Intraspecific responses in crop growth and yield of 20 soybean cultivars to enhanced

Intraspecific responses in crop growth and yield of 20soybean cultivars to enhanced ultraviolet-B radiation under field conditionsLi Yuan a,b,*,Zu Yanqun b ,Chen Jianjun b ,Chen Haiyan baThe Center for Agricultural Biodiversity Research and Training of Yunnan Province,Yunnan Agricultural University,Kunming 6502011,PR ChinabEco-environment Research Institute,College of Resources and Environment,Yunnan Agricultural University,Kunming 6502011,PR ChinaReceived 22October 2001;received in revised form 15April 2002;accepted 18April 2002AbstractField studies were conducted to determine the potential for intraspecific responses in crop growth and grain yield of 20soybean cultivars to enhanced ultraviolet-B (UV-B,280–315nm)radiation.The supplemental UV-B radiation was 5.00kJ m À2,simulating a depletion of 20%stratospheric ozone at Kunming (258N,1950m).Out of the 20soybean cultivars tested,17and 15showed significant change in plant height at 80DAP (days after planting)and ripening stages,respectively.Sensitivity in plant height was greater at 80DAP than at ripening.The plant height of 3cultivars increased,and that of 17cultivars decreased.Under UV-B radiation,LAI (leaf area index),biomass and grain yield decreased,respectively.The greatest percent decrease was 95.7,93.9and 92.8,respectively.RI (response index)was the sum of percent change in plant height at ripening,LAI,biomass and grain yield.The results showed that all 20soybean cultivars had a negative RI,indicating inhibition by UV-B radiation on soybean growth.The RI of 6tolerant cultivars was higher than À163.1and 5out of 6originated from south China (low latitude).The RI of the most tolerant cultivars,Yunnan 97801,was À72.4.Meanwhile,the RI of 5sensitive cultivars was lower than À256.9and 4out of the 5originated from north China (high latitude).The RI of the most sensitive cultivar,Huanxianhuangdou,was À295.7.These UV-B tolerant cultivars identified in this study might be useful in breeding programs.#2002Elsevier Science B.V .All rights reserved.Keywords:Soybean;Stratospheric ozone depletion;UV-B radiation;Response index;Crop growth;Grain yield1.IntroductionThe rapid decline in stratospheric ozone concentra-tions has been confirmed by satellite measurements (Molina and Molina,1992).The most pronounced thinning of the ozone layer has been measured overthe Antarctic continent with up to 71%depletion during the Antarctic spring (Kerr,1993).Recent mathematical models predict a further increase in solar ultraviolet-B (UV-B)irradiation in future years (Madronich et al.,1995).UV-B effects on plants have been the subject of considerable research;approxi-mately 600papers have been published (Caldwell et al.,1998).An examination of more than 200plant species reveals that roughly 20%are sensitive,50%are mildly sensitive or tolerant and 30%are comple-tely insensitive to UV-B radiation (Teramura,1983).Field Crops Research 78(2002)1–8*Corresponding author.Present address:College of Resources and Environment,Yunnan Agricultural University,Kunming 6502011,PR China.0378-4290/02/$–see front matter #2002Elsevier Science B.V .All rights reserved.PII:S 0378-4290(02)00084-9Whilst the impact of enhanced UV-B radiation on plant physiology,morphology,growth,nutrition and biomass has been investigated extensively(Li et al., 1998,1999;Yue et al.,1988),little is known about intraspecific responses of plants to enhanced UV-B. Intraspecific responses in maize and wheat(Biggs and Kossuth,1978;Li et al.,2000a,b),broad bean (Bennett,1981),cucumber(Murali and Teramura, 1986),soybean(Teramura and Murali,1986;Sullivan and Teramura,1990;Teramura et al.,1990a;D’surney et al.,1993),rice(Teramura et al.,1991;Barnes et al., 1993;Dai et al.,1994)have been reported.Plant species and even genotypes within species can differ greatly in their responses to UV-B.Reasons for this are not clear (Caldwell and Flint,1994).Due to our lack of under-standing of the basis of intraspecific responses to UV-B radiation,further studies on its importance would be worthwhile.Most of the UV-B research in the past two decades has been conducted as short-term experiments in growth chambers and greenhouses,where an unna-tural spectral balance of radiation may have substan-tially changed plant sensitivity to UV-B.It is important in experiments to maintain a realistic bal-ance between various spectral regions,since both UV-A(315–400nm)and visible(400–700nm)radiation can have ameliorating effects on responses of plants to UV-B(Caldwell et al.,1995).In growth chamber and greenhouse experiments,the visible and UV-A radiation is usually much less than in sunlight;thus, even if realistic levels of UV-B are used in simulating ozone reduction,the plant response may be exagger-ated relative tofield conditions.Unfortunately,only 15%of the studies have been conducted underfield conditions.While the laboratory and glasshouse stu-dies provide information on mechanisms and pro-cesses of UV-B action,onlyfield studies can provide realistic assessments of what will happen as the stratospheric ozone layer thins(Caldwell et al., 1995).Vegetation communities function differently to iso-lated plants and the behavior of the former is not easily predicted from the latter.Besides,the responses of soybean crops to UV-B are more complicated than those of plants isolated(Teramura and Murali,1986; Sullivan and Teramura,1990;Teramura et al., 1990a,b;D’surney et al.,1993).Community-level field experimentation is needed to evaluate realistic consequences of increased solar UV-B resulting from ozone reduction.Soybean is one of the major world food crops.The effects of enhanced UV-B radiation on photosynthe-tic characteristics,growth,morphology,intraspecific responses,total biomass and yield have already been studied(Sullivan and Teramura,1990;Teramura, 1980,1983;Teramura and Murali,1986;Teramura et al.,1990a,b;Lydon et al.,1980;D’surney et al., 1993).Unfortunately,only few studies have been conducted underfield conditions(Teramura et al., 1990a).In this study we grew20soybean cultivars in thefield under ambient and supplemental levels of UV-B radiation for a season with the objective of evaluating intraspecific responses in crop growth and yield.We hypothesized that enhanced UV-B radiation would affect crop growth and yield and result in intraspecific responses underfield condi-tions.2.Materials and methods2.1.Plant materials and growth conditionsThefield experiment was conducted at Yunnan Agricultural University,Kunming,China.No fertili-zation was needed during the season.Seeds of20 soybean(Glycine max)cultivars were obtained from Yunnan Academic of Agricultural Sciences,Gansu Academic of Agricultural Sciences and Henan Aca-demic of Agricultural Sciences.Seeds were sown in rows spaced0.4m apart at a density of50seeds mÀ2in 120plots of2:0Â1:0m2each on10April1999.Five border rows were sown round each plot in order to minimize heterogeneity in microclimate.The overall experimental design was a randomized complete block with two UV-B treatments and three replica-tions.At30DAP(days after planting),plants were thinned to40mÀ2for uniformity in growth.This planting density is within commonly used sowing practices for the Kunming region.2.2.UV-B radiationSupplemental UV-B radiation was provided by filtered Gucun brand30W sunlamps(Gucun Instru-ment Factory,Shanghai,China)following the2L.Yuan et al./Field Crops Research78(2002)1–8procedure outlined in Lydon et al.(1986).Lamps were suspended above and perpendicular to the planted rows(rows oriented in an east–west direction to minimize shading)andfiltered with either0.13mm thick cellulose diacetate(transmission down to 290nm)for supplemental UV-B radiation or 0.13mm polyester plasticfilms(absorbs all radiation below320nm)as a control(Sullivan and Teramura, 1990).Cellulose diacetatefilters were presolarized for 8h and changed weekly to ensure uniformity of UV-B transmission.The spectral irradiance from the lamps was determined with an Optronics Model742(Optro-nics Laboratories,Orlando,FL,USA)spectroradi-ometer.The spectral irradiance was weighted with the generalized plant response action spectrum(Cald-well,1971)and normalized at300nm to obtain UV-B BE.Six lamps were installed above each plot.Plants were irradiated for7h daily from30DAP to ripening stage,centered around solar noon.Plants under polye-ster-filtered lamps received only ambient levels of UV-B radiation(10.00kJ mÀ2UV-B BE during clear sky conditions on the summer solstice).Plants beneath the cellulose diacetatefilters received ambient plus sup-plemental levels of UV-B.The lamp height above the plants was adjusted weekly to maintain a distance of 0.45m between the lamps and the top of the canopy, and provided supplemental irradiances of5.00effec-tive kJ mÀ2UV-B BE.This supplemental level was similar to that which would be experienced at Kunm-ing(258N,1950m)with a20%stratospheric ozone reduction during a clear day on the summer solstice (10.00kJ mÀ2UV-B BE)according to a mathematical model of Madronich et al.(1995).Total daily photo-synthetic photonfluence(PPF between400and 700nm)under lampfixtures was90%of that above the lamps.2.3.Measurements and statistical analysesFifteen plants per plot were used to observe plant height measured as the distance from the soil surface to shoot tip at80DAP and ripening.Plants in two subplots of0:5Â0:5m2each were harvested from each plot to determine leaf area index (LAI)at55DAP,total biomass(including roots)per subplot and grain yield per subplot at ripening.Total leaves and a subsample of15leaves per subplot were collected at55DAP,and LA(area per leaf)of this subsample was measured with a Li-Cor3100(Li-Cor, Lincoln,NE,USA)area meter.All leaves were then oven-dried at688C for68h and weighed.A regres-sion relationship was developed between leaf weight and leaf areaðP<0:01Þ.This linear regression was used to determine total LA per subplot,then LAI was I is leaf area per unit ground area. Total biomass and grain were oven-dried at688C for 68h and weighed.The response index(RI)according to Dai et al. (1994),Teramura and Murali(1986)and Li et al. (2000a)used to evaluate the overall response of soybean to enhanced UV-B radiation which was cal-culated as follows:RI¼PHtÀPH cPH cþLAI tÀLAI cLAI cþTB tÀTB cTB cþGY tÀGY cGY cÂ100%where RI is the response index,PH the plant height at ripening stage,LAI the leaf area index,TB the total biomass and GY the grain yield under t(UV-B radia-tion)and c(control).Statistical differences between means of control and UV-B radiation treatment of any measured parameter were determined by t-test at the P<0:05or0.01level. Correlation between the percent change of different parameters were determined at the P<0:05or0.01 level.3.Results3.1.Plant heightUV-B radiation had obvious effects on the plant height of20soybean cultivars at80DAP and ripening underfield conditions(Table1).At80DAP,UV-B radiation had a positive effect on Yunnan97929ðP<0:05Þ,and a negative effect on16cultivars (P<0:05or0.01).At ripening,UV-B radiation increased the height of Lanyin20ðP<0:05Þ,and significantly decreased the height of14cultivars (P<0:05or0.01).Out of20cultivars,17and15 showed significant differences(P<0:01or0.05)at 80DAP and at ripening,respectively.Sensitivity in plant height of20soybean cultivars to enhanced UV-BL.Yuan et al./Field Crops Research78(2002)1–83radiation varied with time,being greater at80DAP than at ripening stage.3.2.Leaf area indexUnderfield conditions at55DAP,LAI of20 soybean cultivars decreased significantly by UV-B radiation(Table2).Of20cultivars,Yunnan97801 and Yunnan97501showed significance at P<0:05 while another18cultivars were significant at P<0:01.UV-B radiation did not have positive effects on LAI for any of the cultivars.3.3.Total biomassUV-B radiation significantly decreased total bio-mass of20soybean cultivars(Table2).Of the20 cultivars,Yunnan97801,Yunnan97929and Yudou10 were significant at P<0:05,while another17culti-vars were significant at P<0:01.UV-B radiation did not have positive effects on total biomass for any of the cultivars.3.4.Grain yieldThe effect of UV-B radiation on grain yield of20 soybean cultivars underfield conditions is presented in Table2.Of the20cultivars,UV-B radiation had no significant effect on Lanyin20,Yudou8,Lvgundou, Heidadou and Longdou1,and had a negative effect on the other15cultivars(P<0:05or0.01).3.5.Response indexThe RI is an integration of the effect on plant height, LAI,total biomass and grain yield,which could reflect the overall sensitivity of soybean cultivars to enhanced UV-B radiation.In this case,all20cultivars had a negative RI,indicating an overall inhibition of UV-B radiation on soybean growth(Table3).Huanxian-huangdou was most adversely affected(RI,À295.7),while Yunnan97801was least affected (RI,À72.4).Across all cultivars tested in the present study,five cultivars had negative RI less thanÀ256.9, the most sensitive cultivars were Huanxianhuangdou,Table1Intraspecific sensitivity to UV-B radiation based on plant height(cm)of20soybean cultivars underfield conditions aCultivar name80DAP Ripening stageControlþUV-B%Change ControlþUV-B%Change Yunnan9780141.237.7À8.546.846.3À1.1 Lanyin2021.522.3 3.718.723.324.6* Yunnan9792943.349.514.3*57.058.8 3.2 Yudou1046.441.1À11.4*58.646.4À20.8** Yudou1854.246.4À14.4**61.948.1À22.3** Yudou849.540.3À18.6**58.342.2À27.6** Lvgundou34.934.8À0.349.949.8À0.2 Heidadou85.265.8À22.8**88.866.3À25.3* Yunnan9750162.348.1À22.8**68.247.6À30.2** Pitiaohuangdou64.250.9À18.4**60.451.9À14.1* Yunnan9750645.236.8À18.6**46.939.1À16.6** Yunnan9770153.843.6À19.0**49.043.4À11.4 Longdou176.646.4À39.4**79.852.4À34.3** E013857.140.4À29.3**68.452.3À23.5** Yunnan9651052.538.2À27.2**53.039.4À25.7** Df-159.445.7À23.1**72.144.9À37.7** Lingtaihuangdou39.527.2À31.1**39.732.4À18.4** Kanglehuangdou33.929.6À12.7*34.336.7 6.7 Tuhuangdou166.449.5À25.5**73.640.5À45.0** Huanxianhuangdou37.326.8À28.2**37.431.0À17.1**a Measurements were made on15plants per plot.*Significant differences between control and UV-B radiation at P<0:05according to t-test.**Significant differences between control and UV-B radiation at P<0:01according to t-test.4L.Yuan et al./Field Crops Research78(2002)1–8L.Yuan et al./Field Crops Research78(2002)1–85 Table2Intraspecific sensitivity to UV-B radiation based on LAI,total biomass and grain yield of20wheat cultivars underfield conditions a Cultivar name LAI Total biomass(g mÀ2)Grain yield(g mÀ2)ControlþUV-B%Change ControlþUV-B%Change ControlþUV-B%Change Yunnan97801 1.2870.976À24.2*396.8312.2À21.3*98.973.4À25.8* Lanyin20 1.7090.285À83.3**201.2105.0À47.8**68.556.5À17.5 Yunnan97929 3.455 1.078À68.8**617.5357.1À42.2*133.0105.5À20.7* Yudou10 2.982 1.057À64.5**446.2282.2À36.8*42.233.0À21.8* Yudou18 1.8500.735À60.3**412.4193.8À53.0**36.626.8À26.8* Yudou8 3.290 1.181À68.9**474.2237.6À49.9**45.537.9À16.7 Lvgundou 3.8240.390À89.8**698.5255.5À63.4**82.368.9À16.3 Heidadou 2.2040.551À75.0**615.3272.5À55.7**130.7109.4À16.3 Yunnan97501 1.655 1.069À35.4*833.3185.5À77.7**36.322.5À38.0* Pitiaohuangdou0.5430.269À50.5**283.569.7À75.4**33.817.0À49.7** Yunnan97506 1.8790.621À67.0**593.3294.4À50.4**161.564.3À60.2** Yunnan97701 1.9320.734À62.0**416.9122.1À70.7**39.116.9À56.8** Longdou1 5.4780.238À95.7**1042.4277.4À73.4**142.6125.9À11.7E0138 1.9510.330À83.1**674.4155.0À77.0**50.830.6À39.8* Yunnan96510 2.0820.753À63.8**392.259.6À84.8**10.4 5.2À50.0** Df-1 2.7880.413À85.2**971.6254.8À73.8**195.577.8À60.2** Lingtaihuangdou 2.8730.156À94.6**387.445.3À88.3**42.516.0À62.4** Kanglehuangdou 1.3510.106À92.2**343.446.9À86.3**92.8 6.7À92.8** Tuhuangdou1 1.4550.254À82.5**326.166.3À79.7**105.317.5À83.4** Huanxianhuangdou 2.4170.189À92.2**845.151.7À93.9**292.022.0À92.5**a LAI was determined on three plots at55DAP.Total biomass and grain yield are expressed on a ground area basis.Measurements were made on three subplots per plot.Shoot biomass and grain were determined at ripening.*Significant differences between control and UV-B radiation at P<0:05according to t-test.**Significant differences between control and UV-B radiation at P<0:01according to t-test.Table3RI of20wheat cultivars under enhanced UV-B radiation,together with their origin and habitat aRank Cultivar name Origin Habitat RI1Yunnan97801South China High elevation,uplandÀ72.4 2Lanyin20North China Middle elevationÀ124.0 3Yunnan97929South China High elevation,uplandÀ128.5 4Yudou10South China Low elevation,lowlandÀ143.9 5Yudou18South China Low elevation,lowlandÀ162.4 6Yudou8South China Low elevation,lowlandÀ163.1 7Lvgundou North China Middle elevation,uplandÀ169.7 8Heidadou North China Middle elevation,uplandÀ172.3 9Yunnan97501South China High elevation,uplandÀ181.3 10Pitiaohuangdou North China Middle elevationÀ189.7 11Yunnan97506South China High elevation,uplandÀ194.2 12Yunnan97701South China High elevation,uplandÀ200.9 13Longdou1North China Middle elevation,uplandÀ215.1 14E0138South China High elevation,uplandÀ223.4 15Yunnan96510South China High elevation,uplandÀ224.3 16Df-1South China High elevation,uplandÀ256.9 17Lingtaihuangdou North China Middle elevation,uplandÀ263.7 18Kanglehuangdou North China Middle elevation,uplandÀ264.6 19Tuhuangdou1North China Middle elevation,uplandÀ290.8 20Huanxianhuangdou North China Middle elevation,uplandÀ295.7a Ranking1–20is in the order of increasing sensitivity to UV-B radiation.Tuhuangdou1,Kanglehuangdou,Lingtaihuangdou and Df-1.The6most tolerant cultivars were Yunnan 97801,Lanyin20,Yunnan97929,Yudou10,Yudou 18,Yudou8,with response indices higher than À163.1.4.DiscussionThis is thefirst report identifying intraspecific responses in crop growth and yield of20soybean cultivars to enhanced UV-B radiation underfield conditions.This is supported by earlierfindings of intraspecific responses in soybean(Teramura and Murali,1986;Sullivan and Teramura,1990;Teramura et al.,1990a;D’surney et al.,1993),maize and wheat (Biggs and Kossuth,1978;Li et al.,2000a,b),horse-bean(Bennett,1981),cucumber(Murali and Tera-mura,1986),rice(Teramura et al.,1991;Barnes et al., 1993;Dai et al.,1994).Reduction in plant height has often been used as an index to assess the degree of UV-B radiation sensi-tivity(Biggs and Kossuth,1978).In thisfield study, UV-B radiation had significant effects on the height of 20soybean cultivars at80DAP and ripening(Table1). Sensitivity in height of20soybean cultivars to enhanced UV-B radiation was greater at80DAP than at the ripening.The correlation between height at80 DAP and height at ripening was significant(r¼0:754, P<0:01).Height at ripening was used in the RI.In a greenhouse experiment,UV-B radiation decreased plant height of soybean(Teramura,1980).UV-B radiation significantly dwarfed soybean,primarily due to shorter internodes rather than smaller node number(Teramura,1980).A similar result was observed in wheat(Li et al.,1998).This could be due to photo-oxidative destruction of the phytohor-mone indole acetic acid followed by reduced cell wall extensibility as demonstrated in sunflower seedlings (Ros and Tevini,1995).The levels of ethylene,which promote radial growth and reduce elongation,are increased after irradiation with UV-B(Caldwell et al.,1995).However,the mechanism for UV-B radiation to increase plant height is still not clear. Although UV-B radiation had no significant effects on leaf area and total leaf number of soybean in an early study(Teramura,1980),in this study,LAI of20 soybean cultivars responded significantly to UV-B radiation(Table2).Under UV-B radiation,sensitivity in LAI was significant.So,LAI may be used as a parameter for selection of UV-B tolerance.UV-B radiation may directly affect cell division and some intrinsic growth characteristics(Beggs et al.,1985).It may be associated with many of the changes observed following UV-B exposure,such as the changes in leaf area I was contributed by area per leaf and leaf number per subplot.In this study,leaf number per subplot and area per leaf decreased under UV-B radiation,respectively.Similarly,UV-B radiation decreased LA of soybean(Sullivan and Teramura, 1990)and this was the reason why soybean LAI decreased.Total biomass is a good indictor of the effects of UV-B radiation on growth(Teramura,1983).Total biomass represents a long-term integration of all biochemical,physiological,and growth parameters. Therefore,even subtle UV-B induced effects on phy-siological processes could accumulate and result in significant effects on biomass.Other studies indicated UV-B radiation decreased total biomass of soybean (Teramura,1983;Teramura et al.,1990b;Sullivan and Teramura,1990).Decrease in total biomass of soy-bean in this study are similar to those found in both greenhouse(Teramura,1980,1983)andfield(Sullivan and Teramura,1990)studies.No greenhouse experi-ments have reported increases or no changes in bio-mass of soybean with enhanced UV-B radiation (Teramura,1983).The effect of UV-B radiation on total biomass may be dependent on the concentration of UV-absorbing compounds that attenuate incoming radiation and affectively limit damage to cellular components including the genetic material(D’surney et al.,1993).Early experiments on effects of UV-B radiation on crops were conducted in growth chambers and green-houses.Owing to space limitations,it is impractical to grow plants to reproductive maturity in such facilities. Thus,very little is known about UV-B effects on grain yield of soybean(Teramura,1983).No changes were reported in a greenhouse study(Teramura et al., 1990b)and afield study(Sullivan and Teramura, 1990),grain yield of soybean cultivars Essex and Williams decreased when combined over the6-year field study(Teramura et al.,1990a).Reduced grain yield has important economic implications;conse-quently,using grain yield as a parameter in a RI is6L.Yuan et al./Field Crops Research78(2002)1–8necessary.In addition,comparisons between green-house andfield observations revealed that UV-B sen-sitivity based on seed yield of soybean differs markedly,indicating thatfield validations are ulti-mately necessary for assessing the potential impacts of increased solar UV-B radiation(Teramura and Murali,1986).The RI was a good indicator to assess plant sensi-tivity to enhanced UV-B radiation(Dai et al.,1994).In previous studies,parameters used in response indices were different.Only in the soybeanfield study of Teramura and Murali(1986),was grain yield included as a parameter,and response sensitivity of soybean to UV-B radiation differed markedly when based on seed yield.Additionally,these parameters were only observed with isolated plants of cucumber and rice during short-term experiments in the greenhouse (Murali and Teramura,1986;Teramura et al.,1991; Barnes et al.,1993;Dai et al.,1994).Because plants respond differently to UV-B in different environments (Teramura and Murali,1986)and inherent genetic differences in different cultivars also contribute,varia-bility between experiments could also be due to differences in growth conditions,length of UV-B radiation,stage of growth and the ratio of incident PPF to UV-B radiation,all of which have been demon-strated to greatly modify UV-B responsiveness(Ter-amura et al.,1991).Besides,the responses of soybean crops to UV-B were more complicated than those of isolated plants(Teramura and Murali,1986;Sullivan and Teramura,1990;Teramura et al.,1990a,b; D’surney et al.,1993).Intraspecific differences of soybean to UV-B radia-tion were observed in the greenhouse and thefield (Teramura and Murali,1986;D’surney et al.,1993).In this study,the parameters of the RI included plant height,LAI,total biomass and grain yield.The corre-lations between LAI and total biomass(r¼0:500, P<0:05),total biomass and grain yield(r¼0:672, P<0:01)were significant,respectively.And the parameters were measured in crops underfield con-ditions.Then the results of this study might provide realistic assessments of intraspecific responses among 20soybean cultivars to enhanced UV-B radiation. As ambient UV-B radiation level is greater at lower latitudes than that at higher latitudes,it is generally assumed that crop cultivars originating near the equa-tor are more tolerant to UV-B radiation.In this study,out of the6tolerant cultivars,5cultivars originated from south China(low latitude).The most sensitive cultivars,i.e.Huanxianhuangdou(RI-295.7),Tuhuang-dou1(RI-290.8),Kanglehuangdou(RI-264.6)and Lingtaihuangdou(RI-263.7),originated in north China(high latitude).Similar results were observed in wheat in afield study(Li et al.,2000a).However, rice cultivars originating from regions with high ambi-ent UV-B were not necessarily more tolerant to enhanced UV-B radiation(Barnes et al.,1993;Dai et al.,1994).The UV-B tolerant cultivars identified in our study might be used as possible donors in breeding programs;however,the genetic basis for these differ-ences must be further examined.In conclusion,enhanced UV-B radiation had sig-nificant effect on plant height,LAI,total biomass and grain yield of soybean crops underfield conditions. The RI presented here showed that soybean is poten-tially a UV-B sensitive species.This result was similar to the previous study(Biggs and Kossuth,1978).UV-B sensitivity might be associated with UV-B radiation fluences and long-term accumulation(Li et al.,1998). Soybean is an economically important crop and its intraspecific response to UV-B radiation underfield conditions is still not clearly understood.Effects of UV-B radiation on plants are related to other environ-mental factors,including PPF,CO2,drought,phos-phorus nutrition,temperature,ozone fumigation and heavy metal(Teramura et al.,1990b;Caldwell et al., 1995).Furtherfield studies are needed to improve the understanding of the relationship between UV-B radiation effectiveness and other environmental vari-ables.This would greatly enhance our ability to more realistically assess intraspecific responses of soybean to increased levels of UV-B radiation. AcknowledgementsThis work was supported by the National Natural Science Foundation of China,the National Research Foundation for Post-Doctorate Studies in China, the Applied Base Research Foundation of Yunnan Province,China,and the Research Foundation for Academic Leaders in Yunnan Province,China.We wish to thank Tan Lingling of the Department of Biology,Lanzhou University,for supporting the experimental work.L.Yuan et al./Field Crops Research78(2002)1–87ReferencesBarnes,P.W.,Maggard,S.,Holman,S.R.,Vergara, B.,1993.Intraspecific variation in sensitivity to UV-B radiation in rice.Crop Sci.33,1041–1046.Beggs, C.J.,Schneider-Zeibert,R.,Wellman, E.,1985.UV-B radiation and adaptive mechanisms in plants.In:Worrest,R.C.(Ed.),Stratospheric Ozone Reduction,Solar Ultraviolet Radia-tion and Plant Life.Springer,Berlin,pp.235–250. Bennett,J.H.,1981.Photosynthesis and gas diffusion in leaves of selected crop plants exposed to ultraviolet-B radiation.J.Environ.Qual.10,271–275.Biggs,R.H.,Kossuth,S.V.,1978.Impact of solar UV-B radiation on crop productivity.Final report of UV-B biological and climate effects research.Terr.FY77,11–77.Caldwell,M.M.,1971.Solar UV-B irradiation and the growth and development of higher plants.In:Giese,A.C.(Ed.),Photo-physiology,V ol.6.Academic Press,New York,NY,pp.131–171. Caldwell,M.M.,Flint,S.D.,1994.Stratospheric ozone reduction, solar UV-B radiation and terrestrial ecosystem.Climate Change 27,375–394.Caldwell,M.M.,Teramura,A.H.,Tevini,M.,Kulandaivelu,G., 1995.Effects of increased solar ultraviolet radiation on terrestrial plants.Ambio24,166–173.Caldwell,M.M.,Bjorn,L.O.,Bornman,J.F.et al.,1998.Effects of increased solar ultraviolet radiation on terrestrial ecosystem.J.Photochem.Photobiol.Biol.46,40–52.D’surney,S.J.,Tschaplinski,T.J.,Edwards,N.T.,Shugart,L.R., 1993.Biological responses of two soybean cultivars exposed to enhanced UV-B radiation.Environ.Exp.Bot.33,347–356. Dai,Q.J.,Peng,S.B.,Chavez, A.Q.,Vergara, B.S.,1994.Intraspecific responses of188rice cultivars to enhanced UV-B radiation.Environ.Exp.Bot.34,422–433.Kerr,R.A.,1993.The ozone hole reaches a new low.Science262, 501.Li,Y.,Yue,M.,Wang,X.L.,1998.Effects of enhanced ultraviolet-B radiation on crop structure,growth and yield components ofspring wheat underfield conditions.Field Crops Res.57,253–263.Li,Y.,Yue,M.,Wang,X.L.,Hu,Z.D.,petition and sensitivity of wheat and wild oat exposed to enhanced UV-B radiation at different densities underfield conditions.Environ.Exp.Bot.41,47–55.Li,Y.,Zu,Y.Q.,Chen,H.Y.,Chen,J.J.,Yang,J.L.,Hu,Z.D., 2000a.Intraspecific responses in crop growth and yield of20 wheat cultivars to enhanced ultraviolet-B radiation underfield conditions.Field Crops Res.67,25–33.Li,Y.,Zu,Y.Q.,Chen,J.J.,Chen,H.Y.,Yang,J.L.,Hu,Z.D., 2000b.Intraspecific differences in physiological responses of 20wheat cultivars to enhanced ultraviolet-B radiation under field conditions.Environ.Exp.Bot.44(2),95–103. Lydon,J.,Teramure, A.H.,Summers, E.G.,1986.Effects of ultraviolet-B radiation on growth and productivity offield-grown soybean.In:Worrest,R.C.(Ed.),Stratospheric Ozone Reduction,Solar Ultraviolet Radiation and Plant Life.Springer, Berlin,pp.313–325.Madronich,S.,Mckenzie,R.L.,Caldwell,M.M.,Bjorn,L.O.,1995.Changes in ultraviolet radiation reaching the earth’s surface.Ambio24,143–153.Molina,M.J.,Molina,L.T.,1992.Stratospheric ozone.ACS Symp.Ser.483,24–35.Murali,N.S.,Teramura,A.H.,1986.Intraspecific differences in Cucumis sativus sensitivity to ultraviolet-B radiation.Physiol.Plant.68,673–677.Ros,J.,Tevini,M.,1995.Interaction of UV-radiation and IAA during growth of seedling and hypocotyl segments of sun-flower.J.Plant Physiol.146,295–302.Sullivan,J.H.,Teramura,A.H.,1990.Field study of the interaction between solar ultraviolet-B radiation and drought on photo-synthesis and growth in soybean.Plant Physiol.92,141–146. Teramura, A.H.,1980.Effects of ultraviolet-B irradiances on soybean.I.Importance of photosynthetically active radiation in evaluating ultraviolet-B irradiance effects on soybean and wheat growth.Physiol.Plant.48,333–339.Teramura,A.H.,1983.Effects of ultraviolet-B radiation on the growth and yield of crop plants.Physiol.Plant.58,415–427. Teramura,A.H.,Murali,N.S.,1986.Intraspecific differences in growth and yield of soybean exposed to ultraviolet-B radiation under greenhouse andfield conditions.Environ.Exp.Bot.26, 89–95.Teramura,A.H.,Sullivan,J.H.,Lydon,J.,1990a.Effects of UV-B radiation on soybean yield and seed quality:a6-yearfield study.Physiol.Plant.80,5–11.Teramura,A.H.,Sullivan,J.H.,Ziska,L.H.,1990b.Interaction of elevated ultraviolet-B radiation and CO2on productivity and photosynthetic characteristics in wheat.Plant Physiol.94,470–475.Teramura, A.H.,Ziska,L.H.,Sztein, A.E.,1991.Changes in growth and photosynthetic capacity of rice with increased UV-B radiation.Physiol.Plant.83,373–380.Yue,M.,Li,Y.,Wang,X.L.,1988.Effects of enhanced ultraviolet-B radiation on plant nutrients and decomposition of spring wheat underfield conditions.Environ.Exp.Bot.40, 187–196.8L.Yuan et al./Field Crops Research78(2002)1–8。