Physiological and biochemical responses of facultative halophytes to differential hydroponic solution salinities

Salinity is a major abiotic stressor limiting crop production globally. Unlike obligate halophytes, facultative halophytes adapt to saline environments by enhancing their secondary metabolism, particularly under moderate salinity, offering great potential for Controlled Environment Agriculture (CEA). However, the metabolic thresholds of specific edible halophytes under hydroponic salinity management remain unclear. To evaluate their growth performance, osmolytes, and antioxidant activity in CEA, we hypothesized that moderate salinity would stimulate antioxidant and soluble carbohydrate accumulation without severely compromising growth parameters: two facultative Amaranthaceae species, Chenopodium giganteum D. Don and Amaranthus tricolor cv. Red Aztec, were grown in a deep-water culture system tested with different salinity levels (0, 150, and 300 mM NaCl). Our study revealed that increased salinity resulted in declines in growth rate, leaf area, and dry weight, whereas leaf mass and leaf area mass increased, indicating structural adaptations. Salinity triggered metabolic processes to increase glucose and fructose concentrations, indicating salinity-associated metabolic responses. Moreover, the responses of maltose and raffinose were species-specific, and the antioxidant patterns varied on an individual basis. Organic acid profiles in response to salinity were also species-specific, revealing divergent metabolite adaptation. 150 mM NaCl partially maintained growth relative to 300 mM but did not fully preserve it compared with the control, and increased metabolite accumulation, whereas DPPH rose with increasing salinity level. TPC showed no significant change, while FRAP increased at 150 mM NaCl in A. tricolor . Our research findings revealed that the tested species exhibited salinity-dependent changes in selected metabolites and antioxidant activity, and that 150 mM NaCl was not lethal to either species.