Potential of Producing Salicornia bigelovii Hydroponically as a Vegetable at Moderate NaCl Salinity

Salicornia bigelovii is a halophyte that is capable of growing under high salinity. To evaluate the potential of producing S. bigelovii hydroponically as a vegetable at moderate NaCl concentrations, plants were grown in nutrient solutions with 6, 8, and 10 mM NaCl, and with 200 mM NaCl as a control. Results showed that plants had a reduced main stem length, canopy width, stem diameter, and root system length in 6 to 10 mM NaCl compared with those in 200 mM. Also, fresh weight increase, fresh and dry weights of individual plants, marketable yield, and water use efficiency of the plants grown in solutions with 6 to 10 mM NaCl were significantly lower than those grown in 200 mM. Associated with the reduced growth attributes, remarkable decreases in sodium uptake by the plants were also obtained in 6 to 10 vs. 200 mM NaCl. The results suggest that S. bigelovii is not a good candidate for hydroponic production as a vegetable at moderate NaCl salinity resulting from reduced growth attributes, which are possibly associated with decreased sodium uptake. Hydroponic vegetable growers are searching for ways of reducing discharge and increasing on-farm reuse of drainage (Shannon and Grieve, 2000), because discharge wastes both water and fertilizer, pollutes the environment, and sometimes even results 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 accumulations attributable to differential ion uptake by these crops (Kronzucker and Britto, 2011; Savvas et al., 2005). An alternative strategy is to reuse these salinized solutions for hydroponic production of another economically valuable and more salt-tolerant crop (Grieve and Suarez, 1997; Kong and Zheng, 2014; Pardossi et al., 1999). It requires that this species has 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). Salicornia bigelovii has been listed as one of the most salt-tolerant species among 1560 halophytes and has been shown to maintain normal growth even when soil NaCl concentration exceeds 1.3 M, two times greater than full-strength seawater salinity ( 500 mM NaCl; Glenn et al., 1999). Also, S. bigelovii has been introduced to U.S. and European fresh produce markets as a specialty vegetable where its succulent young shoots are sold as ‘Samphire’ or ‘Sea asparagus’ and are in high demand in gourmet kitchens not only for their salty taste, but also for their high nutritional value (Ventura et al., 2011; Ventura and Sagi, 2013). S. bigelovii has been evaluated and cultivated commercially as a vegetable in Mexico, the Middle East, and Africa (Jaradat and Shahid, 2012; Ventura and Sagi, 2013; Zerai et al., 2010). Can S. bigelovii be used for hydroponic production as a vegetable in the salinized discharged solutions mentioned before? Currently, limited information is available to answer this question. Grattan et al. (2008) has found that S. bigelovii has the potential to reuse hypersaline drainage water and that it is able to grow well at one-third strength to full-strength seawater salinity. In the previously mentioned study, the plants were evaluated under relatively high NaCl concentrations, i.e., 150 to 500 mM (Grattan et al., 2008). However, the NaCl concentration of most nutrient solutions discharged from hydroponic production of common vegetables ranges from 6 to 8 mM, a moderate NaCl salinity (Van Os, 1998). Some studies indicate that S. bigelovii inhabits the broadest range of salinity and has very little phenotypic response to salinity gradient (Dagar, 2005; Jaradat and Shahid, 2012). Also, the plants of this species have not shown significant differences in the growth rate and biomass accumulation at a salinity between 0.5 and 10 ppt ( 9 and 180 mM NaCl, respectively) until 35 ppt ( 630 mM NaCl; Brown et al., 1999). Conversely, other studies suggest the optimal NaCl concentration for S. bigelovii growth ranges from 170 to 200 mM (Ayala and O’Leary, 1995; Webb, 1966) or 100 to 400 mM (Parks et al., 2002), and a suboptimal NaCl concentration of 5 mM can cause decreased plant growth associated with reduced sodium uptake (Ayala and O’Leary, 1995). Because the reported results differ, further research is needed to determine whether there will be significant reductions in plant growth and sodium uptake for S. bigelovii grown hydroponically at moderate NaCl concentrations (i.e., 6 to 10 mM). The overall objective of this study was to evaluate the potential of producing S. bigelovii hydroponically as a vegetable at moderate NaCl concentrations, the salinity level of nutrient solution discharged from hydroponic production of common vegetables. Materials and Methods Plant materials and growing conditions. The study was conducted in a greenhouse in the Edmund C. Bovey building at the University of Guelph, Guelph, Ontario, Canada (lat. 43 33́ N, long. 80 15́ W). Seeds of S. bigelovii (Serra Maris Company, Ninove, Belgium) were sown in a medium with a peat-perlite ratio of 1:1 by volume on 21 June 2013. One month after sowing, seedlings with a height of 3 cm were transplanted into rockwool cubes (1.5$ Starter Plugs; Grodan Inc., Ontario, Canada), which were embedded in a Styrofoam disk floating on the nutrient solution in a plastic pot (14-cm top and 12-cm bottom diameters · 15 cm high). Each pot had one Styrofoam disk with four seedlings growing in individual rockwool cubes, except for the control pot, which had only a Styrofoam disk and four unplanted rockwool cubes. Each pot contained 1.5 L of solution with the following nutrients (mg·L): 278.3 nitrogen, 36.7 phosphorus, 420 potassium, 97.6 calcium, 1.58 magnesium, 100.8 sulfur, 0.53 copper, 0.21 boron, 0.53 manganese, 0.53 zinc, 0.16 molybdenum, and 1.05 iron. Five days after transplanting, the nutrient solutions were changed for all the pots, and NaCl was added to the solutions to achieve four treatment concentrations: 6, 8, 10, and 200 (control) mM, which indicated the start of treatment. The initial Na concentrations of the four salt-treated nutrient solutions were measured as 6.8 ± 0.1, 8.7 ± 0.0, 10.6 ± 0.1, and 213.9 ± 3.3 mM, respectively, with the electrical conductivity of 3.38 ± 0.03, 3.52 ± 0.05, 3.75 ± 0.04, and 21.65 ± 0.70 dS·m, respectively. The nutrient solutions were changed every 7 d to reduce nutrient and NaCl concentration variability during the experiment and to maintain a nutrient solution pH range of 5.5 to 7.0. Pots were arranged in a randomized block design with four blocks and four NaCl concentrations within each block. Received for publication 29 Apr. 2014. Accepted for publication 3 July 2014. We thank Mary Jane Clark, Eric Rozema, and Katherine Vinson for their technical support. Thanks also to the anonymous reviewers for their very useful suggestions on the revision of this manuscript. To whom reprint requests should be addressed; e-mail [email protected]. 1154 HORTSCIENCE VOL. 49(9) SEPTEMBER 2014 The greenhouse conditions were set at 18-h light/6-h dark by supplementing natural sunlight with high-pressure sodium lamps to achieve a photosynthetically active radiation at the canopy level averaging no less than 397 ± 34 mmol·m·s and 22 to 28 C light/20 to 22 C dark with a relative humidity between 50% and 60%. Measurements of plant growth and sodium uptake. Two plants were randomly chosen from each pot within each block for growth measurements. Every week the following attributes were measured: main stem length, node number and side branch number on the main stem, and canopy width. Four weeks after the start of treatment, seedlings were taken out of the solutions for photographing. The lengths of root systems and diameters of main stems and side branches were measured at the final harvest. The fresh weight (FW) increase rate was determined weekly after the start of treatment until harvest. At harvest (28 d after the start of treatment), FW and dry weight (DW) of individual plants, marketable yield, and water use efficiency were estimated following the methods described by Kong and Zheng (2014). Also at harvest, the total Na removal amount (mmol/plant), water consumption (L/plant), Na removal efficiency (mol·kg DW), and Na uptake concentration (mmol·L H2O) were evaluated following the same methods used by Kong and Zheng (2014). Statistical analysis. Data were subjected to analysis of variance using DPS (Data Processing System) Software (Version 7.05; Refine Information Tech. Co., Hangzhou, China) and were presented as mean ± SE; separation of means was performed using Duncan’s new multiple range test at the P # 0.05 level. Correlation analysis was used to determine the relationship between the parameters of Na uptake and biomass accumulation.