| Soybean [Glycine max(L.) Merr.] is one of the most important crops in the human diet, animal feeds, and industrial products because of its high protein and oil contents. It is difficult to make a breakthrough in the breeding for new resistance in soybean plants, because for greatly reduces in genetic diversity and less resistance to abiotic stress. So the genetic diversity of soybean must be increased to overcome the lack of salt tolerance gene in cultivated soybean using exotic soybean germplasm. Therefore, the wild soybean(Glycine soja Sieb. et Zucc.), which have more genetic diversity than cultivated soybean, been adopted to improve the resistance of cultivated soybean to environmental stresses. So, 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 Na Cl concentrations in this study. By comparing analyzsis of photosynthesis, photochemical efficiency, ion concentration and distribution, antioxidant enzymes activities between the two soybean plants, we explored the salt resistance mechanism of the Dongying wild soybean. The main results obtained are as follows:(1) The results showed that Na Cl stress inhibited growth, and decreased chlorophyll content(Chl a+b), photosynthesis(PN), stomatal conductance(gs), apparent quantum yield(AQY) and carboxylation efficiency(CE) in the leaves of the two soybean plants, but the inhibition was more severe in the cultivated soybean. The limitation of the CO2 assimilation decreases the consumption of ATP and NADPH, which leads to accumulation of a major acceptor of electrons in PSI, and then feedback inhibits electron transport in chloroplasts and decreases the PSII photochemistry. The reduction of actual photochemical efficiency of PSII(ΦPSII) indicated that less light energy was utilized by photochemical reaction and more excessive excited energy was produced in leaves after treated with different concentrations of Na Cl. The maximum quantum yield of primary photochemistry(φPo), the probability that a trapped exciton moves an electrom into the elector transport chain beyond QA(ψEo), the probability with which an electron from the intersystem electron carriers moves to reduce end electron acceptors at the acceptor side(δRo), the performacnce index(PIabs), and the amount of active PSII RCs per CS(RC/CSo) in the leaves of cultivated soybean decreased with the increasing Na Cl concentration, whereas there is no changes in the wild soybean. And the inhibition of the electron transport leads to products of ROS. So the increased activities of superoxide dismutase(SOD), ascorbate peroxidase(APX), catalase(CAT) in the wild soybean can scavenge ROS effectively, than avoid the damage to membrane and other cellular structures. But the activities of APX and CAT were decreased and the activity of SOD increased in the leaves of cultivated soybean plants after treated with Na Cl.(2) Na Cl stress decreased the relative water content(RWC) in the leaves of cultivated soybean plants, but has no changes in wild soybean, which indicate that the wild soybean can avoid the osmotic stress caused by Na Cl stress. The Na+ concentrations and the ratio of Na+/K+ in leaves of the wild soybean plant 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. The wild soybean is able to maintain normal water absorption via osmo-adjustments by accumulating higher levels of Na+ in its roots and by absorbing more K+ in its leaves avoid the osmotic stress caused by Na Cl stress. 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.(3) 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 Na Cl 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 200 m M Na Cl, Fv/Fm、ΦPSII、q P、PIabs、RC/CSo、φPo、RC/ABS 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 200 m M Na Cl. That’s mean the photosynthetic apparatus in the wild soybean serious injury by salt stress, comparing to the cultivated soybean.(4) According to analyze the correlations between the Na+ concentrations and photosynthetic activities(PN, chl a+b, gs, CE, ФPSII, q P, PIabs, RC/CSo, φPo, and RC/ABS) 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 photosynthetic apparatus in the wild soybean doesn’t have higher salt resistance than that in the cultivated soybean. And the Na+ concentrations in leaves are mainly responsible for determining salt resistance in the two soybean species. But the wild soybean prevents the accumulation of higher concentrations of Na+ in leaves, which could protect its photosynthetic apparatus from salt damage. It is one of the salt resistance mechanisms of the wild soybean.(5) It is noteworthy that the response of photosynthetic apparatus was different in the two soybean leaves of the grafted plants after Na Cl treatment. Na Cl stress decreased PN and gs in the leaves of the two soybean plants, but the inhibition was more severe in wild soybean leaves. However, the Ci in the cultivated soybean leaves decrease with increasing Na Cl concentration, but it increases in the wild soybean leaves after 100 m M Na Cl treatment. Meanwhile, there is no change in the CE in the cultivated soybean leaves, but the CE decreased significantly in the wild soybean leaves. The limitation of the CO2 assimilation feedback inhibits electron transport in PSII and PSI. It was observed that the shape of chlorophyll a fluorescence transients(OJIP) were changed greatly by 100 m M Na Cl treatment in the wild soybean leaves. And the Na Cl stress lead to damage to oxygen evolving complex(OEC), lower energetic connectivity of the PSII unit, reduce the pool size of the end electron acceptors, and decrease the re-reduction of P700 and PC. So the root of wild soybean is helpful to prevent the accumulation of higher concentrations of Na+ in leaves, through acclimated more Na+ in it.In conclusion, the wild soybean is able to maintain normal water absorption via osmoadjustments by accumulating higher levels of Na+ in its roots and by absorbing more K+ in its leaves, prevent the accumulation of higher concentrations of Na+ in leaves, and enhance the activity of antioxidant under salt stress, which could protects its photosynthetic apparatus from salt damage, helping it to survive in saline soil. |
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