Alleviating Mechanism Of Exogenous Nitric Oxide On Alkaline Stress Of Cucumber Seedlings

Cucumber （Cucumis sativus L.） is one of the most important vegetables in our country,which often suffers from alkaline stress in protected and outside culture, resulting in yielddecrase dramatically. Previous studies indicated that nitric oxide （NO） plays important roles inimproving plant resistance to environmental stresses. However, a little was known about theeffects of NO on alleviating alkaline stress. Therefore, an investigation of the impacts ofexogenous NO on cucumber seedlings under alkaline stress has both theoretical and practicalsignificances.In this study, the roles of nitric oxide in alleviating alkaline stress on cucumber seedlingswere investigated from growth, physiology, and gene expression using sodium nitroprusside（SNP） as exogenous NO donor, NaHCO₃treatment as alkaline condition, and nutrient solutionas growth medium. We also explored the mechanism of this alleviating effect to providetheoretical foundation for enhancing cucumber tolerance to alkaline stress. The main resultswere as follows:1.30mmol·L^-1NaHCO₃treatment significantly induced cucumber leaf chlorosis,inhibited the plant growth, and reduced fresh weight and dry weight of cucumber plant rootand shoot, while application of100μmol·L^-1SNP could dramatically reversed the chlorosis ofcucumber leaves, promoted the growth of cucumber seedlings, and increased the fresh weightand dry weight of cucumber plant root and shoot.2.30mmol·L^-1NaHCO₃treatment significantly induced accumulation of H₂O₂andTBARS in cucumber leaves, led to serious electrolyte leakage, enhanced proline and soluablesugar contents, and decreased the activities of antioxidant enzymes and chelating activity offerrous ions. An observation of the ultrastructure of the mesophyll cells suggested that thestructures of chloroplast and mitochondria, as well as the whole membrane system had beenseverely destroyed under alkaline stress. On the contrary, application of SNP dramaticallyimproved the activities of ROS-scavenging enzymes, chelating activity of ferrous ions, andnutrient elements, decreased the degree of lipid peroxidation, as well as effectively protectedthe the structures of chloroplast and mitochondria. Meanwhile, compared with NaHCO₃treatment，there was significant increase in contents of acetate, malate, and citrate after SNPtreatment which had a modest effect on oxalate and succinate contents.3. Solexa sequencing technology, a high throughput DNA sequencing approach, wasutilized to estimate gene expression in libraries prepared from NaHCO₃treatment andNaHCO₃+SNP treatment tissues. The results showed that a total of901genes were responsive to SNP. Among them,437genes were up-regulated and464genes were down-regulated. GOenrichment analysis for different expression genes was conducted. According to the biologicalprocess,34GO terms were enriched, such as “photosynthesis”,“organic acid metabolicprocess”, and “Lipid biosynthetic process” etc.; according to the molecular function,13GOterms were enriched, which were “Catalytic activity”,“oxidoreductase activity”, and “iontransmembrane transporter activity” etc.; According to the cellular components,41GO termswere enriched, like “Chloroplast”,“peroxisome”,“vacuolar membrane”, and“Light-harvesting complex” etc..