Different Responses Of Photosynthetic Apparatus Between Wild Soybean (Glycine Soja) And Cultivated Soybean (Glycine Max) To NaCl Treatment

Salt stress is one of the major stress factors limiting growth of plant, so understanding the salt resistance mechanism of plant is very important to develop cultivars with increased salt tolerance. In this study, a wild soybean (Glycine soja Sieb. et Zucc. ZYD 03262) and a cultivated soybean (Glycine max (L.) Merr. Shanning 11) plants and their detached leaves were used as materials to study responses of their photosynthetic appratus to treatments with different NaCl concentrations (0, 100, 200mM). By comparing analyzsis of photosynthesis, photochemical efficiency, ion concentration and distribution, antioxidant enzymes activities between the two soybena plants, we explored the salt resistance mechanism of the wild soybean.The results showed that NaCl stress inhibited growth, photosynthesis (Pn), and decreased actual photochemical efficiency of PSII (ΦPSII), chlorophyll content (Chl a~+b) and relative water content (RWC) in the leaves of the two soybean plants, but the inhibition was more severe in cultivated soybean. The Na~+ concentrations and the ratio of Na~+/K~+ in leaves of the wild soybean were significantly lower than that of the cultivated soybean, while the Na~+ concentrations in roots of the wild soybean were higher than that of the cultivated soybean. However, according to the above results, it is impossible to clarify whether the photosynthetic apparatus in the wild soybean is more salt tolerant than that in the cultivated soybean because the selective absorption of Na~+ and K~+ through the roots and the translocation of Na~+ and K~+ through the stems can effectively decrease the Na~+ concentrations in the leaves, avoiding the Na~+-induced damage to the photosynthetic apparatus. To elucidate whether the photosynthetic apparatus of the wild soybean is more tolerant to salt stress, detached fully expanded leaves were directly subjected to NaCl treatments, which eliminated the effects of the roots and stems on the photosynthetic apparatuses in the leaves. When the detached leaves of the both soybeans were treated with 100 and 200mM NaCl, the maximal photochemical efficiency of PSII (Fv/Fm),ΦPSII, active PSII reaction centers per excited cross section (RC/CSo) and photosynthetic performance index (PI) decreased more significantly in the wild soybean than that in the cultivated soybean. Meanwhile the Na~+ concentrations in the detached leaves of the wild soybean were higher than that of the cultivated soybean when treated with 100 and 200mM NaCl. The correlations between the Na~+ concentrations and photosynthetic activities demonstrated that increasing Na~+ concentrations in leaves, regardless of whether attached or detached leaves, causes a more pronounced decrease in photosynthetic activities in wild soybean compared to cultivated soybean, that's mean the Na~+ concentrations in leaves are mainly responsible for determining salt resistance in the two soybean species. According to the Non-invasive Micro-test Technique (NMT) data, the Na~+ effluxes in leaves of the wild soybean were significantly greater than those in leaves of the cultivated soybean; however, the Na~+ effluxes were more remarkably higher in roots of the cultivated soybean than in the wild soybean. The activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) in leaves of the wild soybean were increased, while the activities of APX and CAT were decreased and the activity of SOD increased in leaves of the cultivated soybean after treated with NaCl.In conclusion, the wild soybean is able to maintain normal water absorption via osmo-adjustments by accumulating higher levels of Na~+ and by absorbing more K~+ in its roots to avoid the osmotic stress caused by NaCl stress. The enhanced of activity of antioxidant enzymes can effectively eliminate the reactive oxygen species (ROS) to alleviate damage to photosystems and enzymes involved in CO2 assimilation, protecting photosynthetic apparatus against salt stress. Our results suggest that the photosynthetic apparatus in the wild soybean doesn't have higher salt resistance than that in the cultivated soybean. However, the wild soybean prevents the accumulation of higher concentrations of Na~+ in leaves by certain mechanism, which protects its photosynthetic apparatus from salt damage, helping it to survive in saline soil.