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Uranium in plants of southern Sinai
Journal of Arid Environments (1992) 22: 363-368
Uranium in plants of southern Sinai H. Zafrir*, Y. Waiselt, M. Agamij'], J. Kronfeld§ & E. Mazor]
* Department for Field Applications, Nahal Soreq NuclearResearch Center, Yavne,
t:j: Department ofBotany, Tel Aviv University, Tel Aviv 69978,
§ Department ofGeophysics and PlanetarySciences, Tel Aviv University, Tel Aviv 69978 and II§ Geo-isotope Group, Weizmann Institute ofScience,
Rehovot, Israel
(Received 16January 1991, accepted 29 January 1991) Analyses of 220 individual plants, representing 40 different species from the southern Sinai desert have shown that the species' average uranium concentrations range from O'OS to 4'8 p.p.m, with the large preponderance fallinginto the narrower range of O'S to 2'0 p.p.m. Uranium accumulation was observed to be independent of such variables as location, rock types, uranium content or chemistry of the waters, or the age of the plants sampled. The concentrations encountered in southern Sinai seem to be similarto the average data that are available for temperate zone plants despite the lower average concentration of uranium in the waters of such regions. Desert plants, which usually rely upon limited water sources of high mineral content are highly selective in their ion uptake, with uranium being merely one of the rejected elements. Introduction The Sinai is a triangular peninsula, bordered by the gulfs of Suez and Eilat. The south central part is made up of a crystalline mountain complex, with abundant granitic-type rocks and lesser amounts of basics , and metamorphics (Eyal, Bartov et al., 1985). Southern Sinai is a land with a harsh desert climate. Rainfall is infrequent, usually limited to a few showers during the winter. Soil cover of the mountain region is restricted, leaving the mountain slopes exposed. Sand flats and thick alluvial fills are typical to the bottom of the ephemeral streams (wadis). The plant cover is sparse. Trees and large shrubs are generally restricted to the wadis, rock crevices, or wherever shallow ground water is available (Waisel, Pollak etal., 1978), with date palms (Phoenix dactylifera L.), tamarisks (Tamarix sp.), and acacias (Acacia raddiana Savi etc.) being prevalent (Waisel & Alon, 1980). A survey of the uranium concentration of the local plants was conducted in order to understand the mechanisms of tolerance of desert plants. Uranium is one of the elements that are readily soluble in oxidizing environments, and is available for uptake in the soluble form similarly to all other ions. It is found in all the water sources of the Sinai in concentrations ranging from a few to tens of parts per billion (Kronfeld, Navrot et al., 1975). These values are on the average considerably higher than those for most freshwater sources from the wetter temperate regions, which can be generally classified as being below the seawater value of 3'3 fig 1-1 (Rogers & Adams, 1969; Mangini, Sonntag et al., 1979). :j:
Author to whomcorrespondenceshould be addressed.
0140-1963/92/040363
+ 06 $03'00/0
© 1992 Academic Press Limited
364
H. ZAFRIR ET AL.
The concentration of soluble uranium in the root zone is an important factor in determining the uptake of this element by plants (Kinzel, 1982). It might be more important than the uranium concentration of soils or the bedrock (Kronfeld & Zafrir, 1982); in the bedrock, uranium may be chiefly incorporated into the crystal lattice by various minerals, and may not be released despite the highly corrosive micro-environment that is set up around plant roots (Dunn, Ek et al., 1985). Because of the harsh conditions, desert plants have developed efficient biological barriers against non-essential ion uptake, to prevent mineral accumulation in the shoots from reaching toxic levels (Waisel, 1972; Waisel & Agami, 1979). In a study of the uranium concentration of plants in the Soviet Union, Kovalevskii (1984) concluded that most plants possess a biological barrier that limits the uptake of uranium. A few plant species are known to have the ability to concentrate uranium to levels of several hundreds p.p.m. or greater (Dunn, 1981; Shacklette & Erdman, 1982). Plants whose uranium concentration is positively correlated to the uranium concentration in the geologic environment are searched for as biogeochemical indicators. In contrast, there are two other types ofplants that cannot be used for prospecting; those that limit the uptake of the specific metallic ion from the environment (hypoaccumulators), and those that preferentially accumulate a specific element from the environment with no relation to its concentration there (hyper-accumulators). These latter two groups of plants do not reflect the environmental spatial concentrations of the prospected element in a fashion similar to the indicator plants, and therefore are useless as biogeochemical indicators. A study was undertaken to study how the plants of southern Sinai accumulate uranium. Sampling and results During the course of a 2 year survey, plant samples were collected throughout southern Sinai representing most of the habitat types of the southern part of the peninsula. Two hundred and twenty plants representing 40 different species were collected. Samples included leaves and stems together. Major ions, as well as uranium concentrations were measured in adjacent water sources wherever possible. The chemistry of the ground water was found to depend largely upon the rock types of the specific drainage basin. This data will be presented in detail in a separate paper (H. Zafrir et al., in preparation). After washing the sampled plants with distilled water, the leaves and stems were dried, ground and ashed at 550°C. The uranium concentration of the ash was measured by delayed neutron activation (DNA) at the Soreq Reactor. Replicate analyses were performed for each sample, and the analytical error is between 5 and 10% of the values reported in Table 1. One common plant species, Lindenbergia sinaica (Decne) Benth., whose average uranium concentration was among the highest of the southern Sinai plants (Table 1), was further investigated in a controlled greenhouse experiment, in order to clarify its potential as an indicator plant. Four groups of plants of this species were grown from seeds in sand culture and were irrigated with Hoagland's nutrient solution. Mter the plants were established, three of the groups were irrigated with 1,5 and 20p.p.m. concentrations of uranyl nitrate respectively. The control group was supplied only with the nutrient solution. All solutions were made up in tap water, which contained 2p.p.b. uranium. After several months, when the plants began to flower, they were harvested. The uranium concentrations were then determined in the plants' stems and leaves in a manner similar to that of the field samples. The results of this experiment are presented in Table 2. Discussion It is seen (Table 1, Fig. I) that most of the species exhibit average concentrations in the range off):5-2'0 p. p.m. When considering the average uranium concentrations, with their
365
URANIUM IN PLANTS OF S. SINAI
Table I, Average uranium content of shoots of various plant species of southern Sinai Sample no.
Plant species
I 2 3 4 5 6 7* 8 9 10 11 12 13 14* 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Phoenixdactylifera L. Salvadorapersica L. Hammadasalicornica (Moq.) Iljin Cucumis prophetarum L. Trichodesma africana (L.) Lehm. Senna italica Mill. SchouwiaschimperiJaub. &Spach Pituranthos tortuosu (Desf.) Benth. ex Aschers. Helianthemumlippii (L.) Dum.-Cour. Blepharis ciliaris (L.) Burt Pycnocyla tomentosa Decne Lyciumshawii Roem. & Schultes Chrozophora sp. Schouwiathebaica Webb Launaeaspinosa (Forssk.) Sch. Bip. ex Kuntze Acaciaraddiana Savi Ochradenus baccatus Del. Aervapersica (Burm. F.)Juss. Artemisiajudaica L. Astragalus sp. Zygophyllum coccineum L. Fagonia schimperi Presl Heliotropium arbainense Fresen. Lavandulacoronopifolia Poir. Retama raetam(Forssk.) Webb Crotalaria aegyptiaca Benth. Iphionascabra DC. Tephrosia apollinea (Del.) Link Capparis cartilaginea Decne Zilla spinosa (L.) Prantl Pulicaria ctispa (Forssk.) Oliv. Solenostemma oleifolium (Nect.)
33 34 35 36 37 38 39 40
Cleome drosenfolia (Forssk.) Del. Fagonia mollis Del. Pergularia tomentosa L. Orostegia schimperi (Benth.) Boiss. Teucrium sinaicum Boiss. Resedasp, Lindenbergia sinaica (Decne) Benth. Kickxia aegyptiaca (Dum.) Nabelek
Bullock & Bruce
No, of Plants
Average uranium concentration in ash (ppm)
6 I 2 I 2 3 2 I I I 2 4 I 7 2 12 2 2 2 2 6 4 4 I I 2 6 5 7 I 9
0'05 0'10 0'25 0'30 0'30 0'40 0'45 0'50 0'50 0'50 0'60 0'60 0'80 0'85 0'85 0'95 0'95 1'00 1'10 1'l5 1'30 1'35 1'40 1-40 1'40 1'50 1'50 1'55 1'60 1'60 1'65
I 11 I 1 2
1'70 1'85 1'90 2'20 2'25 2'25 2'50 2'60 4'80
3
8 90 1
S.D.
0'05 0'05 0'20 0'25 0'20
0'40 0'55 0-55 0'35 0'55 0'65 0'20 0'20 1'05 0'90 0'70 0'80 0'60 1'00 1'40 1'40 0'35 0'35 1'65 1'20 2'10 1'70
• Sclwuwia is regarded by many taxonomists as a monotypic genus with S. porputl4 (Forssk.) Schwcinf. as the sole species.
associated standard deviations, it is apparent that though variability among species does exist, there is a considerable overlap in concentration ranges among the species. Thus, a rather discrete range of uranium concentrations seems to be characteristic for the species studied in southern Sinai. The average values are not higher, but surprisingly similar to those plants analysed in wetter regions (Cannon, 1952,1964; Konstantinov, 1963; Botova, Malyuga et al. 1963; Dean, 1966; Walker, 1976; Beus & Grigorian, 1977; Dunn, 1981), where the regional uranium concentrations in the ground waters tend to be significantly lower. No hyperaccumulators have been encountered in southern Sinai. On the contrary,
H. ZAFRIR ET AL.
366
Table 2. The effects ofuranium concentration in thenutrient solution on theconcentration ofuranium in theleaves and stems ofL. sinaica (p.p.m.) The uranium concentration in solution
Leaves (uranium in ash) Stems (uranium in ash)
0·OO2p.p.m. (tap water)
Ip.p.m.
5p.p.m.
20p.p.m.
0'17 ± 0'45 (10) 1'85 ± 0'02 (10)
3'1±1'7 (9) 5'0 ± 2'6 (9)
6'3±8'0 (8) 12'4± 20'4 (8)
33'4± 29'8 (7) 39'7± 28'0 (7)
• Number of replicatesin parentheses.
the plants show evidence of retardation in the uptake of uranium. Moreover, the uranium content within a species is quite homogeneous (Table I) despite the fact that a geographical separation of many kilometers (which may be between 10-I00km) between a species collection sites, and may encounter order of magnitude or greater differences in the uranium concentrations of their associated ground waters. For example, L. sinaica, a common plant species in southern Sinai, exhibits an average uranium concentration of 2'6 ± 1'7 p.p.m, for 90 samples distributed widely throughout southern Sinai, and whose associated ground waters exhibit widely varying uranium concentrations (up to one order of magnitude differences). While the variability associated with the plants' mean concentration is not great, it represents one of the wider spreads encountered in the region. It is for this reason that the species was investigated in the laboratory for its potential as an indicator plant. It is observed (Table 2) that an increase in the plants' uranium content,
40
30
'" u Q) Q)
a.
co'" 0
a:
20
~
2
3
Uranium concenlrolion lpp.m.l
Figure 1. A plot of the mean uranium concentrations in the plant species listed in Table 1 of the southern Sinai desert, demonstrating that most of the average values fall within the range of the 0.5 to 2 p.p.m. uranium in the ash. The values exhibited by the Sinai desert plant species do not differ from those of temperate zones, whose associated waters contain appreciably lower uranium concentrations.
URANIUM IN PLANTS OF S. SINAI
367
both in the leaves and in the stems, increases with an increase in the uranium concentration of the irrigation waters. The southern Sinai water sources are almost all considerably greater than the control's uranium concentration but significantly less than the 1 p.p.m. rate. It is seen (Table 2) that the uranium in the stems of the plants of the control samples take up uranium to concentrations not dissimilar (1'85 ± 0'02 p.p.m.) to those of the field samples exposed to higher uranium concentrations. The leaves of the control group are able to maintain lower uranium concentrations. Even at the high 1 p.p.m. rate, the biological barrier is able to maintain uranium concentrations not greatly different from the field samples. At the lowest rates the uranium concentration exhibited within the group is homogeneous; but, as the uranium concentration of the irrigation water increases, the variability among samples rises correspondingly. This is a phenomenon that is typically expressed by plants under stress conditions. It is seen in the present study (Table 2) that increasing uranium uptake is concomitant with increased variability within the plant population as the dosage of soluble uranium is increased. This is particularly manifested by those plant groups that received 5 p.p.m. and 20 p.p.rn. treatments. At such high concentrations of uranium (which are only rarely encountered in natural waters), the biological barrier is being breached and the selective capability of the plants for ion uptake is lost. Therefore, L. sinaica, like the rest of the southern Sinai plants, is a hyperaccumulator at relatively low to moderate environmental levels. At high uranium concentrations its potential as a biogeochemical indicator would increase. The amount of uranium that this type of potential indicator plant is able to accumulate will be less than true indicator plants which have low selectivity to start with. Conclusion The search for good indicator plants for uranium and other heavy metals would appear to be limited among the plants of the Middle Eastern deserts. Because of the harsh conditions, desert plants have developed efficient biological barriers which enable the plants to exist in the desert and to utilize its saline water sources. This barrier prevents the average concentrations of uranium in the southern Sinai plant species from rising over that typical for plants which inhabit wetter regions and where the general uranium burden in the water sources is lower. References Beus, A. A. & Grigorian, S. V. (1977). Geophysical Exploration Methods for Mineral Deposits. Wilmette, Ill: AppliedPublishingCompany. 287pp. Botova, M. M., Malyuga, D. P. & Moiseyenko, U. Z. (1963). Experimental use of the biogeochemical method in prospecting for uranium under desert conditions. Geochemisrry, 3: 379-388. Cannon, H. L. (1952). The effectof uranium-vanadium depositson the vegetation of the Colorado plateau. American Journal of Science, 250: 735-770. Cannon, H. L. (1964). Geochemistry of rocks and related soils and vegetation in the Yellow Cat Area, Grand County, Utah. UnitedStares Geological Survey Bulletin, 1976: 1-127. Dean, M. H. (1966). A survey of the uranium content of vegetation in Great Britain. Journal of Ecology, 54: 589-595. Dunn, C. E. (1981). Reconnaissance level and detailed surveysin the exploration for uranium by a biogeochemical method. In Christopher, J. E. & MacDonald, R. (Eds) SummaryofInvestigations 1981, Saskatchewan Geological Survey. Saskatchewan Department of Mineral Resources Miscellaneous Report, 81-4, pp. 117-126. Dunn, C. E., Ek, J. & Byman, J. (1985). Uranium Biogeochemisrry: A Bibliography andReporton the StareoftheArt. Atechnicaldocumentissuedby the InternationalAtomic EnergyAgency. 83pp. Eyal, M., Bartov, Y., Shimron, A. & Benter, Y. K. (1985). Geological map of Sinai, 11500,000 and explanations. Geological Survey ofIsrael Report, GSU1I8S.
368
H. ZAFRIR ET AL.
Kinzel, H. (1982). Pflonzenokologie undMineralstoffwechsel. Stuttgart: Verlag Eugen Ulmer. S34pp. Konstantinov, V. M. (1963). Feasibility of using a biochemical survey for prospecting for uranium in arid territories. Soviet Geology, 3: 151-155 [in Russian]. Kovalevskii, A. L. (1984). Biogeochemical prospecting for ore deposits in the USSR. Journal of Geochemical Exploration, 21: 63-72. Kronfeld, J., Navrot, J., Zach, R. & Zafrir, H. (1975). Uranium in the water sources of southern Sinai. Sixth Scientific Conference oftheIsraelEcological Society, pp. 167-176. Kronfeld, J. & Zafrir, H. (1982). The possibility of using desert palms in hydrologic reconnaissance prospecting for uranium. Journal ofGeochemical Exploration, 16: 183-187. Mangini, A., Sonntag, C., Bertsch, G. & Muller, E. (1979). Evidence for a higher natural uranium content in world rivers. Nature, 278: 337-339. Rogers, J. J. W. & Adams, J. A. S. (1969). Uranium. In Wedepohl, K. H. (Ed) Handbook of Geochemistry. Berlin: Springer-Verlag. Shacklene, H. & Erdman, J. A. (1982). Uranium in spring water and bryophytes at Basin Creet in Central Idaho. Journal ofGeochemical Exploration, 17: 221-336. Waisel, Y. (1972). Biology ofHalophytes, New York: Academic Press. 395 pp. Waisel, Y. & Agami, M. (1979). Halophytes ofIsrael, Tel Aviv: Division of Ecology Ltd. 80 pp. Waisel, Y. & Alon,A. (1980). Trees oftheLandofIsrael, Tel Aviv: Division ofEcology Ltd. 128pp. Waisel, Y., Pollak, G. & Cohen, Y. (1978). The Ecology ofthe Vegetation ofIsrael, Tel Aviv: Division of Ecology. 460 pp. Walker, N. C. (1976). Biogeochemical studies over the key lake uranium-nickel ore zone. Saskatchewan Department ofMineralResources, Summmy ofInvestigations, pp. 125-127.
Uranium in plants of southern Sinai H. Zafrir*, Y. Waiselt, M. Agamij'], J. Kronfeld§ & E. Mazor]
* Department for Field Applications, Nahal Soreq NuclearResearch Center, Yavne,
t:j: Department ofBotany, Tel Aviv University, Tel Aviv 69978,
§ Department ofGeophysics and PlanetarySciences, Tel Aviv University, Tel Aviv 69978 and II§ Geo-isotope Group, Weizmann Institute ofScience,
Rehovot, Israel
(Received 16January 1991, accepted 29 January 1991) Analyses of 220 individual plants, representing 40 different species from the southern Sinai desert have shown that the species' average uranium concentrations range from O'OS to 4'8 p.p.m, with the large preponderance fallinginto the narrower range of O'S to 2'0 p.p.m. Uranium accumulation was observed to be independent of such variables as location, rock types, uranium content or chemistry of the waters, or the age of the plants sampled. The concentrations encountered in southern Sinai seem to be similarto the average data that are available for temperate zone plants despite the lower average concentration of uranium in the waters of such regions. Desert plants, which usually rely upon limited water sources of high mineral content are highly selective in their ion uptake, with uranium being merely one of the rejected elements. Introduction The Sinai is a triangular peninsula, bordered by the gulfs of Suez and Eilat. The south central part is made up of a crystalline mountain complex, with abundant granitic-type rocks and lesser amounts of basics , and metamorphics (Eyal, Bartov et al., 1985). Southern Sinai is a land with a harsh desert climate. Rainfall is infrequent, usually limited to a few showers during the winter. Soil cover of the mountain region is restricted, leaving the mountain slopes exposed. Sand flats and thick alluvial fills are typical to the bottom of the ephemeral streams (wadis). The plant cover is sparse. Trees and large shrubs are generally restricted to the wadis, rock crevices, or wherever shallow ground water is available (Waisel, Pollak etal., 1978), with date palms (Phoenix dactylifera L.), tamarisks (Tamarix sp.), and acacias (Acacia raddiana Savi etc.) being prevalent (Waisel & Alon, 1980). A survey of the uranium concentration of the local plants was conducted in order to understand the mechanisms of tolerance of desert plants. Uranium is one of the elements that are readily soluble in oxidizing environments, and is available for uptake in the soluble form similarly to all other ions. It is found in all the water sources of the Sinai in concentrations ranging from a few to tens of parts per billion (Kronfeld, Navrot et al., 1975). These values are on the average considerably higher than those for most freshwater sources from the wetter temperate regions, which can be generally classified as being below the seawater value of 3'3 fig 1-1 (Rogers & Adams, 1969; Mangini, Sonntag et al., 1979). :j:
Author to whomcorrespondenceshould be addressed.
0140-1963/92/040363
+ 06 $03'00/0
© 1992 Academic Press Limited
364
H. ZAFRIR ET AL.
The concentration of soluble uranium in the root zone is an important factor in determining the uptake of this element by plants (Kinzel, 1982). It might be more important than the uranium concentration of soils or the bedrock (Kronfeld & Zafrir, 1982); in the bedrock, uranium may be chiefly incorporated into the crystal lattice by various minerals, and may not be released despite the highly corrosive micro-environment that is set up around plant roots (Dunn, Ek et al., 1985). Because of the harsh conditions, desert plants have developed efficient biological barriers against non-essential ion uptake, to prevent mineral accumulation in the shoots from reaching toxic levels (Waisel, 1972; Waisel & Agami, 1979). In a study of the uranium concentration of plants in the Soviet Union, Kovalevskii (1984) concluded that most plants possess a biological barrier that limits the uptake of uranium. A few plant species are known to have the ability to concentrate uranium to levels of several hundreds p.p.m. or greater (Dunn, 1981; Shacklette & Erdman, 1982). Plants whose uranium concentration is positively correlated to the uranium concentration in the geologic environment are searched for as biogeochemical indicators. In contrast, there are two other types ofplants that cannot be used for prospecting; those that limit the uptake of the specific metallic ion from the environment (hypoaccumulators), and those that preferentially accumulate a specific element from the environment with no relation to its concentration there (hyper-accumulators). These latter two groups of plants do not reflect the environmental spatial concentrations of the prospected element in a fashion similar to the indicator plants, and therefore are useless as biogeochemical indicators. A study was undertaken to study how the plants of southern Sinai accumulate uranium. Sampling and results During the course of a 2 year survey, plant samples were collected throughout southern Sinai representing most of the habitat types of the southern part of the peninsula. Two hundred and twenty plants representing 40 different species were collected. Samples included leaves and stems together. Major ions, as well as uranium concentrations were measured in adjacent water sources wherever possible. The chemistry of the ground water was found to depend largely upon the rock types of the specific drainage basin. This data will be presented in detail in a separate paper (H. Zafrir et al., in preparation). After washing the sampled plants with distilled water, the leaves and stems were dried, ground and ashed at 550°C. The uranium concentration of the ash was measured by delayed neutron activation (DNA) at the Soreq Reactor. Replicate analyses were performed for each sample, and the analytical error is between 5 and 10% of the values reported in Table 1. One common plant species, Lindenbergia sinaica (Decne) Benth., whose average uranium concentration was among the highest of the southern Sinai plants (Table 1), was further investigated in a controlled greenhouse experiment, in order to clarify its potential as an indicator plant. Four groups of plants of this species were grown from seeds in sand culture and were irrigated with Hoagland's nutrient solution. Mter the plants were established, three of the groups were irrigated with 1,5 and 20p.p.m. concentrations of uranyl nitrate respectively. The control group was supplied only with the nutrient solution. All solutions were made up in tap water, which contained 2p.p.b. uranium. After several months, when the plants began to flower, they were harvested. The uranium concentrations were then determined in the plants' stems and leaves in a manner similar to that of the field samples. The results of this experiment are presented in Table 2. Discussion It is seen (Table 1, Fig. I) that most of the species exhibit average concentrations in the range off):5-2'0 p. p.m. When considering the average uranium concentrations, with their
365
URANIUM IN PLANTS OF S. SINAI
Table I, Average uranium content of shoots of various plant species of southern Sinai Sample no.
Plant species
I 2 3 4 5 6 7* 8 9 10 11 12 13 14* 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Phoenixdactylifera L. Salvadorapersica L. Hammadasalicornica (Moq.) Iljin Cucumis prophetarum L. Trichodesma africana (L.) Lehm. Senna italica Mill. SchouwiaschimperiJaub. &Spach Pituranthos tortuosu (Desf.) Benth. ex Aschers. Helianthemumlippii (L.) Dum.-Cour. Blepharis ciliaris (L.) Burt Pycnocyla tomentosa Decne Lyciumshawii Roem. & Schultes Chrozophora sp. Schouwiathebaica Webb Launaeaspinosa (Forssk.) Sch. Bip. ex Kuntze Acaciaraddiana Savi Ochradenus baccatus Del. Aervapersica (Burm. F.)Juss. Artemisiajudaica L. Astragalus sp. Zygophyllum coccineum L. Fagonia schimperi Presl Heliotropium arbainense Fresen. Lavandulacoronopifolia Poir. Retama raetam(Forssk.) Webb Crotalaria aegyptiaca Benth. Iphionascabra DC. Tephrosia apollinea (Del.) Link Capparis cartilaginea Decne Zilla spinosa (L.) Prantl Pulicaria ctispa (Forssk.) Oliv. Solenostemma oleifolium (Nect.)
33 34 35 36 37 38 39 40
Cleome drosenfolia (Forssk.) Del. Fagonia mollis Del. Pergularia tomentosa L. Orostegia schimperi (Benth.) Boiss. Teucrium sinaicum Boiss. Resedasp, Lindenbergia sinaica (Decne) Benth. Kickxia aegyptiaca (Dum.) Nabelek
Bullock & Bruce
No, of Plants
Average uranium concentration in ash (ppm)
6 I 2 I 2 3 2 I I I 2 4 I 7 2 12 2 2 2 2 6 4 4 I I 2 6 5 7 I 9
0'05 0'10 0'25 0'30 0'30 0'40 0'45 0'50 0'50 0'50 0'60 0'60 0'80 0'85 0'85 0'95 0'95 1'00 1'10 1'l5 1'30 1'35 1'40 1-40 1'40 1'50 1'50 1'55 1'60 1'60 1'65
I 11 I 1 2
1'70 1'85 1'90 2'20 2'25 2'25 2'50 2'60 4'80
3
8 90 1
S.D.
0'05 0'05 0'20 0'25 0'20
0'40 0'55 0-55 0'35 0'55 0'65 0'20 0'20 1'05 0'90 0'70 0'80 0'60 1'00 1'40 1'40 0'35 0'35 1'65 1'20 2'10 1'70
• Sclwuwia is regarded by many taxonomists as a monotypic genus with S. porputl4 (Forssk.) Schwcinf. as the sole species.
associated standard deviations, it is apparent that though variability among species does exist, there is a considerable overlap in concentration ranges among the species. Thus, a rather discrete range of uranium concentrations seems to be characteristic for the species studied in southern Sinai. The average values are not higher, but surprisingly similar to those plants analysed in wetter regions (Cannon, 1952,1964; Konstantinov, 1963; Botova, Malyuga et al. 1963; Dean, 1966; Walker, 1976; Beus & Grigorian, 1977; Dunn, 1981), where the regional uranium concentrations in the ground waters tend to be significantly lower. No hyperaccumulators have been encountered in southern Sinai. On the contrary,
H. ZAFRIR ET AL.
366
Table 2. The effects ofuranium concentration in thenutrient solution on theconcentration ofuranium in theleaves and stems ofL. sinaica (p.p.m.) The uranium concentration in solution
Leaves (uranium in ash) Stems (uranium in ash)
0·OO2p.p.m. (tap water)
Ip.p.m.
5p.p.m.
20p.p.m.
0'17 ± 0'45 (10) 1'85 ± 0'02 (10)
3'1±1'7 (9) 5'0 ± 2'6 (9)
6'3±8'0 (8) 12'4± 20'4 (8)
33'4± 29'8 (7) 39'7± 28'0 (7)
• Number of replicatesin parentheses.
the plants show evidence of retardation in the uptake of uranium. Moreover, the uranium content within a species is quite homogeneous (Table I) despite the fact that a geographical separation of many kilometers (which may be between 10-I00km) between a species collection sites, and may encounter order of magnitude or greater differences in the uranium concentrations of their associated ground waters. For example, L. sinaica, a common plant species in southern Sinai, exhibits an average uranium concentration of 2'6 ± 1'7 p.p.m, for 90 samples distributed widely throughout southern Sinai, and whose associated ground waters exhibit widely varying uranium concentrations (up to one order of magnitude differences). While the variability associated with the plants' mean concentration is not great, it represents one of the wider spreads encountered in the region. It is for this reason that the species was investigated in the laboratory for its potential as an indicator plant. It is observed (Table 2) that an increase in the plants' uranium content,
40
30
'" u Q) Q)
a.
co'" 0
a:
20
~
2
3
Uranium concenlrolion lpp.m.l
Figure 1. A plot of the mean uranium concentrations in the plant species listed in Table 1 of the southern Sinai desert, demonstrating that most of the average values fall within the range of the 0.5 to 2 p.p.m. uranium in the ash. The values exhibited by the Sinai desert plant species do not differ from those of temperate zones, whose associated waters contain appreciably lower uranium concentrations.
URANIUM IN PLANTS OF S. SINAI
367
both in the leaves and in the stems, increases with an increase in the uranium concentration of the irrigation waters. The southern Sinai water sources are almost all considerably greater than the control's uranium concentration but significantly less than the 1 p.p.m. rate. It is seen (Table 2) that the uranium in the stems of the plants of the control samples take up uranium to concentrations not dissimilar (1'85 ± 0'02 p.p.m.) to those of the field samples exposed to higher uranium concentrations. The leaves of the control group are able to maintain lower uranium concentrations. Even at the high 1 p.p.m. rate, the biological barrier is able to maintain uranium concentrations not greatly different from the field samples. At the lowest rates the uranium concentration exhibited within the group is homogeneous; but, as the uranium concentration of the irrigation water increases, the variability among samples rises correspondingly. This is a phenomenon that is typically expressed by plants under stress conditions. It is seen in the present study (Table 2) that increasing uranium uptake is concomitant with increased variability within the plant population as the dosage of soluble uranium is increased. This is particularly manifested by those plant groups that received 5 p.p.m. and 20 p.p.rn. treatments. At such high concentrations of uranium (which are only rarely encountered in natural waters), the biological barrier is being breached and the selective capability of the plants for ion uptake is lost. Therefore, L. sinaica, like the rest of the southern Sinai plants, is a hyperaccumulator at relatively low to moderate environmental levels. At high uranium concentrations its potential as a biogeochemical indicator would increase. The amount of uranium that this type of potential indicator plant is able to accumulate will be less than true indicator plants which have low selectivity to start with. Conclusion The search for good indicator plants for uranium and other heavy metals would appear to be limited among the plants of the Middle Eastern deserts. Because of the harsh conditions, desert plants have developed efficient biological barriers which enable the plants to exist in the desert and to utilize its saline water sources. This barrier prevents the average concentrations of uranium in the southern Sinai plant species from rising over that typical for plants which inhabit wetter regions and where the general uranium burden in the water sources is lower. References Beus, A. A. & Grigorian, S. V. (1977). Geophysical Exploration Methods for Mineral Deposits. Wilmette, Ill: AppliedPublishingCompany. 287pp. Botova, M. M., Malyuga, D. P. & Moiseyenko, U. Z. (1963). Experimental use of the biogeochemical method in prospecting for uranium under desert conditions. Geochemisrry, 3: 379-388. Cannon, H. L. (1952). The effectof uranium-vanadium depositson the vegetation of the Colorado plateau. American Journal of Science, 250: 735-770. Cannon, H. L. (1964). Geochemistry of rocks and related soils and vegetation in the Yellow Cat Area, Grand County, Utah. UnitedStares Geological Survey Bulletin, 1976: 1-127. Dean, M. H. (1966). A survey of the uranium content of vegetation in Great Britain. Journal of Ecology, 54: 589-595. Dunn, C. E. (1981). Reconnaissance level and detailed surveysin the exploration for uranium by a biogeochemical method. In Christopher, J. E. & MacDonald, R. (Eds) SummaryofInvestigations 1981, Saskatchewan Geological Survey. Saskatchewan Department of Mineral Resources Miscellaneous Report, 81-4, pp. 117-126. Dunn, C. E., Ek, J. & Byman, J. (1985). Uranium Biogeochemisrry: A Bibliography andReporton the StareoftheArt. Atechnicaldocumentissuedby the InternationalAtomic EnergyAgency. 83pp. Eyal, M., Bartov, Y., Shimron, A. & Benter, Y. K. (1985). Geological map of Sinai, 11500,000 and explanations. Geological Survey ofIsrael Report, GSU1I8S.
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