*2.1. Effect of Exogenous H2O<sup>2</sup> on Plant Height and Biomass under NaCl Stress*

Figure 1 shows that the growth of Tartary buckwheat under the CK treatment (H2O + H2O) and 5H + N (5 mmol·L <sup>−</sup><sup>1</sup> <sup>H</sup>2O<sup>2</sup> + 150 mmol·<sup>L</sup> <sup>−</sup><sup>1</sup> NaCl) treatments was significantly better than that under NaCl (150 mmol·L <sup>−</sup><sup>1</sup> NaCl + H2O) and 10H + N (10 mmol·L <sup>−</sup><sup>1</sup> <sup>H</sup>2O<sup>2</sup> + 150 mmol·<sup>L</sup> <sup>−</sup><sup>1</sup> NaCl) treatments, and the root growth with 5H + N was better than that with the other three treatments.

**2. Results** 

with the other three treatments.

was significantly decreased by 2.49 times under 10H + N treatment (Figure 2B).

*2.1. Effect of Exogenous H2O2 on Plant Height and Biomass under NaCl Stress* 

Figure 1 shows that the growth of Tartary buckwheat under the CK treatment (H2O + H2O) and 5H + N (5 mmol·L−1 H2O2 + 150 mmol·L−1 NaCl) treatments was significantly better than that under NaCl (150 mmol·L−1 NaCl + H2O) and 10H + N (10 mmol·L−1 H2O2 + 150 mmol·L−1 NaCl) treatments, and the root growth with 5H + N was better than that

Under NaCl stress, plant height (PH), stem fresh weight (SFW), stem dry weight (SDW), root fresh weight (RFW), and root dry weight (RDW) of Tartary buckwheat were significantly decreased by 28.22%, 51.90%, 44.63%, 59.60%, and 48.19%, respectively, compared with what they were under the CK treatment (Figure 2A–E). Under the 10H + N treatment, PH, SFW, SDW, RFW, and RDW were significantly decreased by 0.78-fold, 0.43-fold, 0.63-fold, 0.58-fold, and 0.61-fold, respectively, relative to what they were under the CK treatment (Figure 2A–E). The SFW of Tartary buckwheat decreased by 9.80% under the 10H + N treatment compared with what it was under the NaCl treatment (Figure 2B). Under the 5H + N treatment, the PH, SFW, SDW, RFW, and RDW of Tartary buckwheat were significantly higher than they were under the NaCl treatment (Figure 2A–E); SFW and RFW increased by 124.60% and 141.58%, respectively (Figure 2B,D), and SFW

**Figure 1.** Effects of exogenous H2O2 on growth of Tartary buckwheat under NaCl stress. The photos were taken on the 40th day of transplanting. CK: control, H2O foliar spray + H2O irrigation, NaCl: H2O foliar spray + 150 mmol·L<sup>−</sup>1 NaCl irrigation, 5H + N: 5 mmol·L−1 H2O2 foliar spray + 150 mmol·L−<sup>1</sup> NaCl irrigation, 10H + N: 10 mmol·L<sup>−</sup>1 H2O2 foliar spray + 150 mmol·L−1 NaCl irrigation. **Figure 1.** Effects of exogenous H2O<sup>2</sup> on growth of Tartary buckwheat under NaCl stress. The photos were taken on the 40th day of transplanting. CK: control, H2O foliar spray + H2O irrigation, NaCl: H2O foliar spray + 150 mmol·L <sup>−</sup><sup>1</sup> NaCl irrigation, 5H + N: 5 mmol·<sup>L</sup> <sup>−</sup><sup>1</sup> H2O<sup>2</sup> foliar spray + 150 mmol·L −1 NaCl irrigation, 10H + N: 10 mmol·L <sup>−</sup><sup>1</sup> H2O<sup>2</sup> foliar spray + 150 mmol·L <sup>−</sup><sup>1</sup> NaCl irrigation.

Under NaCl stress, plant height (PH), stem fresh weight (SFW), stem dry weight (SDW), root fresh weight (RFW), and root dry weight (RDW) of Tartary buckwheat were significantly decreased by 28.22%, 51.90%, 44.63%, 59.60%, and 48.19%, respectively, compared with what they were under the CK treatment (Figure 2A–E). Under the 10H + N treatment, PH, SFW, SDW, RFW, and RDW were significantly decreased by 0.78-fold, 0.43-fold, 0.63-fold, 0.58-fold, and 0.61-fold, respectively, relative to what they were under the CK treatment (Figure 2A–E). The SFW of Tartary buckwheat decreased by 9.80% under the 10H + N treatment compared with what it was under the NaCl treatment (Figure 2B). Under the 5H + N treatment, the PH, SFW, SDW, RFW, and RDW of Tartary buckwheat were significantly higher than they were under the NaCl treatment (Figure 2A–E); SFW and RFW increased by 124.60% and 141.58%, respectively (Figure 2B,D), and SFW was significantly decreased by 2.49 times under 10H + N treatment (Figure 2B).

*Int. J. Mol. Sci.* **2021**, *22*, x FOR PEER REVIEW 4 of 21

**Figure 2.** Effects of exogenous H2O2 on PH (**A**), SFW (**B**), SDW (**C**), RFW (**D**), and RDW (**E**) of Tartary buckwheat under NaCl stress. CK: control, H2O foliar spray + H2O irrigation, NaCl: H2O foliar spray + 150 mmol·L<sup>−</sup>1 NaCl irrigation, 5H + N: 5 mmol·L−1 H2O2 foliar spray + 150 mmol·L−1 NaCl irrigation, 10H + N: 10 mmol·L<sup>−</sup>1 H2O2 foliar spray + 150 mmol·L−1 NaCl irrigation. PH: plant height (cm), SFW: shoot fresh weight (mg), SDW: shoot dry weight (mg), RFW: root fresh weight (mg), RDW: root dry weight (mg). All values are expressed as mean ± SD. According to Duncan's multiple comparisons, different letters represent significant differences among different treatments (*p* < 0.05). **Figure 2.** Effects of exogenous H2O<sup>2</sup> on PH (**A**), SFW (**B**), SDW (**C**), RFW (**D**), and RDW (**E**) of Tartary buckwheat under NaCl stress. CK: control, H2O foliar spray + H2O irrigation, NaCl: H2O foliar spray + 150 mmol·L <sup>−</sup><sup>1</sup> NaCl irrigation, 5H + N: 5 mmol·<sup>L</sup> <sup>−</sup><sup>1</sup> H2O<sup>2</sup> foliar spray + 150 mmol·L <sup>−</sup><sup>1</sup> NaCl irrigation, 10H + N: 10 mmol·L <sup>−</sup><sup>1</sup> H2O<sup>2</sup> foliar spray + 150 mmol·L <sup>−</sup><sup>1</sup> NaCl irrigation. PH: plant height (cm), SFW: shoot fresh weight (mg), SDW: shoot dry weight (mg), RFW: root fresh weight (mg), RDW: root dry weight (mg). All values are expressed as mean ± SD. According to Duncan's multiple comparisons, different letters represent significant differences among different treatments (*p* < 0.05).

#### *2.2. Effect of Exogenous H2O2 and NaCl Stress on Leaf Growth and Relative Water Content 2.2. Effect of Exogenous H2O<sup>2</sup> and NaCl Stress on Leaf Growth and Relative Water Content*

Compared with the CK, the leaf area (LA) and leaf length (LL) of Tartary buckwheat increased significantly by 24.75% and 18.60%, respectively, under 5H + N treatment (Figure 3A,B), and the leaf relative water content (LRWC) decreased slightly (Figure 3C); under 10H + N treatment, LA was significantly decreased by 11.88% (Figure 3A), and LL and LRWC were significantly decreased by 23.97% and 22.84%, respectively (Figure 3B,C). Compared with NaCl treatment, the LA, LL, and LRWC of Tartary buckwheat were significantly increased under the 5H + N treatment (Figure 3A–C), among which LL increased the most (91.33%) (Figure 3C). LA, LL, and LRWC were significantly increased under 10H + N treatment; however, the increasing trend of LRWC was not significant, and all values were significantly lower under 10H + N treatment than under 5H + N treat-Compared with the CK, the leaf area (LA) and leaf length (LL) of Tartary buckwheat increased significantly by 24.75% and 18.60%, respectively, under 5H + N treatment (Figure 3A,B), and the leaf relative water content (LRWC) decreased slightly (Figure 3C); under 10H + N treatment, LA was significantly decreased by 11.88% (Figure 3A), and LL and LRWC were significantly decreased by 23.97% and 22.84%, respectively (Figure 3B,C). Compared with NaCl treatment, the LA, LL, and LRWC of Tartary buckwheat were significantly increased under the 5H + N treatment (Figure 3A–C), among which LL increased the most (91.33%) (Figure 3C). LA, LL, and LRWC were significantly increased under 10H + N treatment; however, the increasing trend of LRWC was not significant, and all values were significantly lower under 10H + N treatment than under 5H + N treatment (Figure 3A–C).

ment (Figure 3A–C).

*Int. J. Mol. Sci.* **2021**, *22*, x FOR PEER REVIEW 5 of 21

**Figure 3.** Effects of exogenous H2O2 on LA (**A**), LL (**B**), and LRWC (**C**) of Tartary buckwheat under NaCl stress. CK: control, H2O foliar spray + H2O irrigation, NaCl: H2O foliar spray + 150 mmol·L<sup>−</sup><sup>1</sup> NaCl irrigation, 5H + N: 5 mmol·L<sup>−</sup>1 H2O2 foliar spray + 150 mmol·L−1 NaCl irrigation, 10H + N: 10 mmol·L<sup>−</sup>1 H2O2 foliar spray + 150 mmol·L−1 NaCl irrigation. LA: leaf area (cm2), LL: leaf length (cm), LRWC: leaf relative water content (%). All values are expressed as mean ± SD. According to Duncan's multiple comparisons, different letters represent significant differences among different treatments (*p* < 0.05). **Figure 3.** Effects of exogenous H2O<sup>2</sup> on LA (**A**), LL (**B**), and LRWC (**C**) of Tartary buckwheat under NaCl stress. CK: control, H2O foliar spray + H2O irrigation, NaCl: H2O foliar spray + 150 mmol·L −1 NaCl irrigation, 5H + N: 5 mmol·L <sup>−</sup><sup>1</sup> H2O<sup>2</sup> foliar spray + 150 mmol·L <sup>−</sup><sup>1</sup> NaCl irrigation, 10H + N: 10 mmol·L <sup>−</sup><sup>1</sup> H2O<sup>2</sup> foliar spray + 150 mmol·L <sup>−</sup><sup>1</sup> NaCl irrigation. LA: leaf area (cm<sup>2</sup> ), LL: leaf length (cm), LRWC: leaf relative water content (%). All values are expressed as mean ± SD. According to Duncan's multiple comparisons, different letters represent significant differences among different treatments (*p* < 0.05).
