Alleviative Effects And Mechanisms Of Silicon On Salt Stress-induced Damage In Cucumber Seedlings

Soil salinity is one of the major abiotic factors that decrease plant growth and production worldwide. How to increase salt tolerance of crops has received worldwide attention. Among strategies to improve the crop salinity tolerance, application of exogenous substances may be a promising approach that saves both time and labor. The beneficial effect of silicon on salt stress has been reported in different plant species, especially in silicon-accumulating monocotyledonous plants. Cucumber(Cucumis sativus L.), which belongs to the cucurbitaceous family, is a worldwide vegetable and it is sensitive to salinity. Increasing salt tolerance of cucumber is requisite for its production in salinized soils. In this research, two cucumber cultivars, grown hydroponically, were chosen as experimental plant materials to study the effects of silicon on the seedling growth, photosynthesis, chlorophyll fluorescence,water metabolism, carbohydrate metabolism, polyamine metabolism and transcriptome of cucumber under both normal and salt stress conditions. The research may help to understand the mechanism for silicon-mediated tolerance to salt stress in plants, which provides a basis for application of silicon fertilizer in cucumber production. The main results are as follow:1. Salt stress decreased the shoot and root dry weights in both cvs. ‘Jin You 1’ and‘Jin Chun 5’. Under salt stress, the leaf chlorophyll content, photosynthetic rate, stomatal conductance, transpiration rate, maximum photochemistry efficiency of photosystem II(Fv/Fm), the efficiency of excitation capture by open PSII reaction centers(Fv’/Fm’), actual photochemical efficiency of photosystem II(ΦPSII) and coefficient of photochemical quenching(q P) were significantly decreased. Exogenous silicon alleviated the stress-induced reduction of plant dry weight and net photosynthetic rate. Fv’/ Fm’, ΦPSII and q P were also increased by silicon addition. Nonphotochemical quenching of the singlet excited state of chlorophyll a(NPQ) was increased by salt stress but decreased by silicon. These results suggest that more PSII reaction centers were in an open state, which allowed more excitation energy to be used for electron transport and improved the capacity to convert photon energy into chemical energy. Besides, silicon addition decreased the accumulation of superoxide(O2.-), hydrogen peroxide(H2O2)and malondialdehyde(MDA), which have contributed to the alleviation of oxidative damage caused by salt stress and thus promoted plant growth and photosynthesis.2. Salt stress significantly increased Na+ concentrations in both roots and leaves but decreased leaf water content in the two cucumber cultivars. Silicon addition only slightly decreased the shoot and root Na+ levels per dry weight in ‘Jin You 1’, but not in ‘Jin Chun 5’after 15 days of stress. When expressed on the basic of water content, the shoot and root Na+concentrations were obviously decreased by added silicon compared with salt stress alone after 10 days of treatment in both cultivars. Besides, the root hydraulic conductivity and leaf-specific conductivity of the seedlings were decreased by salt, but silicon addition alleviated the decreases. Furthermore, the root growth was enhanced to some extent by silicon under salt stress. These were beneficial to improve root water uptake and water transport to the leaves, and thus maintain plant water balance under saline condition. Further study suggest that silicon-mediated up-regulation of aquaporin gene expression and increased root xylem osmotic potential also contributed to the enhanced water uptake. The involvement of aquaporins in water uptake was supported by application of aquaporin inhibitor(Hg Cl2) and restorative(DTT). Besides, silicon application only decreased the root xylem osmotic potential and increased root soluble sugar levels in ‘Jin You 1’, suggesting that the regulatory effect of silicon in osmotic adjustment is cultivar-dependent.3. Salt stress increased the soluble sugar levels in both leaves and roots. Starch was accumulated in the leaves but decreased in the roots under salt stress. Add silicon decreased the soluble sugar levels in leaves through regulating the activities of carbohydrate metabolism enzymes. The starch content was decreased in leaves but increased in the roots by added silicon under stress. The regulatory effect of silicon on the root sugar content under salt stress was cultivar-dependent: silicon addition increased the root sucrose content in ‘Jin You 1’ but decreased it in ‘Jin Chun 5’ under salt stress. The increased soluble sugar in the roots of‘Jin You 1’ might contribute to osmotic adjustment of cucumber under salt stress, which facilitated root water uptake. These results indicate that under salt stress, silicon could decrease carbohydrate accumulation in the leaves by regulating the activities of related enzymes and thus alleviated the feedback repression of photosynthesis in cucumber plants.Moreover, silicon-enhanced assimilate transport from the source leaves to the rootsprovides more energy storage in the roots, which is beneficial for salt stress tolerance.4. Na Cl stress significantly increased the accumulation of free putrescine(Put) in the roots and leaves of both cultivars, while silicon addition inhibited this increase after 10 days of treatment. Compared with salt stress alone, silicon addition increased the contents of free spermidine(Spd) and spermine(Spm) as well as(spermidine+spermine)/ putrescine ratio in both cultivars. In the leaves and roots of ‘Jin You 1’, silicon might decrease the accumulation of free Put through increasing S-adenosylmethionine decarboxylase(SAMDC) activities and enhancing conversion of free Put to free Spd and Spm, as well as conjugated and bound Put,which decreased free Put-induced damage under salt stress. In ‘Jin Chun 5’, the regulatory effect of silicon is different in the leaves and roots. In the leaves, added silicon increased the conjugated and bound Spd as well as bound Spm contents. Moreover, silicon addition significantly inhibited the activity increases of key enzymes in polyamine biosynthesis and degradation pathways under salt stress. In the roots of ‘Jin Chun 5’, silicon decreased free Put accumulation through decreasing the activities of arginine decarboxylase(ADC) and ornithine decarboxylase(ODC) and enhancing the conversion of free Put to conjugated and insoluble bound Put. Moreover, added silicon promoted the conversion of free Spd and Spm to conjugated Spd and insoluble bound Spm, respectively. In addition, application of exogeneous Spd, Spm and inhibitors of polyamine synthesis also showed the alleviative effects of polyamines on salt stress-induced damage in cucumber plants. These results suggest that silicon may partly mitigate salt stress through regulating polyamine metabolism in cucumber seedlings.5. Comparative transcriptome analysis revealed that during salt stress, the expressions of multiple genes functionally associated with metabolism and environmental adaptation were strongly up- or down-regulated. Si plays an important role in shaping the transcriptome of cucumber. Specifically, 2,303 genes in leaves and 233 in roots were differentially expressed after silicon treatment, and these genes were mainly involved in amino acid metabolism,transcription factor, plant hormone signal transduction and stress response. With exogenous silicon added, the transcriptome of cucumber under salt stress showed a tendency of backing to the profile of the control. These results suggest a possibility that silicon works as an elicitor,which helps plants precondition to react to coming stress.In conclusion, Si could increase salt tolerance of cucumber seedlings through alleviating oxidative damage, enhancing root water uptake, regulating photosynthetic assimilate source-sink allocation and polyamine metabolism. Silicon may improve the tolerance of cucumber plants to salt stress via priming. However, the molecular underpinnings still remain to be further explored.