Soil salinization represents a major environmental issue of our time. Being an extensive phenomenom which involves the accumulation of high concentrations of saline and sodic ions, soil salinization is expected to affect more than 830 millions hectares of the continents (Daliakopoulos et al., 2016; Ivushkin et al., 2019). While salinization is known to be primarily caused by natural processes (primary salinization) that are accentuated by global climate changes, it can also be induced or enhanced by human activities, mainly irrigation (secondary salinization; Daliakopoulos et al., 2016). In a same way, many industrial processes (e.g. oil, paper and pulp industries, cement manufacturing, aquaculture, textile treatments, and application of road salts) can promote the salinity of natural ecosystems through the production of saline effluents (for review, see Litalien and Zeeb, 2020). In addition, many anthropogenic activities generate saline solid wastes, including sediment and mud. For instance, harbour dredging is pointed out as a maintenance activity producing large amounts of saline sediments whose management is clearly challenging (Wang et al., 2018). This global context of salinization and the environmental issues raised have led to setting up of new regulations which promote the remediation of saline effluents, saline wastes and marine sediments, with the aim of protecting the environment. Among the ions involved in salinization, chloride is specifically targeted by these regulations, being considered harmful to ecosystems. For instance, chloride can make up more than 1 % of the chemical composition of dredged sediments (Lim et al., 2020). The management of chloride-rich sediments and wastes is challenging as it is inconsistent with many industrial technologies classically used to treat and valorize these materials. High chloride contents are incompatible with most physical, thermal and chemical industrial remediation processes (leading to the degradation of the industrial facilities). Waste storage solutions are also drastically limited by the chloride concentrations (threshold on leachates set at 800 mg.kg− 1 for inert wastes, 15,000 mg.kg− 1 for non-inert and non-hazardous wastes and 25,000 mg.kg− 1 for hazardous wastes according to the French regulations). It would be of great interest to use high-salt accumulating plants to phytoremediate chloride-rich soils, effluents and wastes since this passive treatment process has a low ecological and economic imprint (Jesus et al., 2015).
Salt phytoremediation can simply be defined as “the cultivation of salt accumulating or salt-tolerant plants for the reduction of […] salinity and/or sodicity” (Qadir and Oster, 2002). The phytoremediation of saline environments has thus a special character since it requires the use of plant species able to survive and adapt to saline ions in excess, and which can control saline ion concentrations through various mechanisms. Only halophytes species (i.e. about 1 % of plant species, Flowers, 2008) can be considered for this remediation purposes. Halophytes are salt tolerant species due to various adaptation mechanisms such as the exclusion of salt ions in excess at the root level, the uptake and sequestration of these ions in cells of the aerial organs (accumulator halophytes), or the ion excretion at leaf surfaces (recretohalophytes) (Yensen and Biel, 2006). Most of the halophyte species studied for their ability to remediate salt-affected soils and effluents are accumulators (Brown et al., 1999; Doni et al., 2015; Jesus et al., 2014; Khandare and Govindwar, 2015; Manousaki and Kalogerakis, 2011; Masciandaro et al., 2014; Padmavathiamma et al., 2014; Pouladi et al., 2016; Qadir et al., 2007; Rabhi et al., 2009). These studies focus on the capacity of accumulators to sequester saline ions in the aboveground biomass. In recent years, some studies have dealt with the potential of recretohalophytes to remediate saline environments (Litalien and Zeeb, 2020; McSorley et al., 2016a). Investigations are needed to validate the applicability of this last phenotype for depollution.
Hence, the most studied mechanism involved in the remediation of saline environments is the vacuolar sequestration of excess ions in the cells of the aerial parts of halophytes. Sequestration of excess cations, mainly Na+, is generally studied with respect to their toxicity for plants. In contrast, sequestration of Cl− anions is less well documented. Chloride is a micronutrient useful for plants, e.g. in photosynthesis, but can be phytotoxic at high levels (White and Broadley, 2001). Some halophyte species are known for their capacity to accumulate chloride (Devi et al., 2016, 2008; Krishnapillai and Ranjan, 2005; Rabhi et al., 2009, 2008; Hasanuzzaman et al., 2014). In this context, the most studied species are Atriplex spp., Avicennia spp., Phragmites spp., Salicorna spp., Sesuvium spp., Suaeda spp., Tetragonia spp., and Typha spp. For example, Cl− ion concentrations in shoots can reach 70 mg.g− 1 DW (dry weight) in Suaeda maritima (L.) Dumort., 20 mg.g− 1 DW in Aeluropus littoralis (Gouan) Parl. (Hasanuzzaman et al., 2014), 24 mg.g− 1 DW in Phragmites australis (Cav.) Trin. ex Steud. (McSorley et al., 2016b), 20 mg.g− 1 in Juncus maritimus Lam. (Al Hassan et al., 2016) and range between 24 mg.g− 1 and up to 63 mg.g− 1 in Typha latifolia L. (Morteau et al., 2009; Rozema et al., 2014). By combining their high potential of Cl− accumulation and relatively high biomass, some halophytes can be used in phytoremediation to treat soils and effluents (Fountoulakis et al., 2017; Gorai et al., 2010; Guesdon et al., 2016; Jesus et al., 2015, 2017; McSorley et al., 2016b; Morteau et al., 2009; Calheiros et al., 2009, 2008; Rozema et al., 2014). However, little is known about how the age of a plant affects its aclimation to harmful saline media and its capacity to take up and store ions, and specifically chloride. In this framework, most of the studies deal only with the effect of salinity on seed germination (Boscaiu et al., 2011; Davy et al., 2006; Wu et al., 2016; Yu et al., 2012). To our knowledge, few authors gives any information on salt tolerance and Cl− ion sequestration as a function of plant growth stage (Chartzoulakis and Klapaki, 2000; Zeng et al., 2002). Similarly, phytoremediation studies rarely report the growth stage at which plants are included in experiments. However, this criteria should be determinant in the choice of a phytoremediation model. For instance, Lissner and Schierup (1997) observed a higher mortality of juvenile plants of P. australis (10-weeks old) compared to larger rhizome-grown plants when cultivated in saline solutions. The challenge here is to define at which stage of growth specimens should be implanted in a system to be treated.
This study was performed in order to collect data on the tolerance of macrophyte species to Cl−-spiked substrates as well as on chloride accumulation and removal depending on their growth stage. The aim is to provide sufficient data to discuss the chloride removal potential of a given species as a function of growth stage at the time of its introduction into a saline matrix. Three macrophytes were selected as being tolerant to a large range of salinity and also because they are chloride accumulators used in soil and effluent remediation experiments: P. australis, J. maritimus, and T. latifolia. P. australis is a cosmopolitan glycophyte commonly used in wastewater treatment. Although it is not a true halophyte, it can tolerate environments with salinities up to 23‰ NaCl for haplotype M and 6‰ for others haplotypes (Vasquez et al., 2005). As mentioned above, it does not accumulate high contents of Cl− in its shoots, but compensates with a high aboveground biomass (1 to 5.5 kg.m−²) (McSorley, 2016a; Moore et al., 2012; Rozema et al., 2016). J. maritimus is less well documented, but is known to be tolerant to high salinities up to 30‰ NaCl (Boscaiu et al., 2011, 2007). In contrast, T. latifolia is mostly encountered in habitats with moderate salinity, between 12‰ and 24‰ NaCl but with better salt removal capacity (Rozema et al., 2016; Rozema et al., 2014).
While these three species are good candidates to remediate saline media, effluents or soils (Fountoulakis et al., 2017; Guesdon et al., 2016; Jesus et al., 2015, 2017; McSorley et al., 2016b; Morteau et al., 2009; Calheiros et al., 2009, 2008; Rozema et al., 2014; Prabakaran et al., 2019) it is difficult to determine which one is best suited for a given environment due to a lack of uniformity in the literature data where each species has often been studied separately under different experimental conditions. In the present study, P. australis, J. maritimus and T. latifolia plants were provided by a local producer (Nymphea Distribution, South of France) at two growth stages: 6-months old and 1-year old (time of production) and brought back for culture in the laboratory. Their substrate is composed mainly of peat and clays. It was enriched with NaCl from 3 to 24‰ in order to reach chloride contents from 1,875 mg.kg− 1 up to 15,000 mg.kg− 1, critical threshold concentrations expected for waste storage of chloride-enriched materials according to French regulations. A temporal monitoring of the plant survival rate and chloride accumulation in the shoots was carried out according to plant species, Cl− exposure, and plant growth stage.