## 3 Results  
Effect of Exogenous FA on the Inhibited Growth of Seedlings Induced by Ca(NO\({}_{3}\))\({}_{2}\) Stress  
Superfluous Ca(NO\({}_{3}\))\({}_{2}\) had a remarkable adverse effect on the growth of Chinese flowering cabbage (Figure S1). As the treated concentration of Ca(NO\({}_{3}\))\({}_{2}\) increased, all the growth indices decreased. Ca(NO\({}_{3}\))\({}_{2}\) (80 mM) had an obvious inhibitory effect on plant growth. Compared with the Con, the plant height was obviously reduced by 27.8% with 80 mM Ca(NO\({}_{3}\))\({}_{2}\) (Figure S1a). In addition, there were 45.8% and 51% reductions in the fresh weight of shoots and roots, respectively, when using 80 mM Ca(NO\({}_{3}\))\({}_{2}\) (Figure S1c,d). The shoot dry weight was reduced by 14.9% with 80 mM Ca(NO\({}_{3}\))\({}_{2}\) (Figure S1e). Therefore, 80 mM Ca(NO\({}_{3}\))\({}_{2}\) was chosen as the main concentration to conduct the following experiments. After that, we performed a screening test for FA. As shown in Figure 1, the root lengths, compared with 80 mM Ca(NO\({}_{3}\))\({}_{2}\), and the fresh and dry weights of the shoots and roots, were increased by 26.67%, 49.2%, 56.47% and 58.31%, respectively, with 100 \(\upmu\)M FA treatment. The results revealed that 100 \(\upmu\)M FA could significantly alleviate the growth inhibition of Chinese flowering cabbage induced by Ca(NO\({}_{3}\))\({}_{2}\) stress. Effect of FA Application on the Photosynthetic Capacity of Seedlings under Ca(NO\({}_{3}\))\({}_{2}\) Stress  
As shown in Figure 2, calcium nitrate stress greatly inhibited photosynthesis in Chinese flowering cabbage seedlings. In detail, compared with the Con, the net photosynthetic rate, transpiration rate and stomatal conductance decreased by 24%, 52% and 67%, respectively, with Ca(NO\({}_{3}\))\({}_{2}\) stress (Figure 2a,c,d). However, the application of FA significantly increased the transpiration rate, net photosynthetic rate and stomatal conductance of Chinese flowering cabbage seedlings by 25%, 29% and 36%, respectively. These results revealed that FA had the capacity to alleviate the inhibitory effects on the photosynthetic capacity of seedlings induced by calcium nitrate stress. Figure 1: Effect of FA on the growth indices of Chinese flowering cabbage under Ca(NO\({}_{3}\))\({}_{2}\) stress: (**a**) stands for the phenotype of Chinese flowering cabbage with different treatments. The plant height (**b**), root length (**c**), shoot fresh weight (**d**), root fresh weight (**e**), shoot dry weight (**f**) and root dry weight (**g**) of seedlings with different concentrations of FA treatment. The plants cultivated in quarter-strength Hoagland’s solution with 15 mM Ca(NO\({}_{3}\))\({}_{2}\) were used as the control. The treatment groups were 80 mM Ca(NO\({}_{3}\))\({}_{2}\) with different concentrations of FA, respectively. After different treatments for 5 d, the growth indices were determined with three independent replicates of 20 plants each. Different letters indicate a significant difference at \(p\leq 0.05\). Effect of FA Application on the Oxidation-Reduction System and Root Activity of Seedlings under Ca(NO\({}_{3}\))\({}_{2}\) Stress  
DAB and NBT histochemical staining were used to evaluate the accumulation of O\({}^{2-}\) and H\({}_{2}\)O\({}_{2}\) in the leaves of Chinese flowering cabbage. A deeper brown color was observed in the Ca(NO\({}_{3}\))\({}_{2}\) treatment, but a lighter brown color was observed in the FA + Ca(NO\({}_{3}\))\({}_{2}\) cotreatment (Figure 3a), which indicated that the O\({}^{2-}\) accumulation was reduced by FA. Similarly, the H\({}_{2}\)O\({}_{2}\) staining experiment showed that darker blue was found in Ca(NO\({}_{3}\))\({}_{2}\) treatment alone, but lighter blue was observed in FA + Ca(NO\({}_{3}\))\({}_{2}\) cotreatment (Figure 3b), which revealed that the H\({}_{2}\)O\({}_{2}\) accumulation was decreased by FA. Compared with the Con, nitrate excess induced the accumulation of O\({}^{2-}\), H\({}_{2}\)O\({}_{2}\) and MDA, and the relative electrical conductivity 2- or 3.3-fold (Figure 3c-f).