Growth and Physiological Response of Viola tricolor L. to NaCl and NaHCO3 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 NaHCO3 (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 NaHCO3 stress that V. tricolor can tolerate. The results showed that NaCl and NaHCO3 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 NaHCO3 stress, the height growth of V. tricolor was significantly greater than CK when the concentration of NaCl and NaHCO3 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 NaHCO3 concentration was less than 200 mmol·L-1; soluble sugar (SS) was significantly greater than CK under all NaCl and NaHCO3 treatments; proline (Pro) was significantly greater than CK when the NaCl concentration was 150 mmol·L-1 and the NaHCO3 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 NaHCO3 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 NaHCO3 concentrations were 50, 100 and 150 mmol·L-1, respectively; catalase (CAT) was significantly greater than CK when the NaCl and NaHCO3 concentrations were 25, 50 and 100 mmol·L-1, respectively. Chlorophyll (Chl) was significantly lower than CK when the NaCl and NaHCO3 concentrations were greater than 100 mmol·L-1. Malondialdehyde (MDA) gradually increased with the increase in the NaCl and NaHCO3 concentrations. Membership function analysis showed that the concentrations of NaCl and NaHCO3 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 NaHCO3 when the treatment concentration was lower than 50 mmol·L-1, but higher than that from NaHCO3 when it exceeded 50 mmol·L-1.

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Combined with field observation and indoor water immersion test, water holding characteristics of litters from five typical plantations (Platycladus orientalis, Robinia pseudoacacia, Populus alba var. pyramidalis, P. orientalis+R. pseudoacacia, P. alba var. pyramidalis+R. pseudoacacia) in southern and northern mountains of Lanzhou City were examined. Results showed that litter mass under the plantations ranged from 13.50 to 47.01 t·hm-2, with an order of P. alba var. pyramidalis+R. pseudoacacia＞P. orientalis+R. pseudoacacia＞P. orientalis＞R. pseudoacacia＞P. alba var. pyramidalis. The percentage of un-decomposed litters was greater than that of semi-decomposed litters in all plantations except for P. orientalis plantations. The maximum water-holding rate of litters ranged from 190.8% to 262.7%, with the greatest value in the P. alba var. pyramidalis+R. pseudoa-cacia and the lowest in P. orientalis plantations. The maximum water-holding capacity of litters was 35.29-123.59 t·hm-2, with an order of P. alba var. pyramidalis+R. pseudoacacia＞P. orientalis+R. pseudoacacia＞R. pseudoacacia＞P. orientalis＞P. alba var. pyramidalis. Litter water absorption rate declined linearly within the first hour, and then decreased slowly. Semi-decomposed litters had a higher water-absorption rate than un-decomposed litters. The maximum water retaining amount and effective retaining amount of the litters were P. alba var. pyramidalis+R. pseudoacacia＞P. orientalis+R. pseudoacacia＞P. orientalis＞R. pseudoacacia＞P. alba var. pyramidalis. P. alba var. pyramidalis+R. pseudoacacia had the highest effective retaining rate. P. alba var. pyramidalis+R. pseudoacacia plantation had highest capacity for soil and water conservation in southern and northern mountains of Lanzhou City.