Abstract: 【Objective】In order to provide a reference for soybean cultivation, breeding and artificial regulation of salt stress, the physiological and molecular responses to different genotype soybean seedlings, especially the effects of salt stress on photosynthetic parameters, ions content and the expression of key genes involved in Na⁺ homeostasis of soybean seedlings were studied. Through comparison of the difference in response to salt stress, the study try to reveal the salt resistance mechanisms of different cultivated genotypic soybeans.【Method】The responses of photosynthetic characteristics and ion content of soybean seedlings were studied under salt stress. Salt tolerant cultivated soybean varieties (Y8D6008, Y8D6013) and salt sensitive cultivated soybean varieties (Y8D6132, Y8D6136) were incubated in the 1/2×Hoagland nutrient solution. NaCl was added to nutrient solution incrementally by 50 mmol·L^-1 step per day to provide final concentrations of 150 mmol·L^-1 for salt treatment at 1-compound leaf stage, and the treatment lasted for 7 days. The control plants were cultured with nutrient solution without adding NaCl. The response of soybean seedlings to NaCl stress was assessed by measuring the expression of key genes involved in Na⁺ homeostasis, photosynthetic parameters, and the ion content of soybean. 【Result】The growth of the four cultivated soybean seedlings was significantly inhibited by 150 mmol·L^-1 NaCl stress. And salt stress significantly reduced soybean leaf SPAD value, net photosynthetic rate and transpiration rate. However, the inhibition of NaCl on growth and photosynthetic characteristics was more severe in salt sensitive cultivar than in salt tolerant cultivar. Moreover, 150 mmol·L^-1 NaCl concentration reduced leaf intercellular CO₂ concentration in salt tolerant cultivar, and increased it in salt sensitive cultivar. Thus, the reduction in net photosynthesis rate of salt tolerant cultivar caused by 150 mmol·L^-1 NaCl stress was considered to be a result of stomatal restriction; but increased non-stomatal restriction resulted in more severe reductions in photosynthesis of salt sensitive cultivar. Under salt stress, the Na⁺ content was significantly increased in four cultivars seedlings, and the increase was more pronounced in salt sensitive soybean. In response to salt stress, the K⁺ content of leaves decreased, the magnitude of these changes was greater in salt sensitive cultivars than in salt tolerance cultivars. Compared with the controls, phosphorus (P) accumulation of salt sensitive soybean in leaves was significantly decreased by 150 mmol·L^-1 NaCl treatment. While salt treatment had no significant influence on the P content in the leaves of salt tolerance cultivars. Correlation analysis showed that the relative net photosynthetic rate was strongly negatively correlated with relative Na⁺ content and positively correlated with relative K⁺ content and total phosphorus content. Six key genes, Na⁺ transporter genes (GmSALT3, GmSOS1, GmNcl1, GmNHX1), the gene of signal system for Na⁺ homeostasis (GmCIPK1), and the gene of energetic system for the operation of Na⁺ transporters (GmAVP1), involved in Na⁺ homeostasis were chosen for gene expression analyses. The expression of GmNcl1 was significantly induced by NaCl treatment and the expressions of GmNcl1 induced by salt stress of salt tolerance cultivars were lower than those of salt sensitive cultivars. The expression of GmNcl1 associated closely with salt tolerance features and Na⁺ content of four soybean cultivars. However, the expression of the rest five genes showned no association with Na⁺ content of four soybean cultivars.【Conclusion】Compared with the salt sensitive cultivars, salt tolerant cultivars effectively maintained a lower Na⁺ content, and a higher level of K⁺ and P contents in leaves to ensure a relatively high photosynthetic rate under salt stress condition. This may be one of the mechanisms to keep higher salt resistance in salt tolerant cultivar than in salt sensitive cultivar. In addition, GmNcl1 may associate with the salt tolerance characteristics of the soybean cultivars in this study.

Key words: cultivated soybean')">cultivated soybean, salt stress, photosynthetic characteristic, ion content, gene expression, Na⁺ homeostasis