of the three target species (B. inermis, Figure 3B), where the germination percentage of B. inermis at the intermediate temperature level was higher than at the other two temperature levels, and the germination percentage decreased more slowly with increasing salinity compared with the other two temperature levels. This is in line with the finding of Gorai and Neffati (2007), where the negative effect of salinity on seed germination was alleviated at the optimum temperature. However, the other two species did not show such a pattern.

Results of this study should be interpreted and extrapolated with caution because of the following two reasons. One is that NaCl solutions in this study might evaporate at different rates when they were set under different temperatures during the experiment (Sayer et al., 2017), and this may affect the ultimate salinity level and thus the ensuing results. The other is that each level of temperature was kept constant during the experiment in this study, while previous studies found that variation of temperature can benefit seed germination (Liu et al., 2013, 2017a; Spindelböck et al., 2013; Burghardt et al., 2016). Moreover, soil resources such as soil temperature and salinity vary a lot even at a short distance in natural conditions (Maestre et al., 2003; Lundholm, 2010). Thus future studies on seed germination should consider the heterogeneous distributions of these factors, potentially in combination with other aspects of soil heterogeneity (e.g., Liu et al., 2017b,c, 2019; Liu and Hou, 2021).

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

YL designed the study, conducted the analyses, and wrote the first draft of the manuscript. SZ collected the data. All authors contributed significantly to the manuscript.

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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