Growth and Physiological Response of Viola tricolor L. to NaCl and NaHCO(3) Stress

Soil salinization is an important environmental problem worldwide and has a significant impact on the growth of plants. In recent years, the mechanisms of plant salt tolerance have received extensive attention from researchers. In this paper, an experiment was implemented to assess the potential effect of different NaCl and NaHCO(3) (sodium bicarbonate—an alkaline salt) concentrations (25 mmol·L(−1), 50 mmol·L(−1), 100 mmol·L(−1), 150 mmol·L(−1) and 200 mmol·L(−1)) on the growth, antioxidant enzymes, osmoprotectants, photosynthetic pigments and MDA of Viola tricolor L. to reveal the physiological response and explore the maximum concentrations of NaCl and NaHCO(3) stress that V. tricolor can tolerate. The results showed that NaCl and NaHCO(3) treatments had significant effects on osmoprotectants, antioxidant enzymes, photosynthetic pigments, MDA content and the plant height growth of V. tricolor. On day 14 of the NaCl and NaHCO(3) stress, the height growth of V. tricolor was significantly greater than CK when the concentration of NaCl and NaHCO(3) was less than 100 mmol·L(−1). Soluble protein (SP) was significantly greater than CK when the NaCl concentration was less than 150 mmol·L(−1) and the NaHCO(3) concentration was less than 200 mmol·L(−1); soluble sugar (SS) was significantly greater than CK under all NaCl and NaHCO(3) treatments; proline (Pro) was significantly greater than CK when the NaCl concentration was 150 mmol·L(−1) and the NaHCO(3) concentration were 150 and 200 mmol·L(−1), respectively. Peroxidase (POD) was significantly greater than CK when the NaCl concentration was less than 200 mmol·L(−1) and the NaHCO(3) concentration was less than 150 mmol·L(−1); superoxide dismutase (SOD) was significantly greater than CK when the NaCl concentration was 50 mmol·L(−1) and the NaHCO(3) concentrations were 50, 100 and 150 mmol·L(−1), respectively; catalase (CAT) was significantly greater than CK when the NaCl and NaHCO(3) concentrations were 25, 50 and 100 mmol·L(−1), respectively. Chlorophyll (Chl) was significantly lower than CK when the NaCl and NaHCO(3) concentrations were greater than 100 mmol·L(−1). Malondialdehyde (MDA) gradually increased with the increase in the NaCl and NaHCO(3) concentrations. Membership function analysis showed that the concentrations of NaCl and NaHCO(3) that V. tricolor was able to tolerate were 150 mmol·L(−1) and 200 mmol·L(−1), respectively. Beyond these thresholds, osmoprotectants and antioxidant enzymes were seriously affected, Chl degradation intensified, the photosynthetic system was seriously damaged, and the growth of V. tricolor was severely affected. According to a comprehensive ranking of results, the degree of NaCl stress on V. tricolor was lower than that from NaHCO(3) when the treatment concentration was lower than 50 mmol·L(−1), but higher than that from NaHCO(3) when it exceeded 50 mmol·L(−1).