Climate change will pose major challenges for agriculture in the coming decades, as it is expected to affect plant productivity in certain regions (Leisner 2020). Temperature stress and a limited water supply due to drought can directly impact crop yields. In addition, the effects of climate change on soil functions will also impact crop productivity globally (Hamidov et al. 2018). There is convincing evidence showing that human-induced climate change has already affected global crop productivity (Lone et al. 2017). These effects, combined with a growing human population, will make ensuring food security during the coming decades very challenging (Lobell and Gourdji 2012; Lone et al. 2017).

Soil salinization is the result of the accumulation of soluble salts in the soil due to saltwater intrusion, mineral deposition, irregular precipitation patterns, and increased evaporation, often exacerbated by intensive agriculture (Nachshon 2018; Eswar et al. 2021; Atta et al. 2023). Salinization is an often-overlooked effect of climate change (Rahman et al. 2017; Eswar et al. 2021). Saline soils can contain NaCl levels up to as high as 40 mM (~2300 ppm), as compared to standard soils that have NaCl levels between 0 and 20 mM (~1170 ppm) (Mohamad Yunus et al. 2024b). Nonetheless, farmers still attempt to utilize high-salinity soils for agriculture by cultivating salt-tolerant species (Septiana and Analuddin 2019; Mohamad Yunus et al. 2024a). Salt-tolerant species, or halophytes, can exhibit different patterns of salt accumulation in plant tissues (Noda et al. 2022). Most of the halophytes generally accumulate salts in high concentrations in leaves, which gives evidence for a tissue tolerance mechanism (Shabala 2013).

Crop wild relatives (CWRs) are wild plant taxa that are genetically closely related to crop species (Maxted et al. 2006). Since different CWRs are adapted to growing in different habitat conditions and exhibit diverse ecological strategies, they may harbour important characteristics to cope with diverse stress conditions. Exploiting the diversity of these characteristics is one of the main solutions to develop climate resilience in crops to ensure global food security (Satori et al. 2022). CWRs have evolved traits to cope with different climatic conditions, pest outbreaks, diseases, and abiotic stresses (Castañeda-Álvarez et al. 2016). This genetic diversity has been partially lost in modern cultivated varieties due to years of selective breeding (Dempewolf et al. 2017). Recent advances in plant molecular biology have opened new opportunities for breeding to incorporate these valuable traits of CWRs into crops by overcoming the barriers of traditional breeding programs (Bohra et al. 2022).

The genus Vigna Savi (Fabaceae) comprises a wide range of economically important food legumes, distributed in tropical and subtropical regions, including Sri Lanka (Tomooka et al. 2002; Buddenhagen 2014; Yoshida et al. 2020). Different Vigna species show tolerance to various abiotic stresses, allowing them to thrive in sandy saline soils, limestone rocks, waterlogged lands, mountain tops, and shaded ecosystems (Tomooka et al. 2014a). Moreover, their ability to fix nitrogen, even under abiotic stress conditions, is significant and enables them to grow in low-resource environments (Tomooka et al. 2014b). Soil salinity tolerance has evolved multiple times independently within the Vigna genus, as is shown by the different mechanisms that evolved to cope with salinity (Noda et al. 2022). Shankar et al. (2023) identified 12 potentially salt-tolerant wild Vigna species growing in coastal ecosystems. Vigna marina (Burm.) Merr., also known as the Beach bean, is the Vigna species showing the highest salt tolerance (Yoshida et al. 2020). This species resides in coastal ecosystems and can cope with high salt concentrations and low nutrient levels (Mohamad Yunus et al. 2024a, 2024b). Similar to all other Vigna species, V. marina is a CWR of Vigna crop species like Vigna radiata (L.) R.Wilczek and Vigna unguiculata (L.) Walp. (Horton et al. 2024). Most Vigna crops are sensitive to high salinity in the root environment (Lawn and Cottrell 2016). However, Yoshida et al. (2020) describe unique responses of V. marina to high salinity levels, such as the ability to maintain physiological activity, increased stomatal pore opening, increased transpiration, and tolerating salt inside its tissues. The salt tolerance characteristics of V. marina can be utilized to cultivate this useful food legume (Padulosi and Ng 1993; Chankaew et al. 2014; Septiana and Analuddin 2019) as a cover crop in saline soils (Yoshida et al. 2020; Mohamad Yunus et al. 2024a).

Although the capacity for salt tolerance of Vigna marina and the underlying mechanisms have been well documented (Noda et al. 2022; Wang et al. 2024), there is limited understanding of how different populations of this species respond to salinity stress. This knowledge gap is particularly relevant in Sri Lanka, an island nation with a highly diverse and unique coastal ecosystem. Vigna marina occurs across a range of habitats in Sri Lanka, which possibly have varying salinity levels (Bandara 1989). It is therefore reasonable to hypothesize that different populations of V. marina may exhibit varying degrees of salinity tolerance in response to local environmental conditions. Despite its ecological importance, V. marina has received limited conservation attention in Sri Lanka, even though it is listed as an endangered species in the National Red List of Sri Lanka (Biodiversity Secretariat 2020). Consequently, understanding inter-population variation in salinity tolerance is essential for conservation planning and for identifying populations with valuable phenotypic traits for potential breeding programs.

For plants in particular, inter-population variation and phenotypic plasticity are critical adaptive responses to climate change, given that plants are generally less mobile than animals and cannot easily escape rapidly changing environmental conditions (Lazaridi et al. 2017; Henn et al. 2018; Roybal and Butterfield 2018). For instance, Lazaridi et al. (2017) examined inter-population variation in agro-morphological traits of wild Vigna unguiculata populations and highlighted the presence of a valuable gene pool that can be utilized for crop improvement. Similarly, exploring the genetic and phenotypic diversity of V. marina populations in Sri Lanka could provide essential information for both conservation and sustainable utilization efforts.

The main objective of this study is to determine the inter-population variation in salt tolerance of V. marina seedlings. We assess this by studying the salt tolerance of four wild populations of V. marina from the wet coastal zone in Sri Lanka. In addition, we compared the salt tolerance of the V. marina populations with two commercially available V. radiata populations, one considered salt-tolerant and the other not. Specifically, we studied: (i) the growth response of the different V. marina populations in 100, 1000, 2000, 10,000, and 20,000 ppm salinity levels, and (ii) the amount of accumulation of NaCl inside different plant tissues (roots, shoots, and leaves). We expect that V. marina populations show considerable variation in seedling salt tolerance, as the local habitat conditions of these populations differ substantially. Further, we hypothesized that populations occurring in highly saline locations have evolved a greater salinity tolerance, as natural selection tends to favour such traits under prolonged salt stress. Accordingly, we predict that halophytic V. marina seedlings will outperform their domesticated relative, V. radiata whose seedlings typically inhibit non-saline agricultural soils, when both are subjected to saline conditions.

High salinity concentrations had a strong impact on seedling growth parameters of both Vigna marina and V. radiata. Nonetheless, wild V. marina seedlings clearly performed better than cultivated V. radiata populations in high salt concentrations, even if one of the V. radiata populations was considered more tolerant to high salinity levels. From our results, it is also evident that V. marina populations growing in different locations in Sri Lanka show inter-populational variation for seedling salinity tolerance, which has implications for conservation efforts as outlined below (Table 1).

Seedling development of all the studied V. marina populations improved at slightly higher salt concentrations as compared to growth in distilled water. A similar observation has been made for other halophytes (Alhaddad et al. 2021; Abdellaoui et al. 2023) and confirms the true halophytic nature of V. marina. The growth performance of the four studied V. marina populations differed considerably between the populations, as shown by the different traits measured. The Thalpe and Negombo populations showed the overall best performance at 20,000 ppm NaCl. Given their better performance in saline conditions, these populations may contain salinity tolerance genes, which could be explored for genetic improvement of Vigna crop species. Our experiment showed that the chloride ion content in leaves, stems, and roots of the studied V. marina populations was significantly different among populations. This suggests that plants from different populations, which are exposed to varying salinity levels, have adapted differently to salinity stress, and these adaptations may be reflected in the traits studied. The highest chloride accumulation was recorded in the leaves and stems of V. marina compared to the roots. When the chloride content is higher in the soil solution, Cl- ions influx into the roots and accumulate in the aerial parts of the plants, which reduces plant growth, while the first toxicity symptoms can be seen in leaves as chlorotic patches (Geilfus 2018). Yoshida et al. (2020) showed that the Na⁺ concentration of V. marina also tended to be higher in the stems and leaves than in the roots (roots < stems = leaves). However, in contrast to this report, another study reported that V. marina mainly allocated Na+ to the roots but not to the shoot apex. Na⁺ was significantly higher in the root than in the stem or the leaf (Noda et al. 2022). Noda et al. (2025) showed that V. marina effectively restricted sodium uptake, maintaining low internal sodium allocation even at 300 mM NaCl. Notably, sodium accumulation was predominantly confined to the roots at higher salinity levels, particularly at 200 and 300 mM NaCl.

Recent studies showed that V. marina had the ability to suppress sodium (Na⁺) uptake under salt stress by upregulating genes related to Casparian strip formation and developing a multi-layered, lignified apoplastic barrier around the endodermis. This structural adaptation significantly limits Na⁺ allocation to the shoots. This reinforced endodermal barrier is a key factor in V. marina’s dominance in high-salinity environments and represents a promising trait for improving salt tolerance in crop plants (Wang et al. 2025). In our results, there was no significant difference in the root: shoot ratio at the highest NaCl concentration (20,000 ppm) and control (0 ppm NaCl). Further, the root dry mass was lower compared to the shoot dry mass at the 20,000 ppm salinity level. In contrast, Wang et al. (2024) reported that under salt stress conditions, V. marina exhibited a distinctive growth strategy by allocating more resources to root development than shoot growth. Specifically, the genotype produced a greater number of fine roots that were shorter in length, as well as fewer but longer thick roots. This adaptation led to a significant increase in root dry weight, even under saline conditions. Moreover, V. marina effectively limited sodium accumulation in the stems and leaves, instead retaining some sodium within the root tissues, indicating a potential mechanism for salt tolerance. This root-focused biomass allocation strategy was consistent with observations in V. marina, the most salt-tolerant species in the genus, which also prioritized root over shoot dry matter production under salt stress. These findings suggest a conserved adaptive response among salt-tolerant Vigna species, with implications for improving stress resilience in crop breeding programs.

Inter-population variation in plant traits and responses to environmental conditions can be caused by various factors (Moreira et al. 2012), with genetic variability (Nicotra et al. 2010; Cochrane et al. 2015), maternal effects (Roach and Wulff 1987), and environmental effects (Cochrane 2016) being the three main factors. Since we performed the experiment in standardized conditions, we can exclude variation due to environmental effects as a potential explanation. Any differences we observed between the populations are therefore related to genetic variation or maternal effects. Chankaew et al. (2014) reported that certain alleles, from V. marina and V. luteola (Jacq.) Benth., which are genetically closely related species, can increase salt tolerance. Further, they also observed that the salinity tolerance of V. marina is controlled by the same QTL at the seedling and vegetative stages. Iseki et al. (2016) suggested that the genetic variability in a few genes can considerably affect the salt tolerance of Vigna. Besides genetic variation, plants can respond differently to high salt concentrations due to maternal effects. Seeds of Glycyrrhiza uralensis Fisch. ex DC. collected from plants growing in more saline conditions showed a higher germination rate as compared to seeds from plants growing in non-saline conditions (Gu et al. 2024). Further studies are required to determine whether the inter-population differences we observed are related to maternal effects, due to plants growing in different conditions, or whether there is a genetic background. This could be realized by growing plants from different populations in a common garden for at least one generation and testing the performance of these offspring in different salt conditions.

Understanding the sources of trait variation is not only important for basic ecological insight but also critical for conservation and breeding strategies. Preserving populations that represent the functional and agro-morphological diversity within a species is essential, as such variation underpins resilience to environmental stresses and potential use in crop improvement. Vigna marina has been reported to harbour valuable traits like salinity tolerance, that is absent in domesticated crop varieties (Maxted et al. 2012; Warschefsky et al. 2014). To develop a precise conservation plan for V. marina, a broader sampling effort covering its full distribution across Sri Lanka is necessary to capture local adaptations and rare traits (Vincent et al. 2013). Moreover, incorporating additional functional traits such as phenology, root architecture, and physiological response to stress can help identify ecotypes adapted to specific environmental conditions (Funk et al. 2017). Ultimately, determining whether observed trait variation has a genetic background is critical for prioritizing populations for conservation and utilization in breeding programs.

When comparing the performance parameters of V. marina with the two varieties of V. radiata, it was clear that V. marina performed much better. Vigna marina seedlings survived in high salt concentrations (10,000 and 20,000 ppm), while seedlings of both varieties of V. radiata died at these concentrations. Vigna radiata varieties showed necrotic patches during the salt treatment at 1000 and 2000 ppm saline conditions. This indicates that V. marina seedlings have a higher salt tolerance than the commercialized salt-tolerant variety of V. radiata. The root: shoot ratio of V. radiata was also higher than that of the V. marina populations, which indicates that they invested more energy in root formation, potentially indicating that V. radiata suffered more from drought stress in saline conditions than V. marina. The salt-tolerant variety of V. radiata did show a higher performance than the salt-sensitive variety in saline conditions. Hence, our study showed the potential of V. marina as a genetic resource to improve the salinity tolerance of crops such as V. radiata. Vigna marina is an edible plant that is used as a food crop in many places (Padulosi and Ng 1993; Chankaew et al. 2014). Our findings suggest that V. marina exhibits traits that may be valuable for cultivation in saline environments. While our study included only a limited number of populations, the observed inter-population variation in salt-related traits highlights the potential of this species as a genetic resource. With further research and broader sampling, V. marina could be considered for use in breeding programs aimed at improving salinity tolerance or even for neo-domestication. The use of crop wild relatives in improving cultivated varieties or in developing new crops is expected to increase with advancements in breeding tools and a growing understanding of species diversity (Tomooka et al. 2014b). However, realizing this potential depends on the conservation and accessibility of crop wild relatives, underscoring the need to safeguard germplasm in gene banks and facilitate access to it (Castañeda-Álvarez et al. 2016).

Studying the inter-population variation of a species is useful for understanding the potential resilience of a species to climate change and for informing in situ conservation strategies (Hudson et al. 2015; Henn et al. 2018; Samarasinghe et al. 2022). Species exhibiting high levels of inter-population variation may possess a greater adaptive capacity to cope with future climatic challenges. In our study, V. marina showed some inter-populational differences in seedling functional traits under salinity, which may indicate local adaptation. However, given the limited number of populations examined, it is suggested to study a wider range of populations and environmental gradients to confirm these findings. Conserving only a few V. marina populations may not capture the full extent of trait variability. Therefore, both in situ and ex situ conservation measures should be considered to protect the genetic and functional diversity of V. marina. This is particularly important as the populations studied were located in roadside habitats, which may be vulnerable to disturbance or development. Ex situ and in situ methods could be used to conserve the genetic diversity of V. marina. Seed banking is an important ex situ conservation method that can be used to preserve this species. For further steps, it is important to study more V. marina populations covering coastal areas around the country to evaluate the mechanisms underlying this species’s tolerance to salinity.