Stable Silicon Isotope Composition And Fractionation In Different Silicon-aborbing Plants

Silicon(Si)is the second most mass-abundant element after oxygen in the Earth's surface.Si is essential for diatoms.From unicellular algae to vascular plants,numerous organisms are found to produce siliceous structures.As the beneficial effect of Si has been found,dissolved Si is absorbed in large amounts by terrestrial vegetation and weathering of silicates removes CO2 from the atmosphere.Therefore,there is a steadily growing scientific interest in the plant physiology and Si biogeochemistry.Terrestrial higher plants have a very important impact on the global Si cycle.However,to date there have only been very limited systematic research of Si isotope composition on terrestrial higher plants,and the most are focused on Si monocotyledonous plants.There are still many doubts about the Si isotope composition in dicotyledonous plants.The general objective of this study is to investigate Si isotope fractionation in different Si-absorbing plants(rice,maize,cucumber,wax gourd,and tomato)to lay the groundwork for understanding biogeochemical cycle of Si and mechanism of Si acquisition and allocation in plants.The main results are summarized as follows:1.The results of maize,cucumber,wax gourd and tomato collected from different regions in Zhejiang province showed that there were significant variations of Si isotope compositions in different Si-absorbing plants.The ?30Si values varied from-2.7‰ to 3.3‰,from-1.7‰ to 1.5‰,and from-1.0‰ to 1.9‰ among different organs in maize,cucumber and wax gourd,respectively.The ?30Si values of different organs were all accorded with the law of terminal distribution,by the order stem<root<leaf<grain(fruit).The ?30Si values exhibited a significantly gradient with a progressive increase from lower to upper organ except root.The degree of Si isotope fractionation in maize was stronger than that of cucumber and wax gourd.The 530Si values varied from-4.3‰o to-3·0‰ among different organs in tomato,and there was no significant difference between the ?30Si values of different organs.The Si isotope compositions of different parts of single organ of different silicon-absorbing plants also exhibited significant variations.The degree of Si isotope fractionation of different parts of single organ has reached the range of ?30Si values between different organs of the whole plant.The varied order of ?30Si values in stem and leaf of maize,cucumber,and wax gourd was the same,that is,basal part of stems<middle part of stems<top part of stems and basal part of leaf<middle part of leaf<top part of leaf.2.The results obtained from laboratory studies at different external Si concentrations suggested that the ranges of ?30Si values in rice were-1.89‰?1.69‰,-1.81‰?1.96‰,and-2.08‰?2.02‰ at 0.17 mM,1.70 mM,and 8.50 mM silicon concentrations,respectively.The ranges of 830Si values in cucumber were-1.38‰?1.21‰,-1.33‰?1.26‰,and-1.62‰?1.40‰ at 0.085 mM,0.17 mM,and 1.70 mM external silicon concentrations,respectively.The ?30Si values of different organs were also accorded witul the law of terminal distribution,that is,the ?30Si values exhibited a significantly gradient with a progressive increase from lower to upper organ except root.3.The biologically mediated Si isotope fractionation occurred between different Si-absorbing plants and dissolved Si in the external environment.The Si isotope fractionation factor(api-soi)was estimated to be 0.9989 in maize,0.9987 in wax gourd,0.9986 in cucumber,and 0.9980 in tomato.In the hydroponic systems under different external Si concentrations,the Si isotope fractionation factor(?Pl-Soi)was estimated to be 0.9998 in rice and 0.9999 in cucumber.4.The results obtained from experiment of the combination of plant Si physiological characteristics and Si isotope signal suggested that the absorption of Si by rice is likely to be a combination of active and passive absorption.The degree of Si isotope fractionation between depositional Si and dissolved Si in plants would be larger when plants absorb more Si from the external environment(CK treatment|?30Siroot-aboveground|=0.22‰;low-temperature or metabolic inhibitor treatment|?30Siroot-aboveground|=0.04‰).The fractionation effect between the whole plant and the external environment could also be enhanced when there was only one Si absorption mechanism(CK treatment |30?|=0.08‰;low-temperature or metabolic inhibitor treatment |30?|=0.21‰).