Effects Of Exogenous Spermidine On CO₂ Assimilation And Metabolism Of Salt-Stressed Cucumber Seedlings

Salt stress is one of the most important abiotic limiting cucumber productivity. Because crop productivity is closely related to photosynthesis, it is very important to study the effects of salinity on photosynthesis in order to illuminate the photosynthesis internal mechanism of cucumber salt tolerance. Polyamines are ubiquitous low-molecular-weight aliphatic amines that are involved in regulation of plant growth and development. It was reported that polyamines were involved in the photosynthesis of higher plants to various environmental stresses. However, because of the diversity role of exogenous polyamines and the complexity of changes of photosynthesis to various environmental stresses conditions, the specific mechanism on ncreasing photosynthesisthe by polyamines under stress conditions is not clear, it has no report about exogenous Spd on CO2 assimilation and metabolism of cucumber seedlings under salt stress.In the present study, salt-sensitive cultivar'Jinchun No.2'was chosen as experimental materials. We discussed the effect of exogenous Spd added to nutrient solution containing 50 mmol·L-1 NaCl on CO2 assimilation and metabolism using nutrient solution culture method, providing a theoretical basis for further research on exogenous Spd relieving mechanism on photosynthesis of cucumber seedlings under salt stress. Main research results were as follows:1. Seedlings grown in nutrient solution treated with salt stress for 9 days displayed inhibited growth, reduced chlorophyll (Chl) content and net photosynthetic rates (Pn), stomatal conductance(Gs), intercellular CO2 concentration(Ci) and transpiration rate (Tr). Stomatal limitation was the main reason of declining photosynthesis under short-term salt stress, however, under long-term salt stress stomatal limitation and non-stomatal limitation as the dominant factor; Moreover actual efficiency of PSⅡ(ΦPSⅡ) and photochemical quenching(qP) showed a tendency similar to Pn. However, non- photochemical quenching (qN) showed a quite opposite manner as compared to the others as mentioned above. Howeover, salinity had no significant effect on maximum efficiency of PSⅡ photochemistry (Fv/Fm), Growth and photosynthesis of salt-stressed cucumber seedlings improved by Spd application to salinized nutrient solution. Moreover, Spd increasedΦPSII and qP, also decreased qN. It is susgested that exogenous Spd could increase the salt tolerance of cucumber plants, and this was associated with decreasing on stomatal limitation and increaseing heat dissipation by exogenous Spd.2. Salt stress reduced the soluble protein content, as well as the activity and content of Calvin cycle rate-limiting enzyme Rubisco and Ald of cucumber seedlings, impeded the smooth progress of Calvin cycle; Exogenous Spd increased the activity and content of Rubisco and Ald significantly, accelerated the operation of the Calvin cycle, improved the CO2 assimilation capability of cucumber plants under salt stress, then increased the salt tolerance.3. Salt stress induced an increase on total soluble sugar, sucrose and starch levels in cucumber leaves, also the activities of phosphate sucrose synthase (SPS) and sucrose synthase (SS), which are two key enzymes of sucrose accumulation, while a decrease on starch hydrolytic enzyme (Amylase) activity, a key enzyme of starch accumulation. Total soluble sugar and sucrose contents in roots presented a similar trend with leaves, but contrary to starch content. SPS, SS and starch enzyme activity in cucumber roots were all improved by salt stress when subjected to salinity. Exogenous Spd application alleviated the salinity-mediated changes of carbohydrate accumulation in cucumber leaves, together with SPS, SS and Amylase activity. Exogenous Spd caused a decrease of total soluble sugar and sucrose content in roots, together with SPS, SS and Amylase activity, and an increase of starch content. These results suggested that exogenous Spd involved in the regulation of plant stomatal opening, and reduced carbohydrate accumulation in leaves by regulating some key enzymes activity, regulated carbohydrate distribution between source and sink, promoted conversion and utilization of carbohydrates, finally alleviated the damage on photosynthesis and enhanced salt tolerance of cucumber seedlings.4. Short-term salt stress (3 d) inhibited the growth and photosynthetic significantly. Meanwhile, Salt stress increased the gene expression of Rubisco activating enzyme, glyceraldehyde-3-phosphate dehydrogenase and ribulose-5-phosphate kinase, decreased the gene expression of Rubisco small subunit, Rubisco large subunit,3-phosphoglyceric acid kinase, Aldolase, Furetose-1,6-bisphosphate phosphatase and Triose-3-phosphate isomerase, but had no effect on the expression of sedoheptulose-1,7-bisphosphate phosphatase; Growth and photosynthesis of salt-stressed cucumber seedlings improved by Spd application to salinized nutrient solution. Moreover, Exogenous Spd differed in regulating the salt stress-mediated accumulation of transcripts. Combined the result of long-term salt stress, we speculated that salt stress reduced photosynthesis of cucumber seedlings contained both stomatal limitation and non-stomatal limitation. Exogenous Spd could alleviate stomaal limitation, also non-stomatal limitation by increasing the activity and content of Calvin cycle rate-limiting enzyme, regulating the gene expression of key enzymes, reducing the injury on photosynthesis under salt stress, increasing the salt toleration of cucumber seedlings.