| Soil salinization is a global issue. Chlorides, sulfates and salt contents in saline-alkaline soils directly affect germination of seeds and plant survival as well as reducing plant growth and yield, thus becoming the most direct factors limiting the growth and recovery of plants in saline-alkaline soils. Biological measures represent the most cost-effective modalities to improve the use of saline-alkali soils, while the screening and cultivation of salt-tolerant plant resources are the primary link in biological improvement in saline soils. Ulmus pumila, an important native tree strains in North China, are quick in growth, superior in materials and well tolerant of salts, alkali, drought and pollution. Therefore, they can be used for salt and alkali improvement as well as afforestation. However, Ulmus pumila are widely distributed and rich in intra-strains variations. Therefore, it is of great significance to screen Ulmus pumila well tolerant of salts for improving costal saline-alkaline soils and enriching the resources of plant strains therein.In order to select and cultivate excellent Ulmus pumila strains tolerant of salts,1-year-old Y65, Y1 and Y34 Ulmus pumila seedlings were used as experimental subjects in this study, and different concentrations of NaCl (0, 50, 85, 120 and 155mmmol/L) were adopted to simulate the salt stress environment. On this basis, we studied such physiological indicators as growth, biomass, MDA content, soluble sugar (SS) content, soluble protein (SP)content, free proline content, chlorophyll content and photosynthetic parameters (Pn, Tr, Gs,Ci, Ls, WUE) of the 3 Ulmus pumila strains under NaCl stress, and investigated the salt tolerance capacity and the mechanisms of the 3 strains. The results showed that:(1) Under NaCl stress, the growth of the 3 Ulmus pumila strains was inhibited, the accumulation in plant height and ground diameter was decreased significantly.Under different concentrations of NaCl, biomass accumulation of Y65 was significantly higher than that of Y1 and Y34, and damage to Y65 due to the stress was less. The inhibition of the total biomass showed that the salt tolerance threshold of the three plant strains was greater than 155mmol/L.(2) Under NaCl stress, the root/cap ratio of Y34 was lower than that of the control, and absorption and transportation of salts by plants were reduced by increasing the distribution of biomass in the plants above the ground level. The root/cap ratio of Y65and Y1 was lower than that of the control under 120 mmol/L and higher than that of the control under other concentrations. The acquisition of water and nutrient by roots was increased mainly by increasing the distribution of biomass in the root system, thus distilling the salts within the cells.(3) With the increase of the NaCl concentration, the content of SS in the leaves of the 3 Ulmus pumila strains was increased; the contents of MDA and SP were increased and then decreased,with the highest levels reached at 85mmol/L; the content of Pro was decreased and then increased, and the value of Pro was significantly higher than the CK value of the control at >85mmol/L salt stress. SP and SS might be the major organic osmotic regulators of the white elm strains under low salt stress (50 and 85mmol/L), while under high salt stress (120 andl55mmol/L), Pro could coordinate with SS in involvement in osmotic regulation.(4) Salt stress reduced the contents of chlorophyll a, chlorophyll b, total chlorophylls and the ratio of chlorophyll a/b in the leaves of Ulmus pumila seedlings significantly, and the extent of decrease was increased with the increase in the concentrations of salt stress. Besides,salt stress had a greater impact on the content of chlorophyll in Y34 than on that in Y65 and Y1.(5) Under the low NaCl stress (≤85mmol/L), the photosynthetic mechanism of the three strains were dominated by stomatal limitation, and the transpiration was decreased by increasing the stomatal limitation value, so as to improve the water use efficiency (WUE) and adapt to salt stress.Under high-salinity stress (> 85mmol/L), the photosynthetic mechanism was dominated by non - stomatal limitation, and the uptake of ground water and salt ions was reduced by reducing the water use efficiency.The turning point of Y65 and Y1 was 85mmol/L and the turning point of Y34 was 50mmol/L, which was different from that of the non-stomatal limitation of Pn.(6) Collectively, our results indicated that the salt resistance of Y65 was better than that of Y1 and Y34. We could provide a scientific basis for the afforestation of coastal saline alkali soil and the further research on the salinity tolerance mechanism. |
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