1067-4136/01/3202- $25.00 © 2001
MAIK “Nauka
/Interperiodica”0102
Russian Journal of Ecology, Vol. 32, No. 2, 2001, pp. 102–109. Translated from Ekologiya, No. 2, 2001, pp. 117–124.Original Russian Text Copyright © 2001 by Pozolotina.
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).
The purpose of this work was to study the responseof plants to chronic low-dose irradiation in dandelion(
Taraxacum officinale
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.
MATERIALS AND METHODS
The samples of soils, vegetative plant parts, andseeds 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 10–20 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
137
Cs
was measured in a Canberra Packardgamma-spectrometer with a germanium sensor;
90
Sr
was determined radiochemically by conventional meth-ods (
Metodicheskie rekommendatsii…
, 1980).
Taraxacum officinale
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-Arnol’di, 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.
The effect of chronic irradiation at the cell level wasdetermined by analyzing chromosome aberrations in
Analysis of Local Dandelion (
Taraxacum officinale
s.l.) Cenopopulations from Radioactively Contaminated Zones
V. N. Pozolotina
Institute of Plant and Animal Ecology, Ural Division, Russian Academy of Sciences, ul. Vos’mogo Marta 202, Yekaterinburg, 620219 Russia
Received June 16, 2000
Abstract
—Local dandelion (
Taraxacum officinale
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
90
Sr and
137
Cs exceeded the background level by factors of 13–440and 2–500, 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.
Key words
: radioactive contamination, small doses,
Taraxacum officinale
s.l., chromosome aberrations, radi-osensitivity, intraspecific variation.
RUSSIAN JOURNAL OF ECOLOGY
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ANALYSIS OF LOCAL DANDELION 103
anaphase root meristem cells of
å
1
seed progeny. Prep-arations for cytogenetic analysis were made by thesquash method and stained with acetoorcein. In eachvariant, 500–1500 anaphases in 12–25 rootlets wereanalyzed.
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.
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.
RESULTS AND DISCUSSION
Radioecological characteristics of test plots.
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
90
Sr,
137
Cs
, 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
et al.
, 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
et al.
, 1999).This paper deals with the results of studies in four
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 herb–grass community. Plot2 was in a narrow floodplain area near the Bagaryak Riverchannel. Thick ground vegetation (coverage 90–95%)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
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 herb–grass community growing onstratified alluvial soils. The comparison of plotsshowed that the densities of their contamination with
90
Sr and
137
Cs differ by factors of 13–440 and 2–500,respectively (Table 1).
Studies on the distribution of radionuclides acrossthe soil profile in the EURT zone showed that the sur-face soil layer (0–10 cm) accumulates up to 90% oftheir total amount (Aarkrog
et al.
, 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
et al.
,
1999).
Data on the contents of radionuclides in the phyto-mass and the root layer of the soil (0–20 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
137
Cs less actively than
90
Sr. This is explained bythe fact that, even in moist floodplain soils, the bulk of
Table 1.
Densities of soil and plant cover contaminationwith technogenic radionuclides in test plots, kBq/m
2
Plotnumber Location
90
Sr
137
Cs
1 Background area 1.6 5.0
2 EURT axis in the Bagaryak Riv-er floodplain
20.6 19.1
3 EURT axis near Lake Tygish 63.4 10.8
4 Left-bank Techa floodplain near river channel
538.9 1821.1
5 Right-bank Techa floodplain near river channel
711.6 2506.6
Table 2.
Contents of radionuclides in air-dry abovegrounddandelion phytomass and coefficients of their biological ac-cumulation (CBA)
Plot number
137
Cs, Bq/kg CBA (
137
Cs)
90
Sr, Bq/kg CBA (
90
Sr)
2 5.8
±
0.1 0.12 37.4
±
0 1.02
3 6.8
±
0.7 0.14 43.8
±
0.7 1.20
4 121.2
±
33.0 0.018 1060
±
25 0.70
5 148.9
±
41.0 0.017 1210
±
89 0.47
104
RUSSIAN JOURNAL OF ECOLOGY
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No. 2
2001
POZOLOTINA
137
Cs (98.4%) is firmly fixed and is virtually inaccessi-ble to plants. As to
90
Sr, the proportions of its water-solu-ble and exchange forms are 1 and 44% (Pozolotina
et al.
,1999). The higher the radionuclide concentrations inthe soil, the lower the CBA values.
Calculating radiation load.
The background gamma-radiation in plots was measured using a DRG-01Tdosimeter. Its levels were
7
×
10
–2
µ
Sv/h in plot 1 and
15–21
×
10
–2
µ
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(25–94)
×
10
–2
µ
Sv/h but sometimes reached
170
×
10
−
2
µ
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
90
Sr (together with
90
Y, as these twoisotopes are in a dynamic equilibrium) and
137
Cs werecalculated separately. Dose rate
M
was determined bythe formula
M
=
q
1
+
q
2
, where
q
1
and
q
2
were specific activities of each radionuclide mea-sured in the surface soil layer and
L
was the absorbeddose rate (cSv/s) created by a radionuclide within auniformly contaminated volume at
q
Ò
= 3.7
×
10
4
Bq/g
LSr
90Y
90+( )
LCs
137( )
(Gorshkov, 1967). Table 3 shows the results of thesecalculations.
It is seen that the additional radiation load accountedfor by artificial radionuclides was 1.5–4 times higher inthe EURT area than in the background plot. Accordingto published data (
Itogi…
, 1990), the dose of radiationemitted by short-lived radionuclides one year after theaccident was approximately 2000 times higher than thatemitted by
90
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 40–45 but remainwithin the range of small doses for plants.
Cytogenetic analysis of seed progenies formed ina radiation load gradient.
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
et al.
, 1971; Shevchenko
et al.
, 1998).Chromosome fragments prevailed in the spectrum
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
et al.
, 1999; Geras’kin and Sevan’kaev,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
Table 3.
Concentrations of
90
Sr and
137
Cs in the surface soillayer (0–5 cm) and the resulting additional radiation loads onthe meristematic tissues of dandelion
Plot number
Concentration, Bq/kg Dose rate,
n
×
10
–2
µ
Sv/hAnnual dose,
n
×
10
–2
mSv
90
Sr
137
Cs
1 15 31 1.6 14.0
2 68 73 5.3 46.4
3 646 314 31.8 278.7
4 1951 10830 442.0 3872.0
5 4348 9489 472.0 4135.0
Table 4.
Cytogenetic aberrations in anaphase root meristem cells of seedlings that developed from seeds collected in plotsdiffering in the level of radioactive contamination
Plot number Number of cells analyzed Cells with chromosome bridges
Cells with chromosome fragments Aberrant cells, %
1 1442 3 14 1.18
±
0.60
2 784 7 37 5.61
±
1.58*
3 920 6 41 5.11
±
1.62*
4 931 6 49 5.91 ± 2.21*
5 503 1 42 8.55 ± 2.42*
* Differences from background values are significant at P = 0.95.
RUSSIAN JOURNAL OF ECOLOGY Vol. 32 No. 2 2001
ANALYSIS OF LOCAL DANDELION 105
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. The frequency of genetic disturbances becomeshigher but subsequently remains unchanged, forming aplateau in the dose–effect curve. At still higher doses,repair systems are inefficient and the number of aberra-tions increases monotonically. A noteworthy feature ofthe biological effect of small radiation doses is that theyinduce heritable genomic instability (Bychkovskaya,1986; Vilenchik, 1987). The data on cytogenetic distur-bances in dandelion seed progenies formed in a radio-active contamination gradient can be explained withinthe framework of this concept.
Viability of seeds collected in plots with differentlevels of radioactive contamination. According tosome authors, small radiation doses do not cause anydeviations from the physiological norm at the levelsof organisms and their communities (Geras’kin andSevan’kaev, 1999). I consider this conclusion question-able. Let us compare average values and ranges of varia-tion in basic parameters of the viability of seed progeniesformed in the background and impact plots (Table 5).Each sample comprised the seeds of ten plants, experi-ments were performed in five replications. It is seenthat all the parameters studied were higher in the prog-enies of plants from the background cenopopulationthan in those from the impact cenopopulations. One-way ANOVA confirmed that the corresponding dose–effect dependence is statistically significant (Fexp variedfrom 4.7 to 10.3, depending on parameter). Any signif-icant differences in these parameters between samplesfrom impact plots were absent, although the levels ofradioactive contamination in these plots differed byseveral orders of magnitude.
Individual variation in seed quality within each cen-opopulation had its specific limits. The broadest rangeof variation in all parameters was observed in the localsample from the EURT area. ANOVA confirmed thestatistical significance of differences between the plantsin the parameters characterizing the quality of M1 (Fexpvaried from 8.3 to 33.7). These data were analyzed inmore detail using the method of multiple comparisons(Scheffe’s method), which is not sensitive to the type ofdistribution. In samples from the EURT area and the
Techa floodplain, seven out of ten plants demonstrateddifferences from the average background level, havinglower seed vigor, rates of survival, and leaf growth(data statistically significant). These individual differ-ences accounted for differences in the average values ofparameters between the samples.
The rate of root growth is among the basic indicesof growth processes in plants. The root length was mea-sured in each seedling, which allowed the analysis ofboth differences between the maternal plants and thevariation within families. Dandelion is a triploid spe-cies (2n = 24, x = 8) with parthenogenetic seeds, andeach family is the progeny of one plant. In three fami-lies from the background cenopopulation, the distribu-tion of the character “root length” was close to the nor-mal (Gaussian) distribution; the remaining familiesdemonstrated deviations from this pattern. Family sam-ples from the EURT area were characterized by asym-metrical distribution curves displaced to the left. Insamples from the Techa floodplain, approximately halfof the families had similarly displaced distributioncurves and the other half demonstrated a bimodal dis-tribution with respect to this character. The displace-ment of distribution to the left provided evidence forthe prevalence of seedlings with retarded growth in å1;the bimodal curve indicated differentiation within thesample, i.e., its subdivision into the groups of seedlingswith high and low rates of root growth. These differ-ences may be regarded as manifestations of both indi-vidual variation and the maternal effect, as the latterdepends not only on endogenous features of the mater-nal plants, but also on the conditions of plant growth(Roach, 1987).
The analysis of frequencies of abnormal plants inM1 progenies from different plots is of special interest.These frequencies were determined per family (ma) andper sample (mc); the latter index is more convenient andinformative (Table 6). The samples from impact plotsincluded seedlings in which all organs (hypocotyls,roots, and cotyledons) were deformed. Such seedlingswere more numerous in the samples from the plot withthe highest level of radioactive contamination. Manyseedlings had deformed cotyledons, which were growntogether, split in half, had different sizes, etc. In theimpact samples, their number was significantly greaterthan in the background sample. Some disturbances dis-appeared with time. Thus, when cotyledons and leaves
Table 5. Parameters of viability of seeds collected in the background and radioactively contaminated plots, %
Plot number Seed vigor Germination rate Survival rate Number of plants with leaves
1 59.2 (42.8–72.4) 84.1 (37.2–94.0) 65.8 (37.2–94.0) 46.2 (34.8–56.8)
2 33.5 (7.0–55.0) 61.5 (15.5–78.5) 56.2 (11.0–74.5) 28.9 (5.1–50.0)
3 40.4 (3.2–80.8) 49.3 (3.2–85.2) 43.6 (2.8–82.8) 33.9 (2.0–68.0)
4 23.0 (4.4–35.6) 37.1 (12.4–59.6) 34.3 (9.6–53.6) 18.2 (5.3–28.4)
5 44.3 (12.4–54.0) 60.9 (45.2–76.8) 56.8 (18.8–72.8) 36.1 (13.6–44.4)
106
RUSSIAN JOURNAL OF ECOLOGY Vol. 32 No. 2 2001
POZOLOTINA
were coiled because of the imbalance between the ratesof cell division and stretching, they would straighten outin the course of growth. The frequency of chlorophyllmutations tended to increase at higher levels of radioac-tive contamination. Apparently, both morphological andchlorophyll abnormalities occur because the initial tissuecontains cells with various genetic defects. When the lat-ter are lethal to the initial cells, this is phenotypicallymanifested in the development of deformed plant organs.When the defective cells remain capable of dividing,defective sectors are formed in tissues; i.e., irradiatedplants are actually chimeric (Grodzinskii, 1989).
The analysis of families within each cenopopulationshowed that they differed significantly in the number ofabnormal descendants. Thus, in the sample from themost contaminated plot, the proportions of seedlingswith chlorophyll abnormalities of leaves in differentfamilies varied from 1 to 12% and the frequency of pro-found changes in all plant organs varied from 0 to 4.8%.
The results described above can aid in answering thequestion as to how the changes occurring in cells uponexposure to small radiation doses subsequently mani-fest themselves at the ontogenetic and population lev-els. The published data on this problem are contradic-tory. In some studies, an increase in the rates of plantgrowth and development was observed in radioactively
contaminated areas (Cherezhanova et al., 1971; Kuzin,1991; Pozolotina et al., 1992). The authors of otherstudies performed in such areas either observed a sup-pressing effect of the background gamma-radiation orrevealed no definite relationship between the variation inits level and the quantitative parameters of plants(Popova et al., 1992; Pozolotina, 1996). To some extent,this disagreement is accounted for by differences in theradiation load and plant sensitivity to it, but such anexplanation is far from being complete.
Studies on variation in the viability of seed proge-nies obtained from the same plots over several yearscan aid in gaining a deeper insight into this problem. Inthis case, the source of variation is in the specific fea-tures of genotype realization under changing environ-mental conditions. Let us consider the correspondingdata on dandelion populations from plots 1 (back-ground) and 3 (EURT). Note that, in both plots, the veg-etating plants used for collecting seeds were similarwith respect to the number and size of leaves, the num-ber of inflorescences, and the weight and size of seeds.The only morphological difference was as follows.Dandelion is a taxonomically complex species(Sukachev, 1975; Opredelitel’…, 1994). In the Uralregion, it has more than ten morphological varietiesclassified by different authors as microspecies, bio-types, and even individual species. Several forms ofdandelion in the background plot were easy to distin-guish. In the contaminated plot, it was impossible todetermine the form to which the plants belonged, asthey had various combinations of morphological fea-tures characteristic of different forms.
Viability in M1 was estimated by several criteria. Letus consider temporal variation in the survival of one-month seedlings (figure).
Nine-year observations showed that the range ofvariation in the survival of M1 seedlings was signifi-cantly broader in the sample from the contaminatedplot than in the background sample. In 1991, the seed-lings from the EURT area were not only more viablethan the control seedlings (difference statistically sig-nificant, F = 4.2), but also demonstrated higher growthand developmental rates. At the age of one month, eachseedling had two or three true leaves, compared to only
Table 6. Frequencies of abnormal seedlings (mc) in cenopopulations differing in the level of radioactive contamination
Plantnumber
Profound changesin all organs
Changes in cotyledons Changes in leaves
shape color shape color
1 0 24.5 ± 2.2 0.5 ± 0.1 2.5 ± 1.2 1.0 ± 0.5
2 0.6 ± 0.2 38.7 ± 6.3* 1.4 ± 0.6 1.8 ± 0.5 4.7 ± 2.1
3 0.1 ± 0.05 22.6 ± 2.6 1.0 ± 0.5 3.9 ± 1.1 0.3 ± 0.1
4 1.0 ± 0.5 36.1 ± 4.1* 2.1 ± 1.2 3.6 ± 1.2 3.2 ± 1.2
5 1.5 ± 0.5 41.4 ± 2.2* 3.8 ± 1.6* 4.0 ± 1.2 4.9 ± 1.2*
* Differences from background values are significant at P = 0.95.
1991 1993 1994 1995 1996 1997 1998
100
0102030405060708090
1999
1 2
Survival rate, %
Temporal variation in the survival of seed progenies ofplants from (1) the background cenopopulation and (2) theEURT area.
years
RUSSIAN JOURNAL OF ECOLOGY Vol. 32 No. 2 2001
ANALYSIS OF LOCAL DANDELION 107
one leaf in the control seedlings (Pozolotina et al.,1992). In 1999, the seeds from the same plot were sig-nificantly less viable than the control seeds (F = 8.79).In other years, no significant difference in the quality ofM1 between the samples from the contaminated andbackground areas was revealed (Pozolotina, 1996).Thus, the entire spectrum of low-dose radiation effectswas observed in the cenopopulation from the EURTarea in different years: radiation exposure either stimu-lated or inhibited plant growth and development or hadno apparent effect. In all cases, however, the impactsample was characterized by a high level of chromo-some aberrations in root meristem cells and anincreased frequency of abnormal plants.
Thus, long-term exposure to low-intensity radiationincreases the frequency of chromosome aberrations inplant cells and the proportion of abnormal seedlings inthe seed progeny; moreover, it broadens the range ofvariation in morphological and physiological parame-ters, which exceeds the limits of the reaction normcharacteristic of the background population. Thesefacts provide evidence for genomic instability mani-fested at the level of individuals and their communities.This phenomenon is the main result of exposure to lowradiation doses at the level of cenopopulations.
Response of seed progeny to challenging irradia-tion. To analyze the adaptation potential of plants, it isimportant to estimate their response to challenging irra-diation at high doses. The corresponding data obtainedby different authors are diverse and concern either theincreased tolerance of plants to additional irradiation(Cherezhanova and Aleksakhin, 1971) or the increasedseverity of radiation damage (Popova et al., 1992) orthe absence of any differences between the control andchronically irradiated cenopopulations (Shevchenko et al.,1998). In this work, experiments on additional irradia-tion were performed with a mixture of seeds obtained
from the same plants (see above). To reveal theresponse of plants to irradiation more clearly, theresults were presented as the percentages of corre-sponding parameters in the nonirradiated (control) partof each sample (Table 7). It is noteworthy that challeng-ing irradiation of seeds from the background plot didnot inhibit the development of seedlings at early stages.They developed similarly to the control seedlings,although the frequency of chromosome aberrations intheir root meristem cells increased by an order of mag-nitude upon irradiation at a dose of 100 Gy (data statis-tically significant, tst. = 4.1) and by a factor of 30 uponirradiation at a dose of 250 Gy. These results can beexplained by the elimination of damaged meristemcells and the restoration of their pool owing to theaccelerated proliferation of surviving cells. It is highlyprobable that a weak ontogenetic response to the doseof 250 Gy is a temporary phenomenon and the effect ofirradiation will manifest itself at later stages of plantdevelopment.
In M1 plants from the EURT area, irradiation at adose of 250 Gy caused an increase in the frequency ofchromosome aberrations; the germination rate and thesurvival of seedlings were lower than in the nonirradi-ated control. The sample from plot 3 should be consid-ered in more detail. The viability of seed generationsfrom this cenopopulation proved to vary in time (seeabove). In 1991, when å1 plants were characterized byvery high rates of growth and development, they alsoproved to be more resistant to challenging irradiation thanplants from the background sample (Pozolotina et al.,1992). From 1993 to 1996, no such difference in theresponse to additional irradiation was revealed. In1999, the viability of the seed progeny was so low thatvirtually all seedlings perished after challenging irradi-ation. On the whole, the response of the seed progeny
Table 7. Parameters of radioresistance of seed progenies formed in a radiation load gradient
Dose, Gy Plotnumber
Seed vigor Germination rate Survival rate Plants with
leaves Root lengthAberrant cells, %
Proportion relative to nonirradiated control, %
100 1 119* 107 87 102 95 11.1 ± 2.1*
2 85 89 76* 62* 55* 10.2 ± 2.2
3 112 91 89 59* 65* 7.3 ± 2.7
4 85 64* 58* 114 157* 7.4 ± 2.2
5 129* 119 121* 152* 119* 6.7 ± 2.4
250 1 117 106 102 101 109 35.6 ± 4.9*
2 73* 83* 66* 59* 69* not studied
3 49* 98 98 98 72* 20.1 ± 3.5*
4 84* 77* 86 124* 111 23.7 ± 3.8*
5 106 117 125* 147* 105 12.0 ± 2.7
* Differences from nonirradiated control are significant at P = 0.95.
108
RUSSIAN JOURNAL OF ECOLOGY Vol. 32 No. 2 2001
POZOLOTINA
to challenging irradiation in this cenopopulation maybe regarded as unstable.
The sample from the most contaminated plotdeserves special attention. Irradiation at high doses hadno inhibitory effect on plants by any criterion, includ-ing the frequency of chromosome aberrations; con-versely, several parameters indicated stimulation of thegrowth processes. Apparently, the seed progeny fromthis plot was better adapted to radiation exposure thanthe progenies from other plots. In fact, challenging irra-diation revealed a “hidden” variation characteristic ofcenopopulations. Under conditions of intense chronicirradiation, selection favors radioresistant organisms.
Summarizing the data on plant response to radioac-tive contamination, it can be concluded that the resultsof cytogenetic, ontogenetic, and population analysisagree with each other. Plants developing in contami-nated areas had a high percentage of chromosome aber-rations in root meristem cells, and the frequency ofabnormal individuals among them was also high. Theparameters of viability in å1 plants varied within therange that exceeded the reaction norm characteristic ofthe background sample. Such a situation is evidence forthe genetic instability of these plants. It is principallyimportant that, although radioactive contaminationincreased by several orders of magnitude, the fre-quency and severity of radiation disturbances did notincrease monotonously along this gradient. The resultsof assessing the adaptation potential of chronically irra-diated plants by means of challenging irradiation areambiguous, but the seed progeny from the maximallycontaminated cenopopulation proved to be mostradioresistant by all cytogenetic and ontogenetic crite-ria. The data obtained in this study can aid in elaborat-ing the concept concerning the effects of low-intensityionizing radiation on living matter at different levels ofits organization.
ACKNOWLEDGMENTS
This work was supported by the Special FederalProgram “Integration” and the Russian Foundation forBasic Research, project no. 00-05-65401.
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