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ACTA ECOLOGICA SINICA Volume 28,Issue 2,February 2008 Online English edition of the Chinese language journal Cite this article as:Acta Ecologica Sinica,2008,28(2),525533.Received date:2006-12-24;Accepted date:2007-08-23*Corresponding author.E-mail: Copyright 2008,Ecological Society of China.Published by Elsevier BV.All rights reserved.RESEARCH PAPER Responses of water use efficiency of 9 plant species to light and CO Responses of water use efficiency of 9 plant species to light and CO2 2 and their modeling and their modeling Wang Jianlin1,Yu Guirui2,*,Fang Quanxiao1,Jiang Defeng1,Qi Hua3,Wang Qiufeng2 1 Qingdao Agricultural University,Qingdao 266109,China 2 Institute of Geographic Sciences and Natural Resources Research,Chinese Academy of Sciences,Beijing 100101,China 3 Shenyang Agricultural University,Shenyang 110161,China Abstract:Photosynthesis coupled with transpiration determines water use efficiency(WUE)at leaf level,and the responses of WUE controlled by gas exchanges through stomata to environment are the basis of carbon and water cycles in the ecosystem.In this paper,by using Li-6400 Portable Photosynthesis System(LI-COR),WUE at leaf level was analyzed under controlled photosynthetic photons flux density(PPFD)and CO2 concentration conditions across 9 plant species including maize(Zea mays),sorghum(Sor-ghum vulgare),millet(Setaria italica),soybean(Glycine max),peanut(Arachis phyogaea),sweet potato(Ipomoea batatas),rice(Oryza sativa),Masson pine(Pinus massoniana)and Schima superba.We had developed a new model to estimate the water use effi-ciency in response to the combined effects of light and CO2 concentration.Our measured data validated that this model could simu-late the changes of water use efficiency very well under combined effect of light and CO2 concentration.It could be used to estimate contribution of photosynthesis increase and transpiration decline on water use efficiency with the rising of CO2 concentration.Great differences in water use efficiency occurred in these different plant species under various CO2 concentration levels.Based on water use efficiency at regional scale,we concluded that plants should be separated into C3 plants and C4 plants,and furthermore,C3 plants should be separated into herbaceous plants and woody plants.Our separation criteria would do a great favor in modeling the evapo-transpiration of terrestrial ecosystem with carbon and water balance.Key Words:water use efficiency(WUE);photosynthetic photons flux density(PPFD);CO2 concentration;modeling Water use efficiency(WUE)is expressed as assimilation production per unit water.As an important index for plant energy conversion,WUE has been applied widely in many fields 1.The concepts and definitions of WUE among differ-ent fields and scales are different 2.At leaf level,WUE is the ratio of net photosynthesis rate(Pn)and transpiration rate(Tr),which are coupled with one another under stomatal regula-tions.Pn is the primary drive for plant production,evolution and global carbon cycle,while transpiration drives water cycle in soil plant atmosphere continuum(SPAC)and ecosystem.Therefore,modeling plant WUE is the key index to evaluate the interactions between water and carbon cycles in the eco-system.Plant Pn,Tr,stomatal conductance and WUE are in-fluenced by solar radiation and CO2 density and other envi-ronmental factors;on the other hand,they show powerful self regulations and adaptations to these environmental factors 3,4.Stomata regulate the relation between carbon assimilation and water transpiration,but the transfer directions between carbon and water are different.Carbon assimilation can reach maxi-mum by stomatal regulations;on the other hand,the stomatal regulations are constrained by photosynthate59,which results in different synchronizations between CO2 and water exchanges and various WUEs 4,10.Therefore,in understanding the responses of stomatal be-haviors to environmental factors,modeling the adaptations of WUE controlled by gas exchanges to environment changes is the basis of understanding the carbon and water cycles and their interactions in the ecosystem,and is required to establish carbon and water cycle model at different scales 11,12.In this paper,a model of WUE responses to solar radiation and CO2 was developed based on the rules of gas exchanges,then it WANG Jianlin et al./Acta Ecologica Sinica,2008,28(2):525533 was evaluated with 9 plant species(C3 and C4),and the dif-ferences in WUE between species were analyzed.These re-sults can contribute to discovering the mechanism of the in-teractions between water and carbon cycles,and developing water and carbon cycle model under conditions of climate change.1 Model instructions 1 Model instructions WUE is defined as the ratio of Pn to Tr at leaf level as fol-lowing formula:WUE=Pn/Tr (1)1.1 Photosynthesis rate(P1.1 Photosynthesis rate(Pn n)In 1913,Michaelis-Menten brought out the equation be-tween physiochemical reactions and substrate.Light is not the substrate for Pn but the energy resource for Pn,and can be considered as substrate for Pn.Therefore,the response of Pn to light intensity can be described by the following Micha-elis-Menten equation which has been applied widely,max,max,jndjPPFDPPRPPFDP=+(2)where is the model fraction of apparent quantum yield,Pmax,i is the model fraction of maximal photosynthesis rate in constant CO2,and Rd is the dark respiration.CO2 can be considered as the substrate of Pn,and the re-sponses of Pn to CO2 can also be described by the following Michaelis-Menten equation,max,max,scndscC PPRCP=+(3)where Cs is CO2 in leaf surface,is the apparent car-boxylation rate,and Pmax,c is the model fraction of maximal photosynthesis rate in constant PPFD.If light and CO2 are considered as the substrate of Pn,then the responses of Pn to the two factors can also be described by the following Michaelis-Menten equation 13:maxmaxmaxndPPFD CPPRPPFDPCPPPFD C=+(4)where Pmax is the potential maximal photosynthesis rate.1.2 Transpiration rate(T1.2 Transpiration rate(Tr r)Plant transpiration at leaf level is determined by leaf con-ductance(gst)and vapor pressure difference(VPD),and the transpiration can be defined as:()1/1/1/1/iaPWLrstsbsbWWCeTeTg VPDgggg=+(5)where Wa is water vapor in the air,Wi is water vapor in the stomatal space(kg m3),gs is stomatal conductance,gb is con-ductance of the boundary layer,is air density(kg m3),e is water vapor pressure(Pa),eW(TL)is saturated water vapor pressure at leaf temperature(TL),is psychrometric constant (Pa K1),CP is specific heat capacity of the air at constant pressure(J kg1K1),and is latent heat of evaporation(J kg1).1/gst=1/gs+1/gb (6)gb can be calculated from equation(7):1/bbLgKW u=(7)where uL is wind speed at leaf surface,W is width of the leaf,and Kb is constant.Calculation of the transpiration rate by equation(5)requires to estimate stomatal conductance.Therefore,the responses of stomatal conductance to light intensity and CO2 concentration needs to be decided.Many studies have reported the response functions of stomatal conductance(gst)to light 14 using hy-perbola relation,and in this study,the hyperbola was also used for describing the responses of gst to light,max,/1/csictcPPFD PPFDgggPPFD PPFD=+(8)where gmax,i is maximal stomatal conductance while PPFD,PPFDc is the PPFD when gs reaches 1/2 gmax,i,and gct is cutin conductance.Pn can alter CO2 concentration and subsequently stomatal conductance.The responses of stomatal conductance to CO2 concentration can be described by hyperbola 15:max,01/csssggCC=+(9)where gmax,c is maximal stomatal conductance in constant PPFD,and Cs0 is constant.By applying Jarvis model 16 structures,and combining the responses of stomatal conductance to light and CO2 concen-tration,the model of the responses of stomatal conductance to light and CO2 concentration can be described as follows:max01/1/1/csctsscPPFD PPFDgggCCPPFD PPFD=+(10)where gmax is potential maximal stomatal conductance.By combining equations 7,10 and 2,the model of re-sponses of Tr to light and CO2 can be obtained as follows:max0()1/1/1/1/PrWLcctbLsscCTeTePPFD PPFDggKW uCCPPFD PPFD=+(11)1.3 Water use efficiency(WUE)1.3 Water use efficiency(WUE)Based on the above models,Pn and Tr can be described by equations 4 and 11 under coupled light and CO2,and the model of WUE can be described as follows:WANG Jianlin et al./Acta Ecologica Sinica,2008,28(2):525533 2 Materials and methods 2 Materials and methods 2.1 Materials and experiment sites 2.1 Materials and experiment sites Experiments were carried out at Shenyang Agricultural University(4148N;12324E;L:42 m)and Qianyanzhou Station of the Chinese Academy of Sciences(CAS)(2645N;1154E;L:107 m).7 plant species included were used for ex-periment.Maize,sorghum,millet,soybean,peanut and sweet potato were planted in mid-May with 225 kg N hm2,82.5 kg P2O5 hm2 and 187.5 kg K2O hm2;rice was planted on 23 May with 200 kg N hm2 and 20 kg P2O5 hm2,and a raw spacing of 30 cm 13.3 cm.Other managements were controlled as the field practices used by local farmers.The soil at Qianyanzhou Station was characterized as red earth zone and planted with Masson pine and schima(18 year old).2.2 Measurements 2.2 Measurements Pn,Tr and gs and other environmental factors were meas-ured by using Li-6400 Portable Photosynthesis System(LI-COR,Ltd.,USA).During the measurement,the flow velocity was set as 500 m2 s1 and the leaf temperature was controlled at 30 centigrade.The photosynthetic photon flux density(PPFD,mol m2s1)controlled by red and blue resources were set into 15 levels:2000,1800,1600,1400,1200,1000,800,600,400,200,100,60,20,10,0,and CO2 concentration in the reference chamber controlled by CO2 injector system were set into 9 levels:0,50,100,200,300,400,600,800,1000.For Masson pine and schima,PPFD was set into 13 levels:1000,800,600,400,200,100,80,60,40,20,10,5,0,and CO2 concentrations in the reference chamber was set into 11 levels:0,50,100,200,300,400,600,800,1000,1200,1400,1600.For every CO2 level,30 min was used for preparation and the response time was set as 120 sec for crops and 180 sec for trees.3 Response characters and evaluations 3 Response characters and evaluations Equation 12 was used to predict WUE of the 9 plant species,and the relation between simulations and measured values was presented in Fig.1.The statistic results showed that equation 12 could be used to well simulate the responses of WUE to light and CO2 concentrations.4 Results and discussion 4 Results and discussion 4.1 Effects of CO4.1 Effects of CO2 2 concentration on WUE concentration on WUE Global climate changes associated with enrichment in CO2 concentration greatly influenced WUE,which in turn could alter global water and carbon cycles.Therefore,understanding the responses of WUE to elevated CO2 is the important means to predict the responses of global water and carbon cycles to elevated CO2 concentration.Many studies have been carried out on the effects of elevated CO2 on WUE,showing obvious increases in WUE by 50%150%at doubled CO2 concentra-tion mainly because of the increase in Pn and decrease in Tr with the increasing CO2 concentrations 17.But Murray 18 reported that WUE was increased mainly by the decreasing gs,while Rogers et al.19 reported opposite results that the in-creasing WUE with increasing CO2 was mainly due to the decrease in Tr.In another study 20,Kimball reported that the increase in Pn with increasing CO2 was the main reason for the increasing WUE.There has not yet been a conclusion on the reason for the increasing WUE with elevated CO2 concentra-tions.In almost all the experiments,Pn increased with in-creasing CO2 concentration,and then Pn appeared to decline,which was mainly caused by the accumulation of photosyn-thate.In a 1-year experiment,Pn can also be increased by 29.69%.The extent of decrease in Pn suppressed by the accu-mulation of photosynthate is generally small under high CO2 concentration,and Pn can maintain a high level.These results showed that the increase in Pn owing to high CO2 concentra-tion contributed greatly to the increase in WUE.Stomatal conductance as an important index shows sensitive responses to elevated CO2 concentrations,i.e.,stomatal conductance de-creases with elevated CO2 concentration and maintains lower intercellular CO2 pressure by 20%30%than atmosphere CO2 concentration.Under doubled CO2 concentration conditions,33%50%or more of the stomatal resistance was increased2123.The decrease in stomatal conductance can reduce transpiration rate and thus contribute to the increase in WUE.In this study,similar result was obtained that both increase in Pn and de-crease in Tr increased WUE.The contributions of Pn and Tr to the increase in WUE varied with the different light intensities and CO2 densities calculated by the model(Table 1),where great differences in WUE between C3 and C4 plants were found with the elevated CO2 concentration.For C3 plants,the increase in WUE was mainly caused by the increase in Pn,while for C4 plants,the increase in WUE was mainly caused by the decrease in Tr.4.2 Differences in plant WUE among plant species under various CO4.2 Differences in plant WUE among plant species under various CO2 2 concentrations concentrations C3,C4 and CAM plants showed different WUEs owing to different photosynthesis ways and primary production 24.Under current CO2 concentration level,WUE of CAM plants maxmax0maxmax1/1/1/1/()cctbLdsscPWLPPFD PPFDPPFD CPggKW uRCCPPFD PPFDPPFDPCPPPFD CWUECeTe+=(12)WANG Jianlin et al./Acta Ecologica Sinica,2008,28(2):525533 is higher than that of C3 and C4 plants 25,and WUE of C4 is general higher than that of C3 26.Because C3 and C4 plants account for about 99%of the terrestrial vegetation,WUE of C3 and C4 plants can determine the coupled water and carbon cycles in terrestrial ecosystem,and accurately distinguishing the WUE between C3 and C4 plants under elevated CO2 con-centrations is very important for predicting the effects of global changes on water and carbon cycles in terrestrial eco-system.Based on the data in Table 1 and by using hierarchical cluster analysis(Fig.2),it was found that there were obvious differences in WUE between C3 and C4,and among C3,her-baceous and woody plants,mainly associated with different responses of plants to the changes in CO2 concentrations.From above results,study on WUE of regional plants under elevated CO2 conditions should distinguish C4 and C3 plant species as well as herbaceous and woody plants in C3 plants to accurately estimate WUE of regional plants.4.3 Regional application of WUE model 4.3 Regional application of WUE model Fig.1 Relationship between measured and simulated values of water use efficiency under interaction of light and CO2 Table 1 Elevated WUE by increased photosynthesis and decreased transpiration at doubled CO2 concentration(PPFD1000 mol m2s1)CO2=375 mol mol1 CO2=750 mol mol1 Species Pn(molm2s1)Tr(mmolm2s1)WUE(mmolmol1)Pn(molm2s1)Tr(mmolm2s1)WUE(mmolmol1)Increased
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