| Tree rings α-cellulose and long-chain lipids(fatty acids,n-alkanes)of terrestrial higher plants are not susceptible to external interference in the process of biological evolution,and their stable carbon and oxygen isotope compositions(δ13C,δ18O)are widely used for paleoclimate,paleoenvironment,and paleoecological reconstruction.However,we are not very clear about the complex physiological and metabolic pathways in plant organic matter synthesis,and the isotope composition at different positions in the molecule may record different environmental and physiological information,which needs to be mined in combination with intramolecular position-specific isotope analysis(PSIA),to strip metabolic signal and accurately establish the quantitative relationship of isotope composition and environmental factors from biomarkers.Therefore,this paper analyzed the position isotope distribution patterns of oxygen isotope in α-cellulose and carbon isotope in fatty acids,combined the isotopic analysis of the whole sample and molecular level,obtained the metabolic and climatic information recorded during the synthesis of α-cellulose and fatty acids.The main research contents and conclusions are listed as follows:(1)The position-specific oxygen isotope profile of α-cellulose glucose units in plant autotroph/heterotrophic tissues with different photosynthetic pathways by multi-step wet chemical strategy combined with GC/EA-Py-irMS showed heterogeneous distribution characteristics,and these distribution characteristics were similar in autotrophic and heterotrophic tissues.Compared to bulk stable isotope analysis(BSIA)of α-cellulose,O-2,O-3 and O-5 were enriched,O-4 and O-6 were depleted compared with α-cellulose.The extent of O-2 enrichment is not only related to the exchange of carbonyl oxygen with tissue water,but also to the kinetic isotope effect(KIE-18O)associated with the(invertase and/or sucrose synthase)catalyzed glycosidic bond fission.O-3 and O-5 are mainly related to the oxygen isotopic values of the water at the synthesis site.The relative 18O-depletion of O-4 is most likely due to the preferential consumption of 16O during formation of β-1,4-glycosidic bonds.O-6 is most likely to record the signal of the source water,and is also affected by the isotopic effect of multiple P-O bond fission,resulting in a shielding effect on the water source information.(2)By analyzing the oxygen isotopic composition of leaf and cambium watersoluble sugars(WSC),sucrose(Suc),and α-cellulose(bulk and PSIA)of highresolution oak(Quercus palustris Münchh)trees grown on the south slope of the Qinling Mountains and using simple correlation coefficient analysis,over the period of June 2021-June 2022,it was found that δ18OSuc and δ18OWSC of oak leaves and cambium had a positive correlation with temperature(T)and vapor pressure deifict(VPD),but the leaves and cambium δ18Ocell were not correlated with environmental factors,indicating a loss of signal in seasonal environmental variation in α-cellulose.At the same time,the cambium δ18OPG and δ18OO-3～5 were negatively correlated with relative humidity(RH)and positively correlated with VPD,and it was preliminarily concluded that α-cellulose δ18OPG and δ18OO-3～5 could be used as proxy indicators for high-resolution environmental and climate within the year.(3)To tease apart the climate signal from metabolic control that is imprinted in the individual carbon positions of long chain saturated fatty acids(LCFA),a three-step wet chemical strategy of α-bromination—α-hydroxylation—decarboxylation was developed,and the determination of four carbon atoms at the carboxyl end of commercial palmitate(n-C16:0 FA of C3 botanical origin)was accomplished by combining GC-C-irMS,rayleigh fractionation model and isotopic mass balance formula,and it was found that:the distribution pattern of carbon isotope composition in fatty acid molecules was "odd carbon enrichment,even carbon depletion".Similar patterns were also found in field-grown coconut fruits collected from tropical Hainan Island(China).It is believed that the key point affecting the carbon isotope profile of fatty acid molecules is that the process of acetyl-CoA forming malonyl-CoA catalyzed by acetyl-CoA carboxylase leads to a large positive kinetic isotope effect of methyl terminal carbon atoms.The long-held explanation that the kinetic carbon isotope effect(KIE-13C)associated with pyruvate decarboxyltion catalyzed pyruvate decarboxylase complex(PDH)is the reason why fatty lipid is relatively depleted in 13C against glucose,may not be valid.(4)With the PSIC-13C distribution pattern of fatty acids and current knowledge about fatty lipid chain extension mechanisms in higher plants,it is deducible that δ13C of the C2(acetyl-CoA)extension units for fatty lipid biosynthesis likely varies with chain length,and hypothesized that such δ13C-chain length relationship may differ between C3 and C4 plants as the two photosynthetic modes are known to have different carbon resources allocation strategies.To test this hypothesis,we measured 38 kinds of C3 and C4 grass leaves n-alkanes δ13C grown in Hainan Island(China),and found that:with the increase of chain length of C3 plants,the n-alkanes δ13C decreased;In C4 plants,the n-alkanes δ13C increased with the increase of chain length.Using the quantitative relationship between n-alkanes δ13C and chain length and other fatty acyl compound metabolic branches in leaves of higher plants,it was found that the distribution of C3 plant n-alkanes δ13C was mainly affected by the normal kinetic isotope effect when C2n acyl-ACP and malonyl-ACP condensed to form C-C bonds during chain lengthening,while the distribution trend of C4 plant n-alkanes δ13C was mainly related to higher leaf sugar(sucrose)export ratio. |
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