Effects And Regulation Mechanism Of Salt Stress On Photosynthesis And Antioxidant System In Soybean

Salt stress is one of the major abiotic stresses that depress plant growth and drastically limit crop production. Because of the increasing salinization in soils, it has been a great threat to crop productivity. Soybean is one of the main crops in our country. The understanding of the mechanism for its response to salt stress is important to agriculture application and plant improvement. Hybridized soybeans have been selected using asymmetric somatic hybridization method between salt-tolerant but low yield wild soybean(Glycine cyrtoloba ACC547) and salt-sensitive but high yield cultivated soybean(G.max Melrose). A stable somatic descendents(S111—9) has been proved to have inherited saltinity-resistant traits from ACC547. All these materials provide us a novel opportunity to identify physiological factors responsible for this genotypic difference in photosynthesis, to find the key photosynthetic factors related to salt tolerance. All these would help to develop strategies for improving cultivars' response to salt stress. The main results obtained were shown as follows:The net photosynthetic rate (Pn) decreased progressively with the increasing salt concentration in both soybean varieties, but the reduction was greater in Melrose than in S111-9. The decrease in Pn was positively related to the reduction in stomatal conduntance(gs) and intercellular CO₂ concentration(Ci) in S111-9, while only gs in Melrose. Only in the highest NaCl treatment did the maximum PS II quantum yield (Fv/Fm) significantly decreased in both varieties, but photochemical quenching(qP), actual photosystem II (PS II) efficiency (FPSII) , relative electron transport rate(ETR) and Rubisco carboxylation activity were significantly decreased in Melrose under any salt treatments but not in S111-9. The limiting factor for photosynthetic decline was different for different varieties, that was, stomatal limitation for salt-resistant S111-9, but Rubisco activity(RA) limitation for salt-sensitive Melrose. The reduction in RA was closely related to the salt-induced reduction in LSU content which is mainly regulated at posttranscriptional level.Salt stress induced more serious oxidative damage in Melrose than in ACC547. The parameters, such as MDA, EL, O₂^-·and H₂O₂, were induced to increase much greater in Melrose than that in ACC547 by salt stress. And this enhancement was greater in root than in leaf for both Melros and ACC547. The activity of some antioxidant enzymes such as SOD, POD and CAT were induced to increase greatly in ACC547, though the CAT activity was not detected in roots of both varieties. To Melrose, salt stress induced no obvious effect on the activity of these enzymes, except that the highest NaCl concentration induced some increase in root's SOD activity. The salt induced enhancement of enzymes activity was accompanied with the improvement of corresponding isoenzymes, which was especially evident in POD in ACC547 root, which indicates that POD might play an important role in removing H₂O₂, especially in root of ACC547.Salt stress also disturbed the cell's ion balance. Comparatively, the ACC547 holds more efficient regulation mechanism to maintain ion balance than Melrose. ACC547 could control the leaf's Na⁺ content by accumulating more Na⁺ in its roots, while the Melrose accumulated more Na⁺ in its leaf. Furthermore, the over absorbance of Na⁺ in Melrose reduced the normal absorbance of K⁺, while in ACC547, the K⁺ absorbance was not influenced by the Na⁺ absorbance, so it could maintain the normal K⁺/ Na⁺, which was reduced in Melrose.The GR activity was induced to increase by salt stress in both soybean leaves, but decreased in both roots. Compared to Melrose, the GR activity increased more in ACC547 leaf, but reduced less in ACC547 root. The MDHAR activity was greatly improved in ACC547, especially in its root, while the DHAR activity was not changed greatly in both leaf and root. But to Melrose, except that the MDHAR activity was induced to decrease significantly in its root, the MDHAR activity in its leaf, as well as the DHAR activity in both leaf and root were changed little. Compared with these antioxidant enzymes, salt stress induced more significant difference on non-antioxidant enzymes, such as GSH, ASC, GSH/GSSG and ASC/DHA, between these two soybean varieties. Salt stress, especially the later period of stress, improved the content of GSH and ASC, as well as the ratio of GSH/GSSG and ASC/DHA in both leaf and root of ACC547, while salt stress decreased or has less effect on those parameters in Melrose. All these implied that the non antioxidants might play an important role in salt-resistant in ACC547.Analysis of post-illumination transient increase in chlorophyll fluorescence showed that NaCl induced the increase of CEF1 more greatly in ACC547 than in Melrose. The maximum photochemical efficiency was reduced less in ACC547than in Melrose after 10d of NaCl stress. In ACC547, salt-induced increase of CEF1 was associated with the enhancement of LHCII phosphorylation in its isolated thylakoid membrane. NaCl could induce LHCII phosphorylation in ACC547 thylakoid membrane in the dark, but couldn't in Melsore. Furthermore, the LHCII phosphorylation levels in light-stimulated ACC547 thylakoid membrane was enhanced by the presence of NaCl, but decreased in Melrose. Treatment with photosynthetic electron flow inhibitors (DCMU, DBMIB) inhibited LHCII phosphorylation induced by both light and NaCl in Melrose, while they have less effect in ACC547. All these indicating that NaCl could induce state II transition in ACC547 which helps to balance energy distribution between two photosystems. But this efficient regulation mechanism is absent in Melrose.