| Recently, the close relationship between silicon and plant resistance to environmental stress has been discovered. However, the relevant studies were focused on high silicon-acucumulator monocots, such as rice, wheat, sorghum. As dicotyledonous plants, tomato(Solanum lycopersicum L.) is one of the most widely cultivated vegetable crops in the world. There were different views on Si uptake in tomato and physiological effects of silicon on tomato. To make insight into characteristics of silicon absorption and utilization in tomato and the physiological effects of silicon on tomato, on the basis of the analysis of effects of silicon on tomato growth and characteristics of silicon absorption and utilization in tomato, we carryed out a set of studies on effects of silicon on physiological metabolism, tissue structure and regulatory networks of protein in tomato under drought stress. The purpose was to clarify the mechanism of alleviation of drought stress by silicon supplementation in tomato and to provide a theoretical basis for reasonable application of silicon. The main results are as follows:1. Grown in Hoagland hydroponic solution, silicon contents of organs tomato were dramatically increased with the increase of silicon concentration in culture solution, and the differences among leaves were most significant. The SiO2 contents of leaves in Si0.6, Si1.2 and Si1.8 treatment were increased by 250.90 %, 403.59 % and 552.69 % compared with the control. Compared with CK, the plant growth and chlorophyll contents in Si1.2 and Si0.6 treatment were higher, but this is not the case for Si1.8 treatment. Simultaneously, net photosynthetic rates(Pn) in Si0.6 and Si1.2 treatment were also significantly higher than that of the CK, which were increased by 15.36% and 23.12%. At 11:00 o’clock, Pn of all treatments reach their peaks, while Pn of Si1.8 was 5.74% lower than that of the CK. Transpiration rate(Tr) of tomato leaves was decreased as the silicon concentration increased. In contrast with CK, Tr in Si0.6, Si1.2 and Si1.8 treatment were reduced by 7.42 %, 11.47 % and 23.08 % at 13:00 o’clock. The instantaneous water use efficiency(WUEi) of tomato dealing with silicon were significantly higher than that of the CK. At 11:00 o’clock, WUEi of all treatments reach their peaks, while WUEi of Si0.6, Si1.2 and Si1.8 treatment were 22.22%, 35.47% and 17.52% higher than the CK.2. Grown in Hoagland hydroponic solution, with the improvement of silicon level in culture solution, plant biomass and Si absorption of tomato increased remarkably. The improvement of silicon level in culture solution caused the reduction in fruit maturity period by 2-3 d and the increase in fruit set and yield for single plant. But fresh mass of single fruit decreased. While too high silicon level(1.8 mmol·L-1)weakened the increase of growth of tomato plant. Yields of tomato under silicon level of 1.2 mmol·L-1 were highest, which increased by15.57% compared with the control treatment. Meanwhile, fruit quality was significantly improved. The improvement of silicon level in culture solution also resulted in increase of silicon content,silicon absorption rate and silicon accumulation in tomato plants. Silicon content in blade was most abundant, to which root ranked only second. While fruit and stem were both low. The prolongation of growth period led to gradually increases in silicon content of each organ, as well as significantly enhancement of silicon absorption rate which came to peak at fruit flourish stage. Along with improvement of silicon level in culture solution,the silicon distribution ratio gradually reduced in leaves but increased in fruits. Meanwhile, there were small variations in root and stem.3. The drought damage index of tomato seedling increased with the increase of PEG-6000 concentration in Hoagland nutrient solution. On base of a comprehensive analysis, 1% PEG-6000 was selected for simulated drought stress in our experimental research. With drought time prolonged, relative water content(RWC), the photosynthetic pigments, net photosynthetic rate(Pn), stomatal conductance(Gs), maximum photochemical efficiency(Fv/Fm), actual photochemical efficiency(ΦPSⅡ), and photochemical quenching(qP) of tomato exposured to different silicon concentrations all continued to decline. While non-photochemical quenching(NPQ) gradually increased. The stomatal limitation value(Ls) first increased and afterwards decreased. Ho wever, intercellular carbon dioxide concentration(Ci) first decreased and afterwards increased. There were significant differences among the increase range and the decrease range of the parameters relation to tomato exposured to different silicon concentrations. Such as on day 12 of the experiment, in contrast to tomato plant without silicon(CK), the leaf RWC of plant in groups that were applied 0.6 or 1.2 mmol·L-1 silicon were increased 18.03%, 30.25%, respectively. Chlorophyll content were increased by 64.56%、88.24%. Pn were increased by 48.78%、131.71%. ΦPSⅡwere increased by 31.68%, 62.70%. qP were increased by 18.92%、40.54%. NPQ were decreased by 9.54%、13.35%. With dealt with 12 d by 1.8 mmol /L silicon level, however, relevant parameters were significantly lowered compared with the control(CK), except NPQ. For example, RWC, chlorophyll content, Pn, ΦPSⅡ and q P were respectively decreased by 17.53%, 21.79%, 21.95%, 10.16% and 5.41%. It was indicated that, under 1% PEG-6000 simulated drought stress conditions, Hoagland nutrient solution that was added 1.2 mmol·L-1 silicon, helped to maintain the high water content and photosynthetic pigment content in tomato leaves, improved photochemical efficiency in the leaves pigment, and reduced photoinhibition. Simultaneous, photosynthetic reaction ability in dark were enhanced, which contributed to maintain high photosynthetic rate.4. The tomato seedlings were grown in Hoagland solution added 1% PEG-6000. At 12 d, the simulated drought led to the increase in the ra tio of the root cortex in the diameter, and reduction in leaf thickness, palisade tissue thickness and spongy tissue tightness. while application silicon handled the drought- induced adverse change, which weakened water loss in tomato leaves and increased water storage capacity in tomato leaves under drought stress. Thereby water status of tomato leaves under drought stress were improved. During the drought stress,Application of silicon significantly increased zeatin(ZR), gibberellin(GA) content in tomato leaves and roots and reduced abscisic acid(ABA) content. Application of silicon significantly enhanced ZR/ABA, IAA/ABAand GA/ABA of roots and leaves. Throughout the process of drought sress, application of silicon also improved soluble sugar, soluble protein and proline content content. while application of silicon reduced the O 2ˉ. production rate, H2O2 content and MDA content. Therefor, exogenous silicon played a role in maintance of organ and tissue structure, regulation of the osmotic adjustment substa nce accumulation and alleviation of oxidative damage by active oxygen, which can improve drought tolerance in tomato.5. The results showed that with the extension of drought stress(Hoagland solution added 1% PEG-6000), MDA content in roots and leaves were both increased. Superoxide anion(O2-.) productivity rate and hydrogen peroxide(H2O2) content of mitochondria gradually increased. While the activities of superoxide dismutase(SOD), peroxidase(POD) and catalase(CAT) rapidly increased at the early stage of drought stress,and then rapidly decreased after peak.The application of silicon significantly increased SOD activity, POD activity and CAT activity of mitochondria, as well as antioxidase of chloroplast. Silicon also enhanced the content of reductive ascorbate and reductive glutathione, significantly reduced the accumulation of reactive oxygen. Therefor silicon alleviated the damage of cell membrane and maintained the intact structure of mitochondria and chloroplasts.6. To make a systematic and in-depth study on mechanisms of mitigation of drought stress by silicon, a set of differentlially expressed proteins of tomato leaves exploring to drought stress or drought stress plus silicon were analyzed by isobaric tags for relative and absolute quantitation(iTRAQ). we posed a contrast between tomato leaves under drought stress and tomato leaves under drought stress plus silicon.Altogether 468 proteins showed difference greater than 1.2 fold, including 373 up-regulated and 95 down-regulated. Pathway analysis showed that contrast with treatment of drought stress plus silicon, drought stress resulted in more severely suppressio of photosynthesis and photorespiration, oxidative decomposition of substance, stomatal closure by signal transduction, reduction in the active oxygen scavenging capacity, apoptosis and water loss. By iTRAQ technology, this study showed that the majority of the up-regulated or down-regulated protein accounted for five aspects, that is material and energy metabolism, photosynthesis and photorespiration, active oxygen scavenging, defense response. The pathway analysis showed that silicon alleviated drought- induced obstruction of photosynthesis and photorespiration, increase of the material oxidative decomposition, promotion of signal transduction related to stomatal closure, reduction of the active oxygen scavenging capacity, increase of water loss and promotion of apoptosis process. Thus, silicon improved drought tolerance of tomato. |
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