Physiological and phytochemical responses of red amaranth (Amaranthus tricolor L.) and green amaranth (Amaranthus dubius L.) to different salinity levels

Worldwide, soil salinity has adversely affected about 30% of the irrigated and 6% of total land area with a resultant monetary loss of USD 12 billion in agricultural production (Chaves et al., 2009). High salt content affects plant growth by modifying their morphological, anatomical (Hasan et al., 2018) and physiological traits (Khorshid et al., 2016). Such growth impairment is due to osmotic effects and ionic imbalances affecting plant metabolism (Greenway and Munns, 1980). The severity of salt damage has been found to be dependent on species, cultivar (Talukdar, 2011) and growth stages of the plant (Mahdi Dar et al., 2007).
        
A horticultural crop with potential for cultivation under saline condition is amaranth (Amaranthus spp.) (Omami et al., 2006). Amaranth is an edible plant that has been used by humans for over 4000 years. It is grown and consumed as grain or vegetable crop in Australia, Africa and Asia (Corke and Cai, 2016). In Vietnam, amaranth is commonly consumed as a leafy vegetable. It is a valuable food source of nutrient with high quality of proteins, vitamins, minerals and bioactive compounds such as phenolics (Gomes et al., 2016). The possible use of this plant as a nutritious green crop in semi-arid region has been proposed (Myers, 1996).
        
Phenolic compounds are well-described secondary plant metabolites, which can play an important role in absorbing and neutralizing free radicals, quenching singlet oxygen and decomposing peroxides (Ksouri et al., 2009). An enhancement of antioxidant activity and the amount of phenolic compounds can be observed under various environmental factors and stress conditions (Valifard et al., 2014). Hence, salt-stressed plants might represent potential sources of phenolic compounds for economical use.
        
Despite a substantial amount of literature on responses of plants to salinity stress, little information is available on amaranth, especially the change in its phytochemical content in response to salt tolerance. The objectives of this study were to: (1) determine the growth and biomass production of amaranth under saline condition; (2) evaluate the NaCl uptake, phytochemical content and antioxidant activity of amaranth treated with varying levels of NaCl and (3) compare the response of red (Amaranthus tricolor L.) and green amaranth (Amaranthus dubius L.) to salinity stress.

Plant growth
 
The effect of salinity stress on amaranth vegetative growth and biomass production was presented in Table 2 and 3, respectively. Salinity stress reduced plant growth in both red and green amaranth at all treatments except for the 25 mM NaCl treatment. The most detrimental effect was noted in 100 mM NaCl treatment for all plant growth parameters. The root dry weight in red amaranth was affected greatly with up to 75% reduction; whereas in green amaranth, it was the leaf dry weight with up to 90% reduction. The salt- induced growth reduction has also been commonly observed in previous researches. Omami et al., (2006) reported that 100 mM NaCl caused a reduction in plant height (34%), leaf number (40%) and total leaf area (58%) in A.tricolor. In the present research work, red amaranth showed similar response with 42% reduction in leaf number and 40% in total leaf area. However, the reduction in plant height was lower compared to the previous study with only 28% decrease.
 

 

        
The reduction in plant growth of green amaranth was greater than red amaranth at all salinity levels, except for root dry weight. Hence, on the basis of these parameters, A. tricolor was more tolerant than A. dubius at different salt levels. Omami et al., (2006) also stated that A. hypochon- driacus and A. cruentus were more tolerant than A. tricolor at 100 mM NaCl based on the greater values in leaf area, plant height and shoot dry mass.
        
The current study showed that shoot biomass in red and green amaranth was higher than root biomass at all levels of NaCl. This is a particularly important characteristic for genotypes cultivated as leafy vegetable crops under salinity stress. A reduction in root growth with increasing root zone salinity (from 25 mM to 100 mM NaCl) was also observed on A. cruentus, A. tricolor, A. hypchondriacus and Accession’ 83 by Omami et al., (2006) and in Carthatmus tinctorius by Bassil and Kaffka (2002).
 
NaCl content
 
Sodium chloride concentration in different plant parts increased with salinity stress except in the treatment with 25 mM NaCl. Concentrations of Na⁺ in roots were higher than in shoots (Fig 1). In the red amaranth shoot, the highest increase in Na⁺ content relative to the control was noted at 100 mM (5 times), followed by the treatment with 50 mM and saline soil (4 times) and then by 50% garden soil: 50% saline soil (2.8 times). A similar trend of Na⁺ content was recorded in the root. The highest root Na⁺ concentration was found in 100 mM NaCl treatment, yielding 8 times that of the control. Similar response to salt stress was observed in shoot and root Na⁺ concentration of green amaranth except for root Na⁺ content at the 25 mM treatment, yielding 2 times over the control.
 

        
The drastic increase in the concentration of Na⁺ and Cl^- in the tissues of plants exposed to salinity led to toxicity as it was shown by a reduced plant growth. Na⁺ disrupts the cell ion homeostasis by inducing an inhibition in the uptake of essential nutrients in a number of crops, e.g., in wheat (Raza et al., 2007) and in sunflower (Akram et al., 2007).
        
The accumulation of Na⁺ and Cl^- in roots provides a mechanism for amaranth to cope with salinity in the rooting medium. In the present study, NaCl content in the red amaranth root was greater than that of green amaranth. This indicates the greater inhibition of transport of toxic ions to the leaf laminae which contributed to a better growth of A. tricolor. Similar mechanism was reported in previous studies in A. cruentus and A. tricolor (Omami, 2005), sweet pepper (Zandstra-Plom et al., 1998) and olive (Kchaou et al., 2010).
 
Phytochemical content
 
In the current study, salinity stress increased the total phenolic content and total flavonoid content in both genotypes. The increase in total phenolic and flavonoid content resulted in an increase of antioxidant activity with up to 35% in red amaranth and 23% in green amaranth.  The 50 mM NaCl treatment produced the greatest total phenolic content and flavonoid in red amaranth. However, high salt concentration (100 mM NaCl) reduced these parameters. A. tricolor produced a higher value of total phenolic content and total flavonoid content compared to that of A. dubius at all salinity treatments. Total phenolic content of red amaranth ranged from 19 to 27 mg GAE (g DW)^-1; while in green amaranth, from 13.9 to 20.1 mg GAE (g DW)^-1.
        
Salt tolerance seems to be favored by an increased antioxidant activity to detoxify ROS (Kibria et al., 2017). Increase in phenolic content in different tissues under increasing salinity has also been reported in a number of plants such as pepper (Navarro et al., 2006) and tomato (Alves et al., 2018). Phenolic compounds exhibit antioxidant activity by inactivating lipid free radicals or preventing decomposition of hydroperoxides into free radicals (Pokomy et al., 2001). Several flavonoids act as potential inhibitor of the enzyme lipoxygenase, which converts polyunsaturated fatty acids to oxygen-containing derivatives (Nijveldt et al., 2001). These compounds which accumulate in plant tissues could help protect them from damaging effects of ROS and may help inhibit lipid peroxidation in stressed-plants (Potapovich and Kostyuk, 2003).

Salinity stress reduced plant growth of A. tricolor and A. dubius except for the 25 mM treatment. The sensitivity to stress differed with the level of salinity, the measured parameters and genotypes. Treatment with 100 mM NaCl caused the greatest reduction in all plant growth parameters.
        
Na⁺ and Cl^- content in both red and green amaranth increased with increasing salinity level in all plant parts.  The greatest increase was recorded in green amaranth with 8 times increase of Na⁺ content in both shoot and root. NaCl content in the root was greater than in the shoot at all salinity treatments. This provides a mechanism for amaranth to cope with salinity stress by excluding most of the toxic ions from the root zone to the aerial parts. Total phenolics, flavonoid content and antioxidant activity increased with increasing salinity level up to 50 mM NaCl. All these parameters were reduced at 100 mM NaCl. Maximum increases in total phenolics content and flavonoid content were 28% and 54%, respectively, in red and green genotypes. In the current experiments, both A. tricolor and A. dubius growth were reduced under mild (from 25 to 55 mM NaCl) and moderate salinity (from 55 to 100 mM NaCl). According to the classification of crop tolerance to salinity (Mass and Hoffman, 1977), A. tricolor and A. dubius can be rated as moderately sensitivity to salinity stress, similar to other leafy vegetable crop. The decrease in growth of green amaranth was greater than red amaranth at all salinity levels, except for root dry weight. Hence, on the basis of these parameters, A. tricolor was more tolerant than A. dubius. A. tricolor also produced greater amount of phytochemical content than A. dubius under salinity stress which was linked to its free radical scavenging activities.

This research is funded by the ACCCU project, The Netherlands Organization for International Cooperation in Higher Education and Research (NUFFIC) and Hue University.