Analysis of Local Dandelion (Taraxacum officinales.l.) Cenopopulations from Radioactively Contaminated Zones

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<ul><li><p> 1067-4136/01/3202- $25.00 2001 </p><p>MAIK Nauka</p><p>/Interperiodica0102</p><p>Russian Journal of Ecology, Vol. 32, No. 2, 2001, pp. 102109. Translated from Ekologiya, No. 2, 2001, pp. 117124.Original Russian Text Copyright 2001 by Pozolotina.</p><p>Radionuclide migration in biogeocenoses and thebiological effects of ionizing radiation on organismsand their communities are often difficult to studybecause radioactive contamination of landscapes isnonuniform. Radionuclides falling from the atmo-sphere and deposited by water flows are distributedunevenly; the strength of their fixation in the soil and,therefore, their availability to plants strongly vary;hence, the range of radiation loads within contaminatedareas may be broad (Tikhomirov, 1972; Aleksakhin,1982). In natural ecosystems, it is impossible to takeinto account the entire complex of environmental con-ditions and differentiate between the effects of individ-ual factors. Experimental radiobiologists have accumu-lated vast data on the effects of radiation on individualcells and plants. These data are very important butinsufficient for understanding the processes occurringat the population level (Grodzinskii, 1989). The struc-tural and functional hierarchy of living organismsimplies the existence of a multilevel system ofresponses to external influences. Therefore, the com-prehensive analysis of such a systemic phenomenon asthe adaptive response to irradiation at different levels,from individual cells to cenopopulations, should beperformed with great caution (Reimers, 1994).</p><p>The purpose of this work was to study the responseof plants to chronic low-dose irradiation in dandelion(</p><p>Taraxacum officinale</p><p> s.l.) populations growing in theareas of the Eastern Ural Radioactive Trace (EURT)and the Techa River floodplain, which were contami-nated by radioactive discharge from the Mayak Produc-tion Association.</p><p>MATERIALS AND METHODSThe samples of soils, vegetative plant parts, and</p><p>seeds were taken along the central EURT axis and inthe Techa River floodplain near the village of Brodokal-mak in 1998. The background plot was located beyondthe zone of radioactive contamination. The soils weresampled by 5-cm layers to a depth of 30 cm, taking intoaccount the sample area. Samples of the abovegrounddandelion phytomass were taken from the sites adjoin-ing the soil section. In addition, seeds of 1020 individ-ual plants of each cenopopulation were collected. Soilsamples were dried and sifted through a screen (meshsize 0.1 cm); phytomass samples were incinerated. Thecontent of </p><p>137</p><p>Cs</p><p> was measured in a Canberra Packardgamma-spectrometer with a germanium sensor; </p><p>90</p><p>Sr</p><p>was determined radiochemically by conventional meth-ods (</p><p>Metodicheskie rekommendatsii</p><p>, 1980).</p><p>Taraxacum officinale </p><p>s.l., a widespread polycarpicspecies of the family Asteraceae, is convenient for indi-cating biological effects in technogenically disturbedecosystems. Both vegetative reproduction by root suck-ers and seed reproduction are observed. Most authorsindicate that seeds are formed parthenogeneticallywithout chromosome reduction and pseudogamy (Pod-dubnaya-Arnoldi, 1976; Ermakova, 1990). The dande-lion populations studied, growing in contaminatedareas for a long time, have been exposed to theincreased background radiation over several dozens ofgenerations.</p><p>The effect of chronic irradiation at the cell level wasdetermined by analyzing chromosome aberrations in</p><p>Analysis of Local Dandelion (</p><p>Taraxacum officinale</p><p> s.l.) Cenopopulations from Radioactively Contaminated Zones</p><p>V. N. Pozolotina</p><p>Institute of Plant and Animal Ecology, Ural Division, Russian Academy of Sciences, ul. Vosmogo Marta 202, Yekaterinburg, 620219 Russia</p><p>Received June 16, 2000</p><p>Abstract</p><p>Local dandelion (</p><p>Taraxacum officinale</p><p> s.l.) populations were studied in the areas of the EasternUral Radioactive Trace and the floodplain of the Techa River in its upper reaches. In impact plots, the densityof soil and plant cover contamination with </p><p>90</p><p>Sr and </p><p>137</p><p>Cs exceeded the background level by factors of 13440and 2500, respectively; the radiation load exceeded the background level by factors of 1.5 to 45. The seedprogeny of plants from these plots was characterized by a high proportion of abnormal seedlings and anincreased level of chromosome aberrations in meristem cells. In some years, variation in the seedling viability,growth rate, and developmental rate in these plots exceeded the reaction norm of plants from the backgroundplot, demonstrating both stimulation and inhibition of growth processes. The response of seeds to acute irradi-ation at high challenging doses varied depending on the level of background radiation in the plots.</p><p>Key words</p><p>: radioactive contamination, small doses, </p><p>Taraxacum officinale</p><p> s.l., chromosome aberrations, radi-osensitivity, intraspecific variation.</p></li><li><p> RUSSIAN JOURNAL OF ECOLOGY</p><p>Vol. 32</p><p>No. 2</p><p>2001</p><p>ANALYSIS OF LOCAL DANDELION 103</p><p>anaphase root meristem cells of </p><p>1</p><p> seed progeny. Prep-arations for cytogenetic analysis were made by thesquash method and stained with acetoorcein. In eachvariant, 5001500 anaphases in 1225 rootlets wereanalyzed.</p><p>The effects at the ontogenetic and population levelswere estimated by analyzing individual seed progeniesof ten plants from each cenopopulation. Seeds germi-nated in paper rolls, and experiments were performedin five replications. Viability was estimated from theseed vigor and germination rate, survival of one-monthseedlings, and the rates of leaf formation and rootgrowth. The proportions of abnormal seedlings in eachfamily and the entire sample were determined. Thesedata allowed the assessment of both individual varia-tion of all these parameters within each sample andvariation within individual families (with respect toroot growth and the proportion of abnormal plants). Inmaternal plants, the numbers of leaves and flower stalksand the size of the largest leaf were determined.</p><p>At the next stage, the adaptive potentials of the seedprogenies formed under different radiation conditionswere studied. Seeds were gamma-irradiated at doses of100 and 250 Gy (dose rate 15.5 cGy/s) in an Issledova-tel-type unit. Plant tolerance for challenging irradia-tion was estimated by the aforementioned combinationof criteria. The results obtained at each stage ofresearch were processed statistically using the standardSTATISTICA for Windows software package.</p><p>RESULTS AND DISCUSSION</p><p>Radioecological characteristics of test plots. </p><p>Ear-lier, we performed detailed studies on the levels ofradioactive contamination in the Ural region. Theirresults demonstrated that the soils contained techno-genic radionuclides of various origins. The contribu-tions of </p><p>90</p><p>Sr, </p><p>137</p><p>Cs</p><p>, and the transuranium elements dis-charged during the Kyshtym accident (1957) and ofradioactive elements transferred by wind from theshores of Lake Karachai (1967) were estimated qualita-tively (Aarkrog </p><p>et al.</p><p>, 1997, 1998). The floodplain eco-systems of the Techa River, in which radioactive wastefrom the Mayak works was dumped between 1949 and1951, are still heavily contaminated throughout theriver course (Trapeznikov </p><p>et al.</p><p>, 1999).This paper deals with the results of studies in four</p><p>impact plots (two located on the EURT central axis andthe other two, in the Techa River floodplain) and onebackground plot in Beloyarskii raion, beyond the con-taminated zone. The background plot 1 was in thePyshma River floodplain, and its thick ground vegeta-tion was represented by a herbgrass community. Plot2 was in a narrow floodplain area near the Bagaryak Riverchannel. Thick ground vegetation (coverage 9095%)was represented by a community of herbs, annualgrasses, and bluegrass. Plot 3 was located 400 m awayfrom Lake Tygish, in a birch forest outlier with a</p><p>thinned grass cover growing on gray forest soil. Plots 4and 5 were in the Techa floodplain, on gently slopingbanks near the river channel. Ground vegetation con-sisted of several layers with 100% coverage and wasrepresented by a herbgrass community growing onstratified alluvial soils. The comparison of plotsshowed that the densities of their contamination with</p><p>90</p><p>Sr and </p><p>137</p><p>Cs differ by factors of 13440 and 2500,respectively (Table 1).</p><p>Studies on the distribution of radionuclides acrossthe soil profile in the EURT zone showed that the sur-face soil layer (010 cm) accumulates up to 90% oftheir total amount (Aarkrog </p><p>et al.</p><p>, 1997). The complexrelief and specific hydrologic conditions of floodplainlandscapes determine the specific behavior of radionu-clides in their soils and plant cover. Due to regular inun-dation during floods and the inflow of geochemicalmaterial from the catchment area and the river proper,floodplains accumulate large amounts of chemical sub-stances. As a rule, the contents of radionuclides in thesoil profile exponentially decrease with depth, butdeeper soil layers sometimes contain them in fairlylarge amounts (Trapeznikov </p><p>et al.</p><p>,</p><p>1999).Data on the contents of radionuclides in the phyto-</p><p>mass and the root layer of the soil (020 cm) was usedfor calculating the coefficients of their biological absorp-tion (CBA) by the aboveground plant parts (Table 2).The analysis of the results showed that plants accumu-lated </p><p>137</p><p>Cs less actively than </p><p>90</p><p>Sr. This is explained bythe fact that, even in moist floodplain soils, the bulk of</p><p>Table 1. </p><p>Densities of soil and plant cover contaminationwith technogenic radionuclides in test plots, kBq/m</p><p>2</p><p>Plotnumber Location</p><p>90</p><p>Sr</p><p>137</p><p>Cs</p><p>1 Background area 1.6 5.02 EURT axis in the Bagaryak Riv-</p><p>er floodplain20.6 19.1</p><p>3 EURT axis near Lake Tygish 63.4 10.84 Left-bank Techa floodplain near </p><p>river channel538.9 1821.1</p><p>5 Right-bank Techa floodplain near river channel</p><p>711.6 2506.6</p><p>Table 2. </p><p>Contents of radionuclides in air-dry abovegrounddandelion phytomass and coefficients of their biological ac-cumulation (CBA)</p><p>Plot number</p><p>137</p><p>Cs, Bq/kg CBA (</p><p>137</p><p>Cs)</p><p>90</p><p>Sr, Bq/kg CBA (</p><p>90</p><p>Sr)2 5.8 </p><p> 0.1 0.12 37.4 </p><p> 0 1.023 6.8 </p><p> 0.7 0.14 43.8 </p><p> 0.7 1.204 121.2 </p><p> 33.0 0.018 1060 </p><p> 25 0.705 148.9 </p><p> 41.0 0.017 1210 </p><p> 89 0.47</p></li><li><p> 104</p><p>RUSSIAN JOURNAL OF ECOLOGY</p><p>Vol. 32</p><p>No. 2</p><p>2001</p><p>POZOLOTINA</p><p>137</p><p>Cs (98.4%) is firmly fixed and is virtually inaccessi-ble to plants. As to </p><p>90</p><p>Sr, the proportions of its water-solu-ble and exchange forms are 1 and 44% (Pozolotina </p><p>et al.</p><p>,</p><p>1999). The higher the radionuclide concentrations inthe soil, the lower the CBA values.</p><p>Calculating radiation load.</p><p> The background gamma-radiation in plots was measured using a DRG-01Tdosimeter. Its levels were </p><p>7 </p><p> 10</p><p>2</p><p>m</p><p>Sv/h in plot 1 and</p><p>1521 </p><p> 10</p><p>2</p><p>m</p><p>Sv/h in plots 2 and 3; i.e., they werewithin the norm for the Ural region. Dose rates in theTecha floodplain generally varied within the range of(2594) </p><p> 10</p><p>2</p><p>m</p><p>Sv/h but sometimes reached </p><p>170 </p><p>10</p><p>-</p><p>2</p><p>m</p><p>Sv/h. The plagiotropic part of the abovegroundshoot and the apical growth point in dandelions areclose to the ground surface. Hence, the dose receivedby the most radiosensitive meristem cells may be calcu-lated assuming that the growth point is in a uniformlycontaminated medium. Using this simple model, thecontributions of </p><p>90</p><p>Sr (together with </p><p>90</p><p>Y, as these twoisotopes are in a dynamic equilibrium) and </p><p>137</p><p>Cs werecalculated separately. Dose rate </p><p>M</p><p> was determined bythe formula </p><p>M</p><p> = </p><p>q</p><p>1</p><p> +</p><p> q</p><p>2</p><p>, where </p><p>q</p><p>1</p><p> and</p><p>q</p><p>2</p><p> were specific activities of each radionuclide mea-sured in the surface soil layer and </p><p>L</p><p> was the absorbeddose rate (cSv/s) created by a radionuclide within auniformly contaminated volume at </p><p>q</p><p> = 3.7 </p><p>10</p><p>4</p><p> Bq/g</p><p>L Sr90 Y90+( ) L Cs137( )</p><p>(Gorshkov, 1967). Table 3 shows the results of thesecalculations.</p><p>It is seen that the additional radiation load accountedfor by artificial radionuclides was 1.54 times higher inthe EURT area than in the background plot. Accordingto published data (</p><p>Itogi</p><p>, 1990), the dose of radiationemitted by short-lived radionuclides one year after theaccident was approximately 2000 times higher than thatemitted by </p><p>90</p><p>Sr; during four subsequent years, short-lived radionuclides have decayed almost completely. Inthe Techa floodplain, the radiation loads exceed the nat-ural background level by a factor of 4045 but remainwithin the range of small doses for plants.</p><p>Cytogenetic analysis of seed progenies formed ina radiation load gradient.</p><p> The analysis of chromo-some aberrations in root meristem cells is among themost specific and sensitive methods for analyzing theeffects of radiation on plants. Thus, a high level of chro-mosome aberrations in anaphase root cells wasobserved in plants developing from seeds collected inall impact plots (Table 4). Similar phenomena wereobserved in studies on other plant species (Cherezha-nova </p><p>et al.</p><p>, 1971; Shevchenko </p><p>et al.</p><p>, 1998).Chromosome fragments prevailed in the spectrum</p><p>of chromosome aberrations. It should be noted that thelevels of radioactive contamination in the Techa flood-plain were two orders of magnitude higher than those inthe EURT area, whereas the proportions of cells withchromosome aberrations in seed progenies from theseareas differed only slightly, exceeding the control val-ues by factors of 4.3 to 7.3. Therefore, the dependenceof the chromosome aberration frequency on the radia-tion dose was nonlinear. Recently, radiobiologists havebeen elaborating the concept concerning the effect ofsmall radiation doses on the biota, which attributes thenonmonotonic pattern of this dependence to changes inthe ratio of reactions related to cell damage and repair(Burlakova </p><p>et al.</p><p>, 1999; Geraskin and Sevankaev,1999; Rozhdestvenskii, 1999). According to this con-cept, four qualitatively different types of cell responseto small radiation doses are distinguished. At the lowestdoses, the genetic efficiency of irradiation is lower thanthat of the factors accounting for the frequency of spon-taneous chromosome aberrations. Higher radiation</p><p>Table 3. </p><p>Concentrations of </p><p>90</p><p>Sr and </p><p>137</p><p>Cs in the surface soillayer (05 cm) and the resulting additional radiation loads onthe meristematic tissues of dandelion</p><p>Plot number</p><p>Concentration, Bq/kg Dose rate,</p><p>n</p><p> 10</p><p>2</p><p>m</p><p>Sv/hAnnual dose,</p><p>n</p><p> 10</p><p>2</p><p> mSv</p><p>90</p><p>Sr</p><p>137</p><p>Cs</p><p>1 15 31 1.6 14.02 68 73 5.3 46.43 646 314 31.8 278.74 1951 10830 442.0 3872.05 4348 9489 472.0 4135.0</p><p>Table 4. </p><p>Cytogenetic aberrations in anaphase root meristem cells of seedlings that developed from seeds collected in plotsdiffering in the level of radioactive contamination</p><p> Plot number Number of cells analyzed Cells with chromosome bridgesCells with chromosome </p><p>fragments Aberrant cells, %</p><p>1 1442 3 14 1.18 </p><p> 0.602 784 7 37 5.61 </p><p> 1.58*3 920 6 41 5.11 </p><p> 1.62*4 931 6 49 5.91 2.21*5 503 1 42 8.55 2.42*</p><p>* Differences from background values are significant at P = 0.95.</p></li><li><p>RUSSIAN JOURNAL OF ECOLOGY Vol. 32 No. 2 2001</p><p>ANALYSIS OF LOCAL DANDELION 105</p><p>loads activate repair systems, and the resulting fre-quency of genetic disturbances may be lower than thebackground frequency. Further increase in the radiationdose triggers cell switching to a different functionalregime, which involves activation of SOS repair sys-tems. Th...</p></li></ul>


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