| Salinity is an important factor affecting ecological environment in the soil that threatens agricultural production, thus it is a global research focus in recent years. At present, due to the rapid development of protected cultivation and much fertilizer used in process of cultivation, the soil secondary salinization has become one of the most severe problems in the protected cultivation. Cucumber is an economically important horticultural crop and highly sensitive to salinity. The growth and development of cucumber seedlings are inhibited when they are subjected to salinity. The photochemical efficiency of cucumber is significantly declined by salinity, which severely affects yield and quality of cucumber. Polyamines as a new plant growth regulators are low molecular weight ubiquitous nitrogenous compounds with a strong biological activity in all organisms and cells. Under environmental condition, PAs act not only as a direct protective substance, but also a signaling molecule involved in signal transduction, which is conductive to the plant resistance. At present, the view that PAs could increase the photosynthesis under stress has been recognized at home and abroad. However, the mechanism of PAs alleviating the inhibition of photochemical efficiency is still unclear.The present study was conducted hydroponics to investigate the effects of exogenous spraying putrescine (8mM Put) on the structure and function of photo synthetic apparatus in salt-sensitive cucumber cultivar (Jinyou No.4) were exposed to NaCl stress. We discussed the interaction mechanism of adaption variation that exogenous Put regulated the changes of the structure and function of photosynthetic apparatus, chloroplast ultrastructure, thylakoid membrane fatty acid and proteome in salt-stressed cucumber plants. The main results are as follows:1. Spayed application of5-15mM Put could alleviate the inhibition of photochemist-ry efficiency in cucumber. Among of them,8mM Put showed the best performance, which significantly enhanced photosynthetic pigment content and improved the cucumber seedlings growth.2. Exogenous Put significantly enhanced the maximum quantum efficiency (Fv/Fm), the actual efficiency of photosystem Ⅱ (ΦPSⅡ) and protective heat dissipation (ΦNPQ) in the salt-stressed plants. But the destructive heat dissipation (ΦNO) and electron transport rate (ETR) were decreased by salinity. The result showed that exogenous Put could alleviate inhibition of photochemistry efficiency of cucumber through regulating heat dissipation capacity pathway.3. Exogenous Put could significantly increase electron transport per active reaction center (ETo/RC), and promote the primary photochemistry (Ψo) and quantum yield for electron transport (ΨEo). However, it effectively reduced non-photochemical dissipation energy (DIo/RC), the functional size of the antenna per active reaction center (ABS/RC) and light-trapping efficiency per reaction center (TRo/RC), thus promoting total photosynthetic driving force (DFABS) and the structure performance index (PIABS).These results suggested that Put could effectively alleviate the damage of salt stress on the structure and function of photosynthetic apparatus in cucumber seedlings.4. Foliar application of Put significantly reduced the content of tetradecane acrylic, palmitic acid and saturated fatty acids (SFA). The content of hexadecenoic acid, linoleic, linolenic and unsaturated fatty acids (UFA) and the ratio of UFA/SFA were significantly enhanced by exogenous Put treatment. The analysis of chloroplast ultrastructure showed that Put alleviated the disintegration of thylakoid grana lamellae and reduced the number of plastoglobuli in thylakoid membrane. These results indicated that Put could improve the salt tolerance of photosynthetic apparatus through enhancing the content of unsaturated fatty acid composition, thus increasing the liquidity of membrane and maintaining the ordered arrangement of grana thylakoids.5. Blue native polyacrylamide gel electrophoresis (BN-PAGE) and SDS-urea-PAGE were used to separate the membrane proteins, and more than60membrane proteins were visible. Among of them,34protein spots were used to identify by liquid chromatography electro-spray ionization tandem mass spectrometry. Compared with the control, salt stress down-regulated the proteins expression of energy synthesis and PSⅡ reaction center in the thylakoid membrane, such as ATPase CF1, ATPase, CP47, D1, Qb and PsbA. Among of them, proteins spots of D1, CP47, Qb and PsbA had remarkable decrease. However, some membrane proteins were up-regulated by salt stress, such as CP24, D2and LHCⅡ; analysis in transcriptional level showed except the gene of CP47, LHCII, D2and Atpl, the rest of five candidate trancripts exhibited a consistent transcription accumulation changes with the corresponding protein abundance. These results suggested that Put regulated the proteins expression to varying degree in transcription and translation levels.6. Exogenous Put significantly enhanced activities of SOD, POD, and CAT in chloroplast, and also was involved in increasing activities and content of APX, GR, DHAR, MDAR, AsA, GSSG, GSH, DAsA and AsA-GSH cycle, thus the levels of O2-and ROS production were decreased in the chloroplast of salt-stressed cucumber seedlings. In addition, Put treatment significantly decreased the accumulation of Na+and Cl-in chloroplast of cucumber seedlings exposed to salt stress. These results suggested that Put alleviated the oxidative damage and ion toxicity on photosynthetic apparatus of the salt-stressed cucumber seedlings.7. Exogenous Put significantly improved energy-dependent quenching (qE) in NPQfast component, Put also increased the composition of xanthophyll cycle and promoted the de-epoxidation (DEPS) of the xanthophyll cycle in cucumber leaves under salt stress. The result indicated that Put dissipated the excess excitation energy through regulating the protion gradient in thylakoid membrane and promoting xanthophyll cycle.8. Exogenous Put significantly increased free, bound and conjugated polyamines (Put, Spd and Spm) of the chloroplast and thylakoid in salt-stressed cucumber seedlings, especially the conjugated Spd and Spm. Bound and conjugated Put/Spd ratios in the thylakoids were significantly decreased by Put treatment. These results indicated that Put alleviated the salt-induced damage of photosynthetic apparatus structure though regulating levels and forms of endogenous PAs in photosynthetic apparatus. |
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