In duration of
Aegiceras corniculatum treated by 250 mM NaCl, polyphenol content significantly increased more than double as compared to control plants, suggesting that the accumulation of polyphenols played a role as protective metabolites [
110]. In two Tunisian accessions of
Cakile maritima (Jerba and Tabarka), the accumulation of polyphenols in Jerba was significantly increased by 56% and 30% in response to the treatment of 100 mM and 400 mM NaCl respectively, while that in Tabarka declined due to the NaCl treament [
111]. After the treatment with moderate salinity (25–50 mM NaCl), the phenolic content in leaf of
Cynara cardunculus was dramatically enhanced to reach the peak value corresponding to NaCl concentration of 50 mM [
112]. The stresses of various salinity resulted in the accumulation of phenolic compounds in
F. esculentum to be 57%, 121% and 153%, higher than that of the control treated with 10, 50, and 100 mM for 7 d, respectively. Moreover, the accumulation of phenolic compounds was primarily caused by an increase in the contents of four major compounds including isoorientin, orientin, rutin and vitexin [
113]. The increasing salinity was also found to stimulate the biosynthesis of phenols and oleuropein in four olive cultivars, especially in leaves. The increase of total phenols content was abrupt at 125 mM NaCl which was more than double to that of control plants occurred in all cultivars. Due to the highest salinity treatment the concentration of Oleuropein was 18.5, 5.5, 2.5 and 3.8 folds greater than those of the control plants for ‘Zard’, ‘Ascolana’, ‘Koroneiki’ and ‘Arbequina’, respectively. However, the variation trend of leaf hydroxytyrosol concentration was different from that of oleuropein. When exposed to 125 mM NaCl, the hydroxytyrosol in each one of the all cultivars decreased abruptly below the values of the control plants [
114]. Among three chloride salts (NaCl, KCl and CaCl
2), KCltreatment showed a more pronounced effect on the contents of total phenolics and flavonoids in leaves of artichoke (
C. cardunculus) and cardoon (
C. cardunculus var.
altilis) [
115]. Moreover, the rapeseed (
Brassica napus var
oleifera) under increasing salinity was sprouted to evaluate the effect of salinity on total phenolics (TP), non-flavonoids (NF), tannins (TAN), phenolic acids (PAs). In early sprouts, TP increased by 35% with salinity up to 50 mM NaCl, as compared with control and then decreased slightly, the maximum increase of total-NF (30%) was showed in corresponding to the treatment of 25 mM NaCl, and total-TAN increased with salt concentration up to 50 mM and remained such high in response to the treatment of 100 and 200 mM NaCl, while salinity did not give a clear effect on total-Pas content. Overall, a moderate salinity in 25–50 mM NaCl caused the highest relative increase in phenolic concentration [
116]. However, the accumulation of phenolic compounds in plants by salinity stress would also depend on the plant species, so that phenolic compounds failed to be accumulated in some plant species. With respect to the control plants, salinity stressed also led to a decrease of phenolic compounds (chlorogenic and sinapic acid derivatives and flavonoids) in leaves of broccoli (
B. oleracea var.
italica cv. Marathon) and the loss was higher for flavonoids than for sinapic acid derivates [
117]. Furthermore, salinity stress can change the chemical contents of various phenolic compounds in rice cultivars (tolerant and susceptible varieties), causing a large increase in total phenolics and the content of vanillin and protocatechuic acid in tolerant varieties, whereas in contrast, a markedly reduce is found in the susceptible cultivar [
118]. The effect of NaCl concentration on total phenolic content in
S. macrosiphon showed that all the treatments with different concentration of NaCl elicited a remarkable reduction of total phenolic content in the leaf which decreased with the treatment of increasing NaCl concentration. After induced with 6.8 dS m
−1 NaCl, the content of total phenolics was reduced by 2.6 times as compared to control leaves [
118]. However, NaCl salinity increased total antioxidant activity in methanolic extract of the leaf, probably due to the increasing activity of peroxidase (POD) under salt stress conditions [
119].
Jaleel et al. [
98] reported that the content of indole alkaloidin
C. roseus increased due to the treatment of 80 mM NaCl as compared to unstressed control plants. After
C. roseus treated with 150 mM NaCl for 2 months, the content of vincristine in this plant significantly increased as compared with the control sample, but declined with further increasing salinity in a long-term treatment [
120]. The yields of alkaloids in
C. roseus increased gradually with the duration of seawater stress, and the plants treated by 5% seawater gave the yields of alkaloids higher than those treated by 10% seawater. Among the four kinds of alkaloids, the concentration of vindoline, catharanthine and vincristine in the plants treated by 5% seawater significantly increased as compared to the control. In consideration of industrial production, the treatments using 5% seawater can potentially reduce the cost of producing alkaloids [
121]. In the medium of salinity equivalent to 100 mM NaCl, the accumulation of total alkaloids exceeded over that of non-saline control, and found to be maximal in roots of
C. roseus [
122].
Similarly, salinization can significantly alter the accumulation of secondary metabolite of rosemary (
Rosmarinus officinalis), mainly inducing a pronounced effect on monoterpenes composition. It was founded that the solution of NaCl at 100 mM considerably increased the relative abundance of cineole and camphor, but slightly decreased those of borneol, α-terpineol, nopol, and camphene [
123]. Moreover, the root tissues of maize exposed to salt stress can also increase the concentrations of acidic terpenoid phytoalexins, such that the immersion of the root tissues in the solution of NaCl at 500 mM dramedically enhance the quantity of zealexins by about five fold, while the treatment with lower concentration solution of NaCl (100 mM) significantly induce the content of kauralexins to raise by twofold in comparison with the control plants in the medium of 0 mM NaCl [
103]. The effect of salinity stress on the content of plant secondary metabolites is exhibited in
Table 6.