Effects Of Salicylic Acid On Photosynthetic Physiology Of Soybean Seedlings Under Water Stress

Using Sichuan widely cultivated soybean variety Nandou 12 as material, variations in photosynthetic parameters and transcript levels of plastid terminal oxidase （PTOX） gene （PTOX） and its function related gene （pgr5, ndh-h, pds, zds, aoxl,aox2a and aox2b） of soybean seedlings in different root-soaking periods under different concentrations of salicylic acid （0mM,0.1 mM,0.5mM,0.9mM） were evaluated first; And then soybean seedlings were stressed with 4% and 8% PEG after 10h of treatment with salicylic acid （0mM,0.1 mM,0.5mM,0.9mM） pre-culruring. The expression levels of ptox and its function-related gene, photosynthetic parameters, chlorophyll fluorescence characteristics, photosynthetic pigment and MDA contents, activity of POD, SOD, CAT, APX and MDH were determined 0h,4h,8h after water stress. The main results were as follows:1. Salicylic acid induced stomatal closure, and the extent of closure was enhanced with the increasing of SA concentration. As the prolongation of SA treatment, the stomatal conductance （Gs）, transpiration rate （Tr）, Photosynthetic rate （Pn） and intercellular CO₂ concentration （Ci） reduced gradually, but Pn and Ci remained stable after 8h. The changes of transcript levels of ptox and its function-related gene were consistent in different SA concentrations, but changes were most obvious with 0.5mM SA treatment. As the prolongation of SA treatment, the transcript levels of PTOX, PGR5, AOX1 and PDS increased and reached maxlmum after 10h; RbcS’s transcript level increased and reached maxlmum after 6h; the transcript levels of AOX2a, AOX2b and ZDS had little variation; but ndh-h’s transcript levels remained unchanged.2. As the increasing of PEG concentration and the prolongation of water stress, Pn, Gs, Ci, Gs, Fv/Fm, Fv’/Fm’,ΦPSⅡ, qP, total chlorophyll content, chla/chlb, carotenoid content and the activity of NADH-MDH of soybean seedlings decreased; and the activity of POD, SOD, CAT, APX and NADPH-MDH and the content of MDA increased. SA pre-cultivation has prevented the decrease of Pn under water stress. Extremely significantly, SA enhanced Pn of soybean seedlings under water stress （8h after 8%PEG treatment）. Simultaneously, SA pre-treatment enhanced Fv/Fm, Fv’/Fm’,ΦPSⅡ, qP, total chlorophyll content, chla/chlb, carotenoid content and the activity of POD, SOD, CAT, APX and NADPH-MDH、NADH-MDH, and degraded MDA content of soybean seedlings under water stress.3. Exposure to water tress produced a significant increase in the transcript levels of PTOX, AOX1 and AOX2b and the protein levels of PTOX. But the transcript levels of PGR5, RbcS, AOX2a, PDS and zds significantly decreased, and the transcript levels of ndh have not changed. SA pre-treatment increased the transcript levels of PTOX, PGR5, AOX1, AOX2b and ZDS and the PTOX protein levels of soybean seedlings under water stress. But have not changed the transcript levels of NDH-H, RbcS, AOX2a and PDS of soybean seedlings under water stress.Analyzing the results above, we found that SA enhances Pn of soybean seedlings under water stress mainly through:inducing stamatal closure to reduce water desorption; protecting photosynthetic pigment and apparatus by enhancing the activity of antioxidase and MDH; regulating photosynthetic electron transport and enhancing alternative respiratory. PTOX and AOX play important roles in the resistance of soybean seedlings to water stress.