Coleanthus subtilis is characterized by quite a wide intrapopulation morphological variability in response to the environmental conditions, however this relationship has not been analyzed in detail so far. Therefore, we aimed to link plant performance in the model population to abiotic (bottom sediment pH and fertility) and biotic (co-occurring and potentially competing plant species) conditions of growth. We showed that the morphology of C. subtilis specimens was diversified in terms of both the size of individuals and their phenotypes and proved that this variability was related to the pH and macroelement content in the sediment of the site as well as it could be related to the microrelief of the site, which diversified the most such factors as moisture and light availability.
It is already known that the morphotypes of C. subtilis develop in relation to the macroelement contents in the sediment and moisture of the site, with typical forms present in places with organic material deposition and moisture maintained during the life of a plant; on the contrary, when the site is easily drying off (mostly sandy substrate) plants are much smaller (forma nana) (Hejný 1969; John 2011). The latter situation corresponds well to class I of specimens (Figs. 2 and 3). Importantly, a dwarfy phenotype of plants correlated with increased pH and the content of P, as well as the decreased content of N, shown in analyses of bottom sediments (Fig. 3) and by Ellenberg ecological indicators. This further indicates that such conditions are suboptimal for C. subtilis development and growth, what is in agreement with previously published data (Richert et al. 2014; Dajdok et al. 2017). On the other hand, low reaction of bottom sediments (expressed as both pH and the values of Ellenberg ecological indicator of soil reaction, R) as well as the high content of N, favored the bigger size and biomass of C. subtilis (Fig. 3). Moreover, plots with high values of Ellenberg ecological indicators for moisture (M) as well as low values for light indicator (L) and temperature (T) were dominated by bigger specimens of C. subtilis (classes II and III). It is in line with the data of Šumberová et al. (2006) that high temperature, drought and high pH of substrate are the main limiting factors for C. subtilis. It is important to stress here that the light conditions, created locally by the microrelief of the site and shading by co-occurring plants affected strongly C. subtilis morphology. Elongated plants, with limpy stems and leaves (forma laxa) developed, when individuals of C. subtilis were rapidly overgrown by taller plants (e.g., Polygonum spp.) as shown also in other populations of the species (Richert et al. 2014). However, our research showed that such elongated forms could also be observed in patches with high density of individuals, which results from the numerous seeds produced in previous seasons and deposited in bottom sediments. When the conditions are suitable, seeds germinate in a great abundance, as shown by Bernhardt et al. (2013). Our analyses showed that preferences of the model population of C. subtilis (in one water reservoir) are consistent with the general trends discovered previously when the relationship of the C. subtilis abundance and habitat conditions were analyzed in the regional scale (pond complex in SW Poland; Dajdok et al., 2017)). This species was shown to prefer substrates with lower pH and P, and at the same time rich in N, K, Ca, Mg and Na. In the present study, we have found less correlations between element contents in sediment and the size of C. subtilis individuals, which may be related to the lower diversity of habitats due to the smaller scale, and thus somewhat limited ranges of element concentrations in bottom sediments, which were wider (especially for Ca, Mg, P) in the regional scale (Dajdok et al. 2017). Similar relationships were already indicated by many authors (e.g., Hejný and Husák, 1978; Šumberová et al., 2006; Šumberová and Hrivnák, 2013). Also, Hrivnák et al. (2010) showed that the impact of P and nitrate content on macrophyte species composition was greater in a large scale, while in a microscale the correlations weakened. Although differences in the macroelement concentrations in sediments within one pond were less pronounced than in the pond complex studied before (Dajdok et al. 2017), the phytosociological results showed comparable patch diversity in terms of the C. subtilis quantity. Many patches with high cover values (3–5) of this species were observed, but phytocoenoses with only sporadic participation of C. subtilis specimens were also found (Appendix A). This quantitative diversity further suggests that other factors, such as, e.g., the microrelief affecting the humidity and light conditions and, indirectly, the time of the exposure of the bottom part of reservoir enabling seed germination and influencing plant developmental stage, have the impact on the occurrence of C. subtilis specimens.
The characteristic feature of vegetation patches with C. subtilis analyzed in this study, was their low diversity and low species richness, which can result from the relatively uniform substrate conditions, as indicated by the moderately diverse content of basic elements, and the type of the pond's fishery management (which in this case was nursery fishponds). The low species richness of the vegetation that develops on exposed bottoms of nursery fishponds was also previously pointed by Šumberová et al. (2006), who related this feature to the relatively short period of time when the bottom is available for plants, and which also favors the species of the extremely short life cycle (Šumberová and Hrivnák 2013).
Phytosociologically, such patches of vegetation with abundant participation of C. subtilis are usually included into the association Polygono-Eleocharitetum ovatae Eggler 1933, within the alliance Elatini-Eleocharition ovatae Philippi 1968 and the class Isoëto-Nanojuncetea Br.-Bl. et R. Tx. 1943 (Šumberová 2011; Šumberová and Hrivnák 2013; Richert et al. 2016). However, there is a disagreement about the rank and syntaxonomy of the community with C. subtilis, thus some authors classified it as a separate association Coleantho-Spergularietum echinospermae (Vicherek 1972) Hejný 1978 (Hejný and Husák 1978) or subassociation within Cypero-Limoselletum (Oberdorfer 1957) Korneck 1965 as Taran (1995) did in relation to data from eastern Siberia, C.-L. coleanthetosum Taran 1994. In the present study, we classified the analyzed patches of vegetation as a community with C. subtilis (within the alliance Eleocharition soloniensis Philippi 1968, class Isoëto-Nanojuncetea), as was done in the recently published comprehensive classification of ephemeral vegetation in Poland (Kącki et al. 2021). The high values of the substrate moisture index (M), shown in our analysis of the model population, confirmed that patches with C. subtilis have the highest demands on moisture and light in comparison with other plant communities of the Isoëto-Nanojuncetea class as suggested earlier by Šumberová and Hrivnák (2013).
In the framework of the study discussed here, quite a noticeable variation of vegetation patches was detected, affected by the site microrelief. Namely, places which were earlier boar-rooted (during the period when the pond was empty, in November-March) were dominated by Callitriche palustris, the species which is able to develop in shallow water (Šumberová et al. 2006), while C. subtilis and other species were more numerous on the elevations, in places subjected to more rapid exposure as the pond was drying off. However, the role of microrelief in shaping vegetation patches requires more detailed research. Likely, it can account for the development of patches with the absolute dominance of C. subtilis, as while habitats are homogenous in terms of trophic and moisture conditions, then even the subtle advantage as, e.g., an earlier exposed bottom of the reservoir (micro-elevation) speeds up seed germination and seedling growth, and in turn may enable individuals of C. subtilis to outcompete the other plants. On the other hand, lagging biomass of undecomposed plant debris from the previous season may be a factor limiting germination. Such a situation was observed during monitoring of this model population, in the years before the current study was undertaken (Dajdok, Klink & Bartosz, not published). Within the pond, where the present study was conducted, significantly fewer individuals of C. subtilis were recorded on the sites with left biomass of Polygonum lapathifolium individuals than in parts of the pond characterized by an exposed bottom surface. The slower growth rate of seedlings in sites covered by the biomass layer compared to the exposed substrate has also recently been shown by Navratilová and Navratil (2022).
The other important factor determining the composition of phytocoenoses and biomass of C. subtilis can be the allelopathic potential of this species (Inderjit and Callaway 2003). Allelopathy is defined as any process where secondary metabolites of plants, fungi, or microorganisms have the impact (positive or negative) on the growth and/or development of biological systems (excluding animals). It is a form of competitive plant-plant interactions that may increase the ability of a plant to colonize and establish in new ecosystems (Kruse et al. 2000). Experiments performed in this study showed negative allelopathic effects of C. subtilis on germination and initial root growth of Sinapis alba at high concentrations of extract (Fig. 4). Interestingly, the allelopathic effect was positive for hypocotyl growth of S. alba at low concentrations. Positive allelopathy at low concentrations is not very common but was observed previously (Sinkkonen 2006). The allelopathic inhibition may lead to a homogenic composition of phytocoenoses due to selective pressure on susceptible species (Weidenhamer 2006) and facilitate maintaining high density of C. subtilis, primarily conditioned by the number of seeds deposited in the substrate in the previous growing seasons (Dajdok 2012; Münzbergová 2012). The allelopathic potential may be especially important in the early stages of C. subtilis growth and the formation of communities with the species, as it may delay and/or reduce the germination of other species and thus their competition (Sicker et al. 2003). It was shown that even very low concentrations of allelochemicals, if constantly delivered to the habitat, may affect the diversity of plant communities and the distribution pattern of some species (Kruse et al. 2000).
Under natural conditions, the negative effects of allelochemicals usually result from combined impact of several compounds (Kruse et al. 2000). The study of An et al. (2001) showed that the phytotoxic effect is dependent on the proper proportion of compounds. The specific compounds, which could be responsible for allelopathic potential of C. subtilis, were identified for the first time.
The detected compounds in the analyzed plant extract belong to different classes of natural substances including benzoxazinone derivatives (HMBOA, MBOA), phenolic acids (benzoic acid, coumaric acid), as well as dicarboxylic acids (azelaic acid). All of them could be involved in allelopathy either alone or as a complex mixture. Phenolic compounds are known to be of great significance in allelopathy (Inderjit 1996), they are one of the main groups of phytotoxic substances associated with wheat allelopathy (Guenzi and McCalla 1966). Phenolic acids like benzoic acid and coumaric acid are common and widespread phytotoxic agents that are able to affect growth at various stages of plant development (Blum 1996; Chung et al. 2001; Bouhaouel et al. 2019). Likewise, benzoxazinones and their metabolites from corn, wheat, and rye are of great importance in the contest of overall phytotoxicity. Ferulic acid was identified in Medicago sativa, Triticum aestivum and Secale cereale as one of allelochemicals responsible for weed suppressing ability (Kohli et al. 2006). Vitexin was proven to inhibit germination and growth of Tortula muralis and Raphanus sativus (Basile et al. 2003). Phytotoxic activity of these compounds is also considered with respect to allelopathic interaction with other plants (Friebe 2001; Gierl and Frey 2001; Huang et al. 2003; La Hovary et al. 2016). In the study by Ma et al. (2011), the authors determined the main allelopathic substance of Jatropha curcas leaves and roots as azelaic acid. This compound is also mentioned as a one of the allelochemicals in rice Oryza sativa (Rimando et al. 2001; Ma et al. 2011).
To sum up, we may conclude that the compounds mentioned above are responsible for the perceived allelopathic activity of the investigated C. subtilis water extract. On the other hand, the regulation of the expression of biochemical responses is complicated involving external factors and the biological state of plants. In particular, stress, including harsh environmental conditions, competition, and nutrient limitation, may induce qualitative changes in secondary metabolites (Kruse et al. 2000; Siemens et al. 2002; Gawrońska and Golisz 2006) as well as the increase in concentrations of phytotoxins (Tang et al. 1995; Weidenhamer 2006), which may enhance the allelopathic activity. Also, the actual effects of allelochemicals in natural environment are influenced by habitat conditions, such as soil reaction, organic matter, nutrient and moisture content and presence of xenobiotics, and may differ from observed in laboratory conditions (Kruse et al. 2000; Hoagland and Williams 2003; Scavo et al. 2019). Moreover, in the phytocoenoses with C. subtilis, the competition with other species with the allelopathic potential is also possible and could complicate the interspecific interactions (Weidenhamer et al. 2023). In particular, Veronica peregrina which reached coverage in the range of 25–50% in some patches (Appendix A), was showed to have allelopathic potential (Polechońska et al. 2020). However, these issues require further research.