Hydroponic vegetable growers are searching for ways to increase on-farm reuse of nutrient solutions and to reduce discharge into the environment (Shannon and Grieve, 2000). Discharge wastes both water and fertilizer, pollutes the environment, and can result in soil salinization (Varlagas et al., 2010). One of the most important factors limiting the reuse of nutrient solutions for hydroponic production of common vegetables (e.g., cucumber, tomato, and pepper) is elevated salinity (Van Os, 1998), resulting mainly from Na+ and Cl− accumulation attributable to their low uptake rates by these plant species (Kronzucker and Britto, 2011; Savvas et al., 2005).
One alternative strategy to discharging salinized solutions is to reuse them for hydroponic production of other economically valuable and more salt-tolerant crops (Grieve and Suarez, 1997; Kong and Zheng, 2014a; Pardossi et al., 1999). These types of crop species must have the ability to withstand elevated salinity levels without growth inhibition and reduced productivity, while providing a saleable product (Adler et al., 2003; Grieve and Suarez, 1997).
Suaeda glauca, an annual halophyte growing in saline–alkaline soils in North China (Guan et al., 2013), has a high degree of resistance to salt and alkali stresses (Yang et al., 2008). Consequently, it has been suggested S. glauca be used as pioneer plants for ecological recovery of saline and sodic soils (Guan et al., 2013). In addition, due to its good palatability and medicinal value (An et al., 2008; Yang et al., 2008), S. glauca has been identified as a traditional wild vegetable and/or medicinal plant in countries such as China and Korea (Kefu et al., 2002; Kim and Song, 2013). Moreover, S. glauca has recently been cultivated in China as one kind of seawater-irrigated specialty vegetable, which can be served as salad or stir-fried with other ingredients (Hong et al., 2003; Yancheng Green Garden Saline Soil Agricultural Science Co., 2013).
Can S. glauca be used for hydroponic production using salinized nutrient solutions recovered from the greenhouse production of the salt-intolerant vegetable crops mentioned above? To answer this question, it is necessary to assess at first the growth response of S. glauca to different salinity levels under hydroponic conditions. So far studies have been focused on mere evaluation of S. glauca’s extreme salt tolerance and understanding tolerance mechanisms under soil or sand culture conditions while neglecting the crop’s commercial potential (Guan et al., 2013; Sun and Zhou, 2010; Yang et al., 2008; Zhao et al., 2005). S. glauca has been reported to be able to survive at soil salt concentrations equal to or greater than 2% (≈260 mm NaCl) (Sun and Zhou, 2010), but when root-zone Na+ concentrations increased up to 300 and 450 mm under sand cultivation, the relative growth rate (RGR) and biomass began to show significant decrease (Guan et al., 2013; Yang et al., 2008). The high salt tolerance of S. glauca is associated with its high levels of salt accumulation in the plant tissues. In their native habitat, Xingjiang, China, the plant can accumulate Na+ up to 2.2% of its dry weight (DW) (Zhao et al., 2005). Under sand cultivation, when external Na+ concentrations increased from 75 to 300 mm, Na+ accumulation in fresh shoots increased from around 300 to 400 mm (Yang et al., 2008). However, salt tolerance, Na+ uptake, and biomass accumulation in plants can be affected by the culture medium (Liu, 2002) and no information is available for hydroponic conditions.
In the above studies, the Na+ concentrations were relatively high; ranging from 75 to 600 mm (Guan et al., 2013; Yang et al., 2008). However, Na+ concentration of most nutrient solutions discharged from hydroponic production of common vegetables ranges from 6 to 8 mm, a moderate salinity (Van Os, 1998). Many dicotyledonous halophytes show optimal growth in concentrations of 50–250 mm NaCl (Flowers and Colmer, 2008), and suboptimal NaCl concentration may reduce plant growth of some halophytes in hydroponic systems (Rozema and Schat, 2013). For example, 5 mm NaCl has been shown to reduce plant growth, including biomass accumulation, associated with decreased sodium uptake in hydroponic Salicornia bigelovii compared with 200 mm NaCl (Ayala and O’Leary, 1995). Conversely, Brown et al. (1999) found that Suaeda esteroa, which is in the same genus as S. glauca, showed no significant differences in final DW and RGR between the plants treated by ≈9 and ≈180 mm NaCl under sand culture (Brown et al., 1999). However, S. esteroa and S. glauca are different halophyte species, and plant growth responses to salinity gradient may vary greatly among different halophyte species even in the same genus (Ashraf et al., 2010). Therefore, further study is needed to determine whether hydroponic S. glauca, like S. bigelovii, is negatively affected when cultivated in a low-salinity environment.
The overall objective of this study was to evaluate the potential of producing S. glauca hydroponically as a vegetable at moderate (6–10 mm) NaCl concentrations typically found in the discharged solutions from hydroponic production of common vegetables.
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