## 4 Discussion  
Soil secondary salinization is one of the main causes of continuous obstacles to vegetable cropping. Soil secondary salinization may result from the accumulation of soil nitrate nitrogen caused by excess nitrogen fertilizer application [5]. Excessive nitrate can lead to serious adverse effects on vegetable growth and nutrient quality. At present, many exogenous substances and chemical fertilizers, including hydrogen-rich water (HRW), spermidine (Spd) and melatonin (MT), have been used to alleviate nitrate stress [5; 24; 25; 26]. However, the alleviatory effect of exogenous FA on Ca(NO\({}_{3}\))\({}_{2}\) stress in Chinese flowering cabbage has not been analyzed. In this study, 100 \(\upmu\)m FA alleviated the inhibitory effect of 80 mM Ca(NO\({}_{3}\))\({}_{2}\) on the growth of Chinese flowering cabbage (Figure 1). As shown in Figure 7, the alleviation of nitrate stress by FA may result from the positive effect on the photosystem and oxidation-reduction system, but it decreased the accumulation of nitrate in Chinese flowering cabbage seedlings (Figure 7). We found that the salt stress caused by Ca(NO\({}_{3}\))\({}_{2}\) could observably reduce photosynthesis in Chinese flowering cabbage seedlings (Figure 2), which was in accord with the inhibitory effect of nitrate stress on tomato and pak choi seedlings [2; 5]. FA could significantly improve the photosynthetic capacity of Chinese flowering cabbage seedlings under nitrate stress by increasing the transpiration rate, stomatal conductance and net photosynthetic rate. Likewise, the weakened photosynthetic capacity induced by salt stress in tomato can also be alleviated by changing these photosynthetic parameters with far-red light treatment [25]. Many studies have found that Ca(NO\({}_{3}\))\({}_{2}\) can greatly induce O\({}^{2-}\), H\({}_{2}\)O\({}_{2}\) and MDA accumulation, and the activities of antioxidant enzymes are adaptively enhanced to resist nitrate stress in pakchoi and tomato seedlings [5; 26]. In our study, the application of FA significantly reduced ROS levels and membrane lipid peroxidation, as determined via histochemical localization and concentration analyses (Figure 3). In addition, plants can produce enzymatic scavengers, including SOD, POD, CAT and APX, to protect seedlings from oxidative stress. SOD is involved in clearing the ROS, and POD, APX and CAT can remove H\({}_{2}\)O\({}_{2}\). Antioxidant activity could be further increased to fight free radical  
Figure 6: Effect of FA on NO\({}^{3-}\) content, NO\({}^{3-}\)-N ratio (shoot/root) and nitrate relative genes in Chinese flowering cabbage seedlings under nitrate stress: (**a**) represents the content of NO\({}^{3-}\); (**b**) shows the NO\({}^{3-}\)-N ratio (shoot/root) in different treatments; (**c**,**d**) represent the relative expression of _BcNRT1.1_ and _BcNRT1.5_. The plants grown in quarter-strength Hoagland’s solution with 15 mM Ca(NO\({}_{3}\))\({}_{2}\) were the control. Different letters indicate a significant difference at \(p\leq 0.05\). damage [27]. Our results suggested that excess Ca(NO\({}_{3}\))\({}_{2}\) increased antioxidant enzyme activity, which was further improved by FA (Figure 4). This result was consistent with the report by [5; 26; 27]. Many studies have found that FA has great potential in improving stress resistance, increasing crop yield and maintaining sustainable agricultural development [5; 28; 29; 30]. However, the application of FA to Chinese flowering cabbage under nitrate stress has rarely been reported. In this study, 100 uM FA significantly alleviated the growth inhibition of Chinese flowering cabbage caused by Ca(NO\({}_{3}\))\({}_{2}\) stress. In addition, the analysis results of the photosynthetic system showed that the net photosynthetic rate, transpiration rate and stomatal conductance under nitrate stress were improved by the application of FA (Figure 2). This result suggested that FA had the capacity to improve the abnormal photosystem induced by nitrate stress.