4.2. Species Coexistence and Response to Salinity
Interaction between plant species has been assessed mostly from the point of resource (light, water, nutrients) availability [
33] sometimes analyzing also the effect of other environmental factors [
48]. It is usually found that in relatively optimal conditions plants have a stronger tendency for competition due to faster resource acquisition, while in less optimal conditions cooperation will prevail [
49,
50]. However, in the case of interaction between individuals belonging to different species, self/non-self recognition can be an important additional factor [
51]. Therefore, in the present study, we tried to find out how physiological characteristics are affected by the cocultivation of two individuals from different species in comparison to those of individuals from a single species, with a substrate salinity acting as a main environmental factor.
The coexistence of species in natural conditions, including salt marshes, is manifested by both positive and negative interactions [
52]. Within biotic interactions, competition is likely the most important factor for the establishment of zonation in salt marshes [
53]. Moreover, it was demonstrated that a trade-off between the competitive ability of a species and its stress tolerance contributes to plant zonation in salt marshes and other plant communities [
33], but physiological mechanisms of the competition are far from clear [
54]. If two species have different tolerance to some environmental factor, a change of intensity of this factor could result in a shift of competition-related balance between the species. Therefore, it is logical to assume that the competitive ability of more salt-tolerant (or halophytic) species increases with an increase in soil salinity in further positively affecting its distribution [
53]. In the present study, it was hypothesized that plants of potentially more salt-tolerant clover species
T. fragiferum will have better physiological performance and growth in comparison to
T. repens only if coexisting in conditions of elevated soil salinity.
Based on the literature analysis, it was expected that
T. fragiferum will show better salinity tolerance [
5,
55] as compared to
T. repens [
8]. Both plant species indeed differentially responded to NaCl treatment. For
T. fragiferum, NaCl treatment significantly increased shoot biomass both in single species and species coexistence conditions except for dry mass in plants inoculated with rhizobia from
T. repens in species coexistence (
Figure 2). Increased shoot fresh mass in
T. fragiferum by NaCl treatment was partially due to increased tissue water content (
Table 4). It is well known that enhanced tissue water content as a result of salinity is a characteristic adaptive response of both halophytes and glycophytes [
56] and could be related to increased salinity tolerance. In contrast, NaCl treatment resulted in a significant decrease in shoot biomass of
T. repens in non-inoculated plants in single species conditions and, most severely, in plants inoculated with rhizobia from
T. fragiferum during species coexistence (
Figure 2). A contradictory effect of NaCl treatment was evident in respect to shoot fresh mass (decrease) and dry mass (no significant effect) for
T. repens plants in species coexistence conditions inoculated with rhizobia from
T. repens, showing that salinity induced a decrease in tissue water content in this treatment combination. Experiments with more realistic NaCl treatment, mimicking short-term seawater flooding conditions, have revealed that
T. repens plants are negatively affected by soil flooding with saline water, but also indicated that the response may be ecotype-specific [
57].
Species coexistence had a significant negative relative effect on the growth of
T. fragiferum for non-inoculated plants and plants inoculated with rhizobia from
T. fragiferum but only without NaCl (
Table 4). Significant coexistence-dependent growth stimulation of
T. fragiferum was evident only for dry mass in plants inoculated with rhizobia from
T. repens and cultivated without NaCl. In contrast, species coexistence benefited
T. repens plants under NaCl treatment without bacterial inoculation and plants without NaCl treatment but inoculated with rhizobia from
T. fragiferum (
Table 4). A significant negative effect of species coexistence on the growth of
T. repens was evident for plants inoculated with rhizobia from
T. repens without NaCl and for plants inoculated with rhizobia from
T. fragiferum with NaCl. Both morphological and physiological data confirmed that
T. repens is a more dominant species than
T. fragiferum, but the dominance was significantly decreased by NaCl treatment and inoculation with rhizobia that in part could be related to nitrogen-dependent changes in competition characteristics between legume species [
14]. Besides competition for resources, the negative effect of species coexistence could be related also to the production and release of allelopathic compounds [
58].
The most striking effect of species coexistence as affected by rhizobial inoculant was seen in the presence of rhizobia from
T. repens, where shoot water content decreased in
T. fragiferum and increased for
T. repens (
Table 4). The effect was clearly due to species coexistence, as it was not evident in single species conditions. The addition of NaCl fully reversed this hydration-related effect. No similar relationship was seen in the case of inoculant from
T. fragiferum, pointing to some specific characteristics of rhizobial isolates, disappearing in conditions of salinity.
Returning to the initial hypothesis, namely, that rare species T. fragiferum can outcompete more common species T. repens only in conditions of increased substrate salinity, it appeared that NaCl treatment indeed had a higher positive effect on the growth of T. fragiferum in conditions of species coexistence, but the positive effect of bacterial inoculant in coexisting species, especially by rhizobia isolated from T. repens, was significantly more pronounced. We did not directly compare responses of single individual plants with those of two individuals of either species (species coexistence). However, preliminary experiments showed that single individuals of either T. fragiferum or T. repens responded to substrate salinity differentially in comparison to two individuals of the same species (Ievinsh et al., unpublished results). Thus, the physiological response of the individual changed in respect to the identity of the coexisting individual; e.g., the outcome of the response of the individual to environmental cues is predicted by the interaction between species.
Possible physiological mechanisms of more pronounced salinity tolerance in
T. fragiferum, based on the present results, need to be discussed in more detail. The reduction in root to shoot transport of Na
+ is described as one of the mechanisms related to increased NaCl tolerance, at least, for glycophytic species [
59]. In the present study, the compartmentation of Na
+ in different parts of shoots of
T. fragiferum and
T. repens was evident, with a higher accumulation of the ion in leaf petioles. For
T. repens, Na
+ was excluded from flowers, accumulating in flower stalks. NaCl-treated
T. fragiferum plants had significantly higher concentrations of Na
+ in all studied plant parts in comparison to
T. repens (
Table 10). Consequently,
T. repens exhibited a typical feature of partial Na
+ exclusion from shoots, while
T. fragiferum accumulated Na
+ in shoots, representing a higher degree of cellular tolerance possibly due to efficient Na
+ compartmentation in cell vacuoles [
60] and by means of the active antioxidant system [
61].
K
+:Na
+ homeostasis is considered to be one of the major traits for salinity tolerance, at least, for glycophyte species [
62]. In the present study, NaCl treatment had a significant effect on K
+ concentration in leaves in some treatment combinations, but in general, variation in the K
+:Na
+ concentration ratio between treatments was not associated with observed differences in plant growth. It seems that at least
T. fragiferum can efficiently replace K
+ with Na
+ for maintenance of cellular osmotic functions, as already noted in previous studies with potential halophytic species [
32].
The decrease in leaf chlorophyll concentration and photosynthesis, including photochemical efficiency of photosystem II, is a well-documented response of plants to suboptimal NaCl treatment [
63,
64]. In a study with
T. repens, it was found that no decrease in maximum quantum yield of photosystem II (Fv/Fm) by NaCl treatment up to 100 mM was evident [
65]. No general direct effect of NaCl on photosynthesis-related characteristics was found in the present study but NaCl treatment significantly affected the effect of other factors on these parameters (
Table 9).
In a study with a halophytic plant species
Cakile maritima, it was shown that salinity-induced a decrease in plant photosynthetic activity that further led to decreased growth and reproduction [
66]. Quantum yield of photosystem II of
C. maritima increased at optimum salinity (100 mM NaCl) but was significantly decreased at 300 and 500 mM NaCl. In the present study, the Performance Index was not much affected by the gradual NaCl increase in the substrate. For rhizobia-inoculated
T. fragiferum plants under single species conditions, the Performance Index significantly increased by NaCl treatment only at the end of the experiment, but this effect was not evident under species coexistence (
Figure 2). For
T. repens under single species conditions, non-inoculated plants exhibited a temporal increase in the Performance Index by NaCl treatment, but for plants inoculated with rhizobia from
T. repens, the Performance Index level temporarily decreased in NaCl-treated plants.
While biological factors are a less studied type of environmental influence, it is logical to presume that for legume species symbiosis with nitrogen-fixing rhizobial bacteria is a critical factor in interspecific interactions. In a microcosm study with a number of dune grassland species, it was established that three of the four studied legume species require rhizobial symbiosis for successful coexistence with non-legume species [
14]. This aspect will be discussed further (
Section 4.3).
It should be noted also that in natural conditions of a salt-affected meadow,
T. fragiferum plants have a significant level of arbuscular mycorrhizal colonization in roots, reaching both high frequency (99%) and intensity (68%) [
47]. In experiments with
T. repens, it has been shown that the presence of arbuscular mycorrhizal symbiosis can modify how clonal integration affects plant performance in heterogeneous environmental conditions [
67]. Arbuscular mycorrhizal symbiosis is shown to be important also for salinity tolerance [
68]. Consequently, in natural conditions, not only rhizobial symbiosis but also mycorrhiza can have a significant impact on the result of the coexistence of allied species.
4.3. Bacterial Inoculation
Inoculation with native symbiosis-forming N
2-fixing bacteria was a significant factor in the present study affecting the outcome of the interaction between
T. fragiferum and
T. repens. In nutrient-limiting conditions, rhizobial inoculation usually has a positive effect on the growth of clover species and other legume plants, depending on both the compatibility and efficiency of used bacterial strains [
69]. Indeed, both fresh and dry mass of
Trifolium plant shoots increased in all treatment combinations inoculated with rhizobia (
Figure 2). However, it is difficult to unequivocally interpret the mineral availability status of plants in the present experiments. First, both nitrate-N and ammonia-N in equal amounts were present in the fertilizer used here. Both total amount and balance between different chemical forms of N can significantly affect plant growth and development in a species-specific manner [
70,
71], as well as the rhizobial symbiosis of legume species including clover [
72]. At least, the use of equal proportions of both forms of N allowed to exclude any specific effects associated with the use of a particular chemical form of N. Second, nitrogen availability and its chemical forms itself regulates salt tolerance in plants [
73].
Direct comparison of nitrogen-fixation efficiency of different
Trifolium species by the two specific isolates of rhizobia was outside the scope of the present study. However, based solely on growth data, it is possible to assume that general specificity to a particular rhizobial isolate was comparably low. In single species conditions, the accumulation of shoot dry matter of both
T. fragiferum and
T. repens was not statistically different between treatments with either rhizobia (
Figure 2B). The identity of bacterial isolate could affect the fate of species interaction, as, in contrast to what was expected, inoculation with rhizobia from
T. repens in
T. fragiferum plants in species coexistence conditions without NaCl treatment resulted in a two-fold increase in shoot dry matter in comparison to inoculation with rhizobia from
T. fragiferum (
Figure 2B). In the respective treatments,
T. repens plants exhibited a two-fold decrease in shoot dry matter. In conditions of coexistence of different legume species, an individual of one species can benefit from nitrogen fixed by an individual from another species due to more efficient rhizobial symbiosis of the latter, similar to what has been described for interaction between legume and non-legume species [
74]; or due to more salt-tolerant symbiotic bacteria. It is important to note that the presence of NaCl completely abolished the mentioned effect. It is usually thought that nodule-forming rhizobia are more salinity tolerant than their host plants [
75], but it is clear from the present results that differences in salinity tolerance of rhizobial strains can have a significant effect on the outcome of species coexistence in conditions of a salt marsh. Yet, functional aspects of rhizobial symbiosis are highly sensitive to salinity, with nitrogen fixation being more vulnerable in comparison to ammonium assimilation [
76].
It appears that the nitrogen-fixing ability of rhizobia isolated from
T. repens was significantly more pronounced in comparison to that from
T. fragiferum, allowing
T. fragiferum plants to benefit from rhizobial symbiosis in
T. repens plants in conditions of species coexistence, but this needs to be examined further. Specificity of rhizobia could be a less important aspect in rhizobia–clover interaction in natural conditions, as compared to differences in the efficiency of particular strains and by different bacteria–clover combinations [
69]. A comparative study from a subtropical region showed a large diversity in respect to nodule-forming species in roots of
T. fragiferum un
T. repens: both plants can form nodules with bacterial species from genera
Bradyrhizobium and
Rhizobium (predominantly strains of
Rhizobium leguminosarum), but
T. repens plants can also have symbiosis with
Sinrhizobium species, while
T. fragiferum with
Mesorhizobium species [
77]. Moreover, it is not clear whether the particular inoculant consisted only of one rhizobial strain.
Several mechanisms have been proposed for the beneficial effect of bacterial inoculants in conditions of increased soil salinity. Tissue-specific regulation of the Na
+ transporter by a soil bacterium
Bacillus subtilis has been shown to represent a critical mechanism for the maintenance of low Na
+ in
Arabidopsis thaliana in further conferring increased NaCl tolerance [
78]. This mechanism is likely to be effective also in the case of
B. subtilis-induced salt tolerance of wheat [
79]. Inoculation of
T. repens with
Azospirillum brasilense, a free-living soil bacterium, increased the leaf chlorophyll concentration of plants both in control (by 62%) and low salinity (64, 75, and 61%, in 40, 80, and 120 mM NaCl, respectively) conditions [
80]. In addition, inoculation of
T. repens with
A. brasilense significantly reduced Na
+ accumulation in shoots and roots in low salinity conditions [
80]. In the present study, tissue Na
+ concentration decreased by rhizobial inoculation in leaves and stolons of NaCl-treated
T. fragiferum plants in single species conditions but increased by inoculation in leaves and stolons of
T. repens plants except in leaves of single species plants inoculated with rhizobia from
T. fragiferum (
Table 10). A relatively close correlation between Na
+ concentration and a potential indicator of peroxidative membrane damage, TBARS concentration, was evident, indicating that a decrease in tissue Na
+ concentration could be associated with less tissue damage.
Some additional evidence for rhizobial symbiosis-dependent protection against NaCl could be derived from analysis of peroxidase activity in leaves where bacterial inoculation with either rhizobia resulted in a significant increase in relative peroxidase activity. This increase was relatively higher for T. repens in single species conditions.
4.4. Evaluation of Cellular Damage and Physiological Performance
Among biochemical indicators of cellular damage, measurement of malondialdehyde (MDA) concentration in tissues by means of the analysis of TBARS concentration has been widely used as an indicator of the relative degree of lipid peroxidation due to unfavorable conditions leading to endogenous oxidative stress [
81]. In particular, salinity usually increases tissue MDA concentration in sensitive species and cultivars but not in resistant ones [
82]. For a halophytic species
Atriplex portulacoides, no significant increase in leaf MDA concentration was seen even at 1000 mM NaCl salinity [
63]. During the comparison of two accessions of halophyte
Cakile maritima, it was found that growth stimulation of the more salt-tolerant accession occurred at 100 mM NaCl, together with decreased MDA concentration, 66% lower than that of control plants [
61]. In the present study, there were statistically significant differences in TBARS concentration in leaves of both
Trifolium species between different treatments (
Figure 7). While, according to the ANOVA results, there was no general trend of effect on TBARS concentration by any of the experimental factors, this parameter was significantly increased by NaCl treatment in several treatment combinations. Moreover, when only NaCl-treated plants were considered, the leaf TBARS concentration relatively tightly positively correlated with leaf Na
+ concentration. Most interestingly, the coexistence of
T. repens with
T. fragiferum completely eliminated the stimulative effect of NaCl treatment on TBARS concentration in
T. repens, which was evident in single species conditions (
Figure 7). In
T. repens plants not treated with NaCl, high TBARS concentration in leaves was evidently caused by other factors, i.e., coexistence with
T. fragiferum together with rhizobial inoculation.
Relative electrolyte leakage in leaves of a legume
Glycine soja plants (10%) increased up to 30% by 150 mM NaCl treatment [
44], which is in a range of changes documented also in the present study. Electrolyte leakage was highly significantly affected by NaCl treatment in the present study (
Table 13). It is usually thought that electrolyte leakage in NaCl-treated plants is a direct consequence of the effect of Na on membrane permeability [
64], but no significant correlation between TBARS concentration and electrolyte leakage was evident. During the comparison of MDA concentration and degree of membrane leakage in Zea mays plants subjected to salinity, it was concluded that permeability characteristics of membranes could be affected by factors other than lipid peroxidation [
83]. Furthermore, it was argued that electrolyte leakage from senescing leaves does not represent a reliable characteristic of cell membrane damage [
84]. As no inhibition of photochemical efficiency of photosystem II was caused by NaCl treatment in spite of the significant increase in TBARS concentration in several treatment combinations, it seems that in the conditions of the present experiment NaCl treatment did not result in a significant increase in oxidative damage to chloroplastic macromolecular structures over some basal level. Consequently, recorded changes in growth between different treatment combinations were not associated with deleterious effects at the level of tissue damage but rather were reflections of physiological changes due to species coexistence, NaCl treatment, rhizobial inoculation, and interactions between them.
It is usually found that salinity-induced increase in peroxidase activity is a characteristic response of more tolerant species indicating activation of defense mechanisms [
85]. Thus, salt-tolerant species
Plantago maritima showed a significant increase in leaf peroxidase activity at 100 mM NaCl (no decrease in growth), while salt-sensitive species
Plantago media (53% decrease in dry matter) had no response at this salinity [
86]. However, when time-course analysis was performed, it became evident that peroxidase activity increases in salt-tolerant species occurred only at early stages after the start of the treatment (reflecting induction of defense responses), but for sensitive species, more late increase reflects an enzymatic response to cellular damage [
87]. In the present study, peroxidase activity was significantly affected by NaCl treatment, decreasing in leaves of
T. fragiferum, but the particular effect of treatment on peroxidase activity in
T. repens plants depended on the combination of factors (presence or absence of particular rhizobial inoculant as well as single species conditions vs. species coexistence;
Figure 5). It is possible that the effect of NaCl on peroxidase activity was at least partially due to stimulation of increased tissue water content by the treatment, which was more pronounced in
T. fragiferum (
Table 1). According to ANOVA analysis, when leaf peroxidase activity was expressed on a dry mass basis, it was significantly affected only by inoculant and species, in comparison to the significant effect of all factors in the case when the activity was expressed on a fresh mass basis (
Table 13).
However, while NaCl resulted in decreased activity of polyphenol oxidase in several treatment combinations, there was no uniform response of the enzyme activity to NaCl treatment that might be attributed to indirect (or interactive) effects of salinity on this enzyme. Studies on the effect of abiotic factors besides tissue wounding on polyphenol oxidase activity are relatively scarce. It was shown that an increase in polyphenol oxidase activity in leaves of
Trifolium pratense correlated with the degree of cell damage due to cattle grazing [
88].
Decreased growth in the absence of rhizobial inoculation occurred together with a decrease in leaf chlorophyll concentration (
Figure 3) as well as a lowered photochemical capacity of photosystem II as indicated by chlorophyll
a fluorescence measurements (
Figure 4). Total leaf chlorophyll concentration is often correlated to N nutrition, as a decrease in N tissue availability can result in inhibited chlorophyll synthesis [
89]. It seems that the characteristic decrease in leaf chlorophyll concentration in both
T. fragiferum and
T. repens plants in the absence of rhizobial inoculation reflects some common adaptive mechanism of clover species to nitrogen-fixing rhizobia, where active symbiosis is necessary to maintain high rates of chlorophyll synthesis. Similar to the present results, inoculation with rhizobia of
Phaseolus lunatus plants led to a higher leaf chlorophyll content in comparison to uninoculated, nitrogen-supplied plants [
90]. Inoculation with both types of rhizobia resulted in the early increase in the Performance Index in both studied species (
Figure 4). The highest initial increase for both
Trifolium species was caused by rhizobia from
T. repens in single species conditions and that from
T. fragiferum in species coexistence conditions. It is not known though to what extent depression of photochemical activity in non-inoculated plants in the present study can be a consequence of possible N shortage.