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Caesium, potassium and ammonium distributions in different organs of tropical plants
Environmental and Experimental Botany 65 (2009) 111–118
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Caesium, potassium and ammonium distributions in different organs of tropical plants R.M. Anjos a,∗ , B. Mosquera a , N. Sanches a , C.A. Cambu´ı b , H. Mercier b a b
Instituto de F´ısica, Universidade Federal Fluminense, Av. Gal. Milton Tavares de Souza s/n, Gragoat´ a, Niter´ oi Cep 24210-346, RJ, Brazil Instituto de Biociˆencias, Universidade de S˜ ao Paulo, Caixa-Postal 11461, S˜ ao Paulo Cep 05422-970, SP, Brazil
a r t i c l e
i n f o
Article history: Received 29 February 2008 Accepted 2 April 2008 Keywords: Tropical woody fruit trees 137 Cs, 40 K and NH4 concentration ratios K/Cs and NH4 /Cs discrimination ratios
a b s t r a c t In the present work the distribution of ions in aboveground plant parts was studied in order to establish the suitability of using radiocaesium as a tracer for the plant absorption of nutrients, such as potassium (K+ ) and ammonium (NH4 + ). We present the results for the distributions of 137 Cs, 40 K and NH4 + from four tropical plant species: lemon (Citrus aurantifolia), orange (Citrus sinensis), guava (Psidium guajava) and chili pepper (Capsicum frutescens). Activity concentrations of 137 Cs and 40 K were measured by gamma spectrometry and concentrations of free NH4 + ions by a colorimetric method. Similarly to potassium and ammonium, caesium showed a high mobility within the plants, exhibiting the highest values of concentration in the growing parts of the tree (fruits, new leaves, twigs, and barks). A significant correlation between activity concentrations of 137 Cs and 40 K was observed in these tropical plants. The K/Cs discrimination ratios were approximately equal to unity in different compartments of each individual plant, suggesting that caesium could be a good tracer for 40 K in tropical woody fruit species. Despite the similarity observed for the behaviour of caesium and ammonium in the newly grown plant compartments, 137 Cs was not well correlated with NH4 + . Significant temporal changes in the NH4 + concentrations were observed during the development of fruits, while the 137 Cs activity concentration alterations were not of great importance, indicating, therefore, that Cs+ and free NH4 + ions could have distinct concentration ratios for each particular plant organ. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Radiocaesium has been introduced into the environment via nuclear weapons testing, nuclear waste disposal, nuclear or radiological accidents, and authorized discharges from nuclear installations. Caesium tends to be retained in the soil through fixation and adsorption to clay minerals and organic matter, persisting in the plant rooting zone for a long period due to its long physical half-life of 30.2 years. Cs+ can be readily absorbed by roots and can be translocated to the aboveground plant parts. For this reason, 137 Cs is a radioactive pollutant of great concern which can rapidly integrate into biological cycles and accumulate in terrestrial ecosystems. Since the removal of radiocaesium from contaminated forest or agricultural ecosystems is difficult, studies on the distribution and transfer of radiocaesium in these ecosystems are important in order to predict the future contamination of vegetal products (Zhu et al., 2002; Anjos, 2006).
∗ Corresponding author. Tel.: +55 21 26295770; fax: +55 21 26295887. E-mail address: [email protected] (R.M. Anjos). 0098-8472/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2008.04.001
Many studies on biogeochemical cycles of radiocaesium have been carried out, mostly in European forests, after the Chernobyl accident (Antonopoulos-Domis et al., 1990, 1991, 1996; Shaw and Bell, 1991; Myttenaere et al., 1993; Barci-Funel et al., 1995; Schell et al., 1996; Fogh and Anderson, 2001; Carini, 2001; Zhu et al., 2002; Bell and Shaw, 2005). Experimental results for temperate species have provided information for the development of models that explain radiocaesium transport through several compartments in a forest ecosystem. Different types of models were developed, as summarized by Schell et al. (1996). However, these models were unable to provide information significant enough to enable the prediction of radiocaesium behaviour in tropical ecosystems. For this reason, results for tropical plants are also of great concern, since these species do not necessarily follow the same pattern as temperate plants. Chemical behaviour of radiocaesium is expected to be similar to that of other monovalent inorganic cations, such as ammonium and the stable alkali (Na, K, Rb and Cs) elements (Shaw and Bell, 1991; Yoshida and Muramatsu, 1998; Zhu et al., 2002; Sanches et al., in press). Ammonium is a central intermediate in the nitrogen metabolism of plants. An important function of potassium is to adjust the osmotic pressure of cells, being, then, directly related
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to the plant growth. After completion of cell extension, for maintenance of the cell turgor K can be fairly readily replaced in the vacuoles by others solutes such as sodium (Marschner, 1995). However, information on the relationship between radionuclides and the major nutrient elements in forest or agricultural ecosystems is still limited. In addition, the quantitative importance of NH4 + transporting N from root to shoot and the capability of plants to store NH4 + in the leaves are still subjects of substantial controversy. Some of them are related to the use of inadequate analytical procedures for extraction and quantification of NH4 + in plants (Husted et al., 2000). The data for trace stable alkali elements such as Na, Rb and Cs in forest ecosystems are limited because of the lack of simple analytical methods. Even using the gamma spectrometry method, in order to perform the 40 K measurements, it is necessary to use at least 50 g dry weight of plant sample, due to its low activity concentration. Only exiguous amounts of sample (about 3 g) allow performing 137 Cs measurements from samples contaminated by radiocaesium. Therefore, the study of 137 Cs is potentially more useful than just for radioecology, since it is interesting to evaluate the potential role of 137 Cs as a tracer for plant nutrition. In order to deepen the understanding on the behaviour of monovalent cations, the purpose of this work is to examine simultaneously the activity concentrations of 137 Cs and 40 K, and the concentrations of NH4 + in several compartments of tropical plants, verifying the hypothesis that these elements have similar concentration ratios for each set of plant organs.
per samples were collected during the spring of 2005. Lemon and orange samples were collected during the spring–summer of 2006. At that time, the guava trees were 7 years old and around 5.0 m high. The chili pepper plants were 1-year-old and around 1.0 m high. The lemon and orange trees were 4 years old and around 2.5 m high. The ground was composed of a red soil containing construction remains with sandy–clay–loam texture. Among its main features, the total potassium content ranged from 0.5 to 2.9%, the sand content from 70 to 81%, the silt content from 13 to 20%, and the clay content from 5.9 to 6.3%. The pH value ranged from 5.8 to 6.3. Soil pH was measured in 1 M KCl in a solid–liquid ratio of 1:2.5 after 1 h equilibrium. The organic matter content ranged from 1.5 to 1.7%. The organic C content was measured with a wet combustion technique using K2 Cr2 O7 and H2 SO4 followed by CO2 analysis. The cation exchange capacity (CEC) ranged from 5.0 to 5.5 cmol kg−1 soil. CEC was measured using silver thiourea as the index cation in a 0.1-M NH4 OAc buffer. Soils from different depths were also collected. The sampling was performed in layers of 5 cm, reaching around 50 cm depth. Since for each plant species, three plants were sampled, then, 12 soil profiles were measured (one for each plant), each one formed by 10 sub-samples. Thus, for each plant species, 30 soil samples were collected. This procedure allowed obtaining a representative sampling of the caesium present within the rooting zone for each plant species studied. 2.2. Determination of 137 Cs and 40 K
2. Materials and methods 2.1. Sampling One of the worst radiological accidents ever reported occurred at the urban area of Goiania, Brazil, in 1987. A piece of a teletherapy machine, containing a 137 Cs source with an activity of 50.9 TBq, was removed from a derelict clinic and sold as scrap. Attracted by the blue glow emitted by the 137 CsCl salt, children and adults handled and distributed the material, in powder form, among relatives and friends, after destroying the shielding. As a consequence, around 1000 persons were irradiated, 250 contaminated either internally and/or externally and 49 were taken into the hospital. Of these, 28 had serious radiation burn problems, one had his forearm amputated and four died. The decontamination process required the demolition of seven residences and some other buildings, and the removal of the topsoil from large areas. In total, around 3500 m3 of radioactive waste were generated (IAEA, 1988; Anjos et al., 2001). Since 1999, a study on the environmental consequences of this radiological accident has been performed by our group (Facure et al., 2001; Anjos et al., 2002). In particular, at one of the sites involved in the accident and has then been inhabited since 1990. Thereafter, their residents have planted several tropical plants in the garden. Thus, as a development of previous works, measurements of the activity concentrations of 137 Cs and 40 K, and NH4 + concentrations from four tropical plant species were performed: lemon (Citrus aurantifolia), orange (Citrus sinensis), guava (Psidium guajava) and chili pepper (Capsicum frutescens). For each specie studied, samples were taken from three plants, separated into nine plant compartments (root, stem, bark, branch, twig, new leaf, old leaf, green fruit and old fruit), and for each compartment 10 sub-samples were analysed. Thus, for each plant compartment measured 30 samples were collected. In order to evaluate the 137 Cs activity concentration and NH4 + concentration as function of the fruit diameter, the green and mature fruits were also rearranged according to their sizes. Guava samples were collected during the spring of 2003. Chili pep-
Sample preparation and analysis were carried out at the Laboratory of Radioecology (LARA) of the Physics Institute of Federal Fluminense University. The root, main trunk, twig, leaf and fruit samples were first washed with distilled water, while the soil samples were sieved through a 1-mm screen. Then they were submitted to a drying process, at 110 ◦ C. Additionally, the vegetable samples were ground to powder. Afterwards, all samples were packed into cylindrical plastic containers, dry-weighed and sealed. The dry weights of each vegetable sample were about 30–50 g and about 200 g for soil samples. The samples were analysed by the conventional technique of gamma spectrometry, using one NaI detector. Radionuclide activities in plants were calculated from the net fullenergy peaks (661.7 keV gamma-ray line for 137 Cs and 1460.8 keV for 40 K) and the measured efficiency of the detector. The 40 K calibration was performed using a reference material (RGK-1) obtained from the International Atomic Energy Agency. Standards of radiocaesium were made by mixing uncontaminated sawdust with the liquid 137 Cs solution prepared by the Brazilian National Nuclear Energy Commission (CNEN). Their results were inter-compared with two laboratories in Brazil. Technical details of sample preparation and analysis can be obtained from Mosquera et al. (2006) and Carvalho et al. (2006). Hence, using the dry weight of the samples, their respective activity concentrations of 137 Cs and 40 K could be expressed in activity per unit mass (Bq kg−1 ). The uncertainties of the specific 137 Cs activity from each individual measurement of plant samples ranged from 5 to 7% and around 10% for the specific 40 K activity. The detection limit was estimated to be 7.0 Bq kg−1 for 137 Cs and 75 Bq kg−1 for 40 K. 2.3. Determination of NH4 + Concentrations of free NH4 + ions were estimated using the phenol-hypochlorite colorimetric method (Weatherburn, 1967). Sample preparation and NH4 + analysis were carried out at the Laboratory of Plant Physiology of the Biosciences Institute of Sao Paulo University. Frozen samples (1 g fresh mass) were ground in liquid
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Table 1 Mean values of activity concentrations (Bq kg−1 dry weight) of 137 Cs and 40 K and NH4 + concentrations and their stem-to-compartment j concentration ratios (CRs) of lemon trees contaminated by caesium j
Compartmenta
Concentrations 137
Lemon tree (Citrus aurantifolia) 1 Main root 2 Main stem 3 Bark 4 Branch 5 Twig 6 Old leaf 7 New leaf 8 Mature fruitb 9 Green fruit
Cs (Bq kg
−1
)
97 (10) 90 (7) 245 (31) 183 (20) 281 (17) 420 (26) 581 (47) 301 (24) 395 (22)
Stem-to-compartment j concentration ratios 40
K (Bq kg
−1
)
236 (20) 230 (20) 600 (48) 432 (37) 723 (54) 1216 (91) 1249 (106) 759 (76) 1049 (55)
+
NH4 (ppm)
CR[Cs]
CR[K]
CR[NH4 ]
1.5 (0.2) 1.5 (0.1) 2.6 (0.1) 1.7 (0.1) 1.6 (0.1) 3.4 (0.2) 2.4 (0.2) 1.9 (0.1) 3.6 (0.2)
1.1 (0.1) 1.0 (0.1) 2.7 (0.4) 2.0 (0.3) 3.1 (0.3) 4.7 (0.5) 6.5 (0.7) 3.3 (0.4) 4.4 (0.4)
1.0 (0.1) 1.0 (0.1) 2.6 (0.3) 1.9 (0.2) 3.1 (0.4) 5.3 (0.6) 5.4 (0.7) 3.3 (0.4) 4.6 (0.6)
1.0 (0.2) 1.0 (0.1) 1.7 (0.2) 1.1 (0.1) 1.1 (0.1) 2.3 (0.2) 1.6 (0.2) 1.3 (0.1) 2.4 (0.2)
Values in parentheses represent one standard deviation from the mean from 30 measurements. a The main stem samples had diameters of about 7 cm; the branches between 2 and 3 cm; the twigs between 0.5 and 1 cm; 6 cm for mature fruits and between 1 and 4 cm for green fruits. b Average water content of 86%.
nitrogen to a fine powder and homogenized with 4 mL of distilled water. The homogenate was centrifuged and the supernatant collected. 300 L aliquots were used in ammonium quantification. Concentrations of free NH4 + ion were expressed in parts per million (ppm). The colorimetric analysis of NH4 + is based on the classical Berthelot reactions: NH4 + is converted to monochloramine, which subsequently reacts with a phenolic compound having an unsubstituted paraposition. All reactions occur under alkaline conditions in the presence of nitroprusside as a catalyst. Ammonia, hypochlorite and the phenolic compound react and an intensely blue-green chromophore (an indophenol complex) is formed as reaction product, quantified by colorimetry at 640–655 nm (Husted et al., 2000; Endres and Mercier, 2001). The uncertainties of the NH4 + concentrations ranged from 3 to 6%. The detection limit was estimated to be 25 M.
2.4. Soil to plant transfer factor Transfer factor (TF) has been used for many years to predict concentrations of radionuclides that could be expected in food crops after accidental releases of radionuclides into the environment. TF is expressed as the ratio between the activity concentration of the radionuclide in the dried edible part of the plant and that in the oven dried soil (both in Bq kg−1 ). TF values are very useful to help estimate radiation doses to people, which might result from
contaminated food supplies after a release of radionuclides to the environment. 2.5. Stem-to-compartment concentration ratio The capability of plants to store ions can be estimated through the comparison between their respective concentrations in different parts or organs of the plant. In this way, it is possible to compare the concentration of a given radionuclide or stable element between younger and older parts of the plant, using stemto-compartment concentration ratios, defined as: CRj =
Cj
(1)
Cstem
where Cj is the concentration of the element in a given plant compartment j (main root, main stem, bark, branch, twig, new leaf, old leaf, green fruit or mature fruit). For a radionuclide, Cj is given in activity per unit mass (Bq kg−1 ). If the element is stable, Cj can be expressed, for example, in parts per million (ppm). Cstem represents the concentration of the element in the main stem. According to Tables 1–4, the values of the NH4 + concentrations are nearly identical for the main root and the main stem, respectively, which constitute the oldest parts of the plant. The same occurs for the activity concentrations of 137 Cs and 40 K, respectively. Thus, in this work, the main stem was assumed to be the oldest compartment of the plant in the evaluation of the concentra-
Table 2 Mean values of activity concentrations (Bq kg−1 dry weight) of 137 Cs and 40 K and NH4 + concentrations and their stem-to-compartment j concentration ratios (CRs) of orange trees contaminated by caesium j
Compartmenta
Concentrations 137
Orange tree (Citrus sinensis) 1 Main root 2 Main stem 3 Bark 4 Branch 5 Twig 6 Old leaf 7 New leaf 8 Mature fruitb 9 Green fruit
Cs (Bq kg
169 (12) 174 (10) 774 (68) 289 (25) 691 (56) 1178 (88) 1290 (109) 600 (46) 658 (64)
−1
)
Stem-to-compartment j concentration ratios 40
K (Bq kg
100 (10) 102 (9) 400 (32) 177 (23) 331 (26) 675 (60) 711 (69) 336 (36) 353 (38)
−1
)
+
NH4 (ppm)
CR[Cs]
CR[K]
CR[NH4 ]
1.5 (0.1) 1.5 (0.1) 2.5 (0.3) 1.8 (0.1) 1.6 (0.1) 3.2 (0.3) 4.7 (0.2) 0.70 (0.07) 1.6 (0.3)
1.0 (0.1) 1.0 (0.1) 4.4 (0.5) 1.7 (0.2) 4.0 (0.4) 6.8 (0.6) 7.4 (0.8) 3.4 (0.3) 3.8 (0.4)
1.0 (0.1) 1.0 (0.1) 3.9 (0.5) 1.7 (0.3) 3.2 (0.4) 6.6 (0.8) 7.0 (0.9) 3.3 (0.5) 3.5 (0.5)
1.0 (0.1) 1.0 (0.1) 1.7 (0.2) 1.2 (0.1) 1.1 (0.1) 2.1 (0.2) 3.1 (0.2) 0.47 (0.07) 1.1 (0.2)
Values in parentheses represent one standard deviation from the mean from 30 measurements. a The main stem samples had diameters of about 8 cm; the branches between 2 and 3 cm; the twigs between 0.5 and 1 cm; green fruits between 3 and 5 cm and; mature fruits between 7 and 8 cm. b Average water content of 84%.
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Table 3 Mean values of activity concentrations (Bq kg−1 dry weight) of 137 Cs and 40 K and NH4 + concentrations and their stem-to-compartment j concentration ratios (CRs) of guava trees contaminated by caesium j
Compartmenta
Concentrations 137
Guava tree (Psidium guajava) 1 Main root 2 Main stem 3 Bark 4 Branch 5 Twig 6 Old leaf 7 New leaf 8 Mature fruitb 9 Green fruit
Cs (Bq kg
−1
)
860 (70) 835 (65) 2425 (230) 1275 (125) 1216 (102) 1493 (104) 2144 (262) 1933 (162) 2157 (265)
Stem-to-compartment j concentration ratios 40
K (Bq kg
−1
)
195 (17) 174 (16) 436 (28) 244 (33) 277 (19) 265 (19) 394 (36) 413 (33) 466 (52)
+
NH4 (ppm)
CR[Cs]
CR[K]
CR[NH4 ]
15 (1) 13 (1) 23 (2) 15 (1) 14 (1) 16 (1) 12 (1) 1.7 (0.1) 25 (7)
1.0 (0.1) 1.0 (0.1) 2.9 (0.4) 1.5 (0.2) 1.5 (0.2) 1.7 (0.2) 2.6 (0.4) 2.3 (0.3) 2.6 (0.4)
1.1 (0.1) 1.0 (0.1) 2.5 (0.3) 1.4 (0.2) 1.6 (0.2) 1.5 (0.2) 2.3 (0.3) 2.4 (0.3) 2.7 (0.4)
1.1 (0.1) 1.0 (0.1) 1.8 (0.2) 1.2 (0.1) 1.1 (0.1) 1.2 (0.1) 0.9 (0.1) 0.13 (0.01) 1.9 (0.6)
Values in parentheses represent one standard deviation from the mean from 30 measurements. a The main stem samples had diameters of about 15 cm; the branches between 3 and 5 cm; the twigs between 0.5 and 1 cm; green fruits between 3 and 5 cm and; mature fruits between 7 and 8 cm. b Average water content of 82%.
tion ratios of caesium (CR[Cs]j ), potassium (CR[K]j ) and ammonium (CR[NH4 ]j ). 3. Results and discussion 3.1. 137 Cs, 40 K and NH4 + distributions in different plant compartments Concentrations of NH4 + and activity concentrations of 137 Cs and from root to shoot for four woody fruit species were evaluated as well as the capability of such tropical plants to store these ions. Analyses of these values reveal the distributions of nutrients among the different parts of each plant. Given the homogeneity of the results, two clusters were identified of younger (fruits, newly leaves, twigs, branches and bark) and older parts (main stems and main roots) and their respective average and standard deviation are reported in Tables 1–4. These data refers to the mean of 30 samples from each plant compartment and allow correlating potassium, ammonium and caesium distributions to justify these remarks statistically. The results suggest that tissues with low Cs activity concentration (on a dry weight basis) had low K activity concentration as well, in such a way that both 137 Cs and 40 K had simultaneously higher activity concentrations in the new parts than in the older parts. In addition, 137 Cs activity concentration in the different organs of these trees decreased according to foliage > fruits > twigs > branches > main stem. However, the caesium concentration ratio in a specific compartment depended on the plant species. For instance, in the orange, lemon and chili pep40 K
per trees (see Tables 1, 2 and 4, respectively) the 137 Cs concentration ratios (CR[Cs]) in the foliage were 5–7 times higher than in the main stem of the plants, while in the leaves of the guava trees (Table 3) it was only 2.5 times higher. Different behaviours were also observed in caesium accumulation in the barks and fruits of these trees. For instance, in guava (Table 3) bark > foliage > fruits, while in orange (Table 2) foliage > bark > fruits. These results suggest that the 137 Cs distribution can show small variations in different species of tropical fruit trees. Barci-Funel et al. (1995) analysed three conifer trees from different species: a pine tree (Pinus silvestris), a spruce (Pinus picea) and a larch (Larix). The 137 Cs activity concentration in the extremities of these trees (needles, shoots apex, twigs, bark) was found to be larger than those measured in the inner cylinder of the main branches, indicating an accumulation towards the ends, where the exchanges with the ambient air are stronger (driving force of transpiration). The concentrations of 137 Cs in the barks were found to be one or two orders of magnitude higher than those of the inner parts of the plant, whereas the 137 Cs concentrations in the needle and twig were from 3 to 17 times higher. In another study on pine and spruce trees, McGee et al. (2000) found similar results for the 137 Cs concentrations in the needle and twig, but the bark activities were only five times higher. Fogh and Anderson (2001) obtained similar results for pine, birch and oak trees. They found that growing parts (leaves, needles, twigs and barks) had specific 137 Cs activities up to one order of magnitude higher than those of the older parts of the plant. Nevertheless, from Tables 1–4, our present results show that for tropical fruit trees, the extremity part activities were only around two to seven times higher than those of
Table 4 Mean values of activity concentrations (Bq kg−1 dry weight) of 137 Cs and 40 K and NH4 + concentrations and their stem-to-compartment j concentration ratios (CRs) of chili pepper trees contaminated by caesium j
Compartmenta
Concentrations 137
Chili pepper tree (Capsicum frutescens) 1 Main root 2 Main stem 3 Branch 4 Twig 5 Leaf 6 Mature fruitb (red) 7 Green fruitb
Cs (Bq kg−1 )
961 (98) 955 (100) 950 (102) 2158 (211) 5832 (518) 2099 (198) 2548 (239)
Stem-to-compartment j concentration ratios 40
K (Bq kg−1 )
448 (51) 450 (40) 495 (62) 1075 (143) 2200 (221) 1001 (184) 1328 (157)
NH4 + (ppm)
CR[Cs]
CR[K]
CR[NH4 ]
5.5 (0.2) 5.6 (0.2) 5.4 (0.6) 6.2 (0.6) 5.4 (1.8) 3.1 (0.6) 12 (5)
1.0 (0.2) 1.0 (0.2) 1.0 (0.2) 2.3 (0.3) 6.1 (0.8) 2.2 (0.3) 2.7 (0.4)
1.0 (0.2) 1.0 (0.2) 1.1 (0.2) 2.4 (0.3) 4.9 (0.6) 2.2 (0.3) 2.9 (0.4)
1.0 (0.1) 1.0 (0.1) 1.0 (0.1) 1.1 (0.1) 1.0 (0.3) 0.6 (0.1) 2.1 (0.9)
Values in parentheses represent one standard deviation from the mean from 30 measurements. a The main stem samples had diameters of about 3 cm; the branches between 1 and 2 cm; the twigs between 0.2 and 0.5 cm; green fruits between 1.8 and 2.5 cm and; above 2.8 cm for red fruits. b Average water content of 85%.
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the inner parts of the plant, suggesting that the 137 Cs distribution in tropical fruit trees shows small alterations when compared with conifers or temperate forest trees. Similarly to caesium, potassium showed a high mobility within the plants, exhibiting the highest values of activity concentration in the growing parts of the tree (fruits, new leaves, twigs, and barks). In addition, the distributions of caesium and potassium have been shown to be weakly dependent on the age of specific plant parts. The values of the 137 Cs and 40 K concentration ratios did not exhibit any statically significant difference between the newly grown plant parts, such as new leaves and green fruits, and the respective older parts, such as old leaves and mature fruits (Tables 1–4). For instance, Table 2 shows that CR[Cs] and CR[K] for new leaves from orange trees were 7.4 ± 0.8 and 7.0 ± 0.9, respectively, while their old leaves had CR[Cs] and CR[K] values equal to 6.8 ± 0.6 and 6.6 ± 0.8. Green fruits from orange trees had CR[Cs] and CR[K] values equal to 3.8 ± 0.4 and 3.5 ± 0.5, respectively, while their mature fruits had CR[Cs] and CR[K] values equal to 3.4 ± 0.3 and 3.3 ± 0.5, respectively. Similar effects can be observed from leaves and fruits from lemon, guava and chili pepper trees, as shown in Tables 1, 3 and 4, respectively. These results suggest that 137 Cs and 40 K have similar behaviour in the studied tropical plants and thus, 137 Cs could trace the transport or accumulation of potassium in such plants. This evidence is in agreement with Zhu et al. (2002), according to whom potassium, used in soils in relatively large quantities as fertilizer, is believed to be effective in inhibiting the uptake of radiocaesium, due to the ability of their ions to block radiocaesium uptake by plant roots. K is not the only soil-based chemical treatment that increases the plant growth rate and also reduces radiocaesium uptake by plants. In contrast, there are some fertilizing treatments that produce the opposite effect. The application of acid or ammonium-based fertilizers, for instance, has been found to increase the availability and hence the transfer of radiocaesium to plants, showing that the presence of NH4 + ions in the soil could affect caesium transfer from soil to plants (Shaw and Bell, 1991; Nisbet et al., 1993; Willey and Tang, 2006). NH4 + is taken up by roots and can be produced in several biochemical processes including nitrate (NO3 − ) assimilation, photorespiration, phenyl propanoid metabolism, degradation of transport amides or protein catabolism. It is generally assumed that most of the NH4 + absorbed by or generated in roots becomes assimilated in the roots and that only minor amounts are translocated to the shoot (Siebrecht and Tischner, 1999; Husted et al., 2000). Similarly, it is generally believed that the NH4 + generated in the leaves is rapidly and efficiently assimilated and that NH4 + concentration in the leaf tissue, therefore, always remains very low. However, several experiments have shown the occurrence of high NH4 + concentrations in plant tissues (Husted et al., 2000). Thus, it is interesting to investigate the existence of a correlation between NH4 + and 137 Cs concentrations in aboveground plant parts and so to evaluate whether caesium would be an efficient element to trace ammonium. From Tables 1–4, it is possible to observe that, just like Cs+ , free NH4 + ions were possibly absorbed and translocated to the aboveground plant parts, exhibiting the highest values of concentration in their growing parts. This result discloses that Cs+ and free NH4 + ions have similar behaviour and thus, 137 Cs could also trace the transport or accumulation of ammonium ions in the plants. Moreover, the NH4 + concentration ratios (CR[NH4 ]) for these plant compartments exhibited a considerable dependence on the plant species. For instance, for the lemon (Table 1) and orange (Table 2) trees, the NH4 + probably absorbed in the roots and presented in the leaves was about twice higher than that accumulated in the guava or chili pepper trees (Tables 3 and 4, respectively). On the other hand, lemon tree fruits showed twice higher NH4 + accumulation than the orange tree fruits. According to Husted et al. (2000), this behaviour
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could be explained by the fact that plant species exhibit considerable difference in their capability to reduce NO3 − and to assimilate NH4 + in the roots. Thus, plants with limited assimilatory capacity may translocate NH4 + to the shoot and may differ both in their capacity to assimilate NH4 + in the leaf tissue and its tolerance to NH4 + accumulation in the leaves. In contrast to 137 Cs and 40 K behaviours, NH4 + concentration ratios have been shown to be significantly dependent on the age of specific plant parts. Fig. 1 shows the average values of the 137 Cs and NH4 + concentration ratios during the maturing process of lemon, orange, guava and chili pepper fruits. In all these cases there was a higher NH4 + concentration in the fruit formation phase, followed by a decrease in its level as the fruits became mature, suggesting that ammonium could have been assimilated in amino acids during the maturation process. A similar process could also occur in the leaves. Thus, Tables 1–4 and Fig. 1 suggest that the assumption of a physiological equivalence between the Cs+ and NH4 + ions does not hold, since they had distinct concentration ratios in different organs of trees. This result is reinforced by comparison between Cs and K concentrations ratios and Cs and NH4 + concentration ratios. 3.2. K/Cs discrimination ratio Although the caesium concentration ratios depend on the plant species, Tables 1–4 indicate that both caesium and potassium have similar concentration ratios in different organs of same trees, since the CR[Cs] and CR[K] values are practically identical for each particular plant compartment. This result can be better observed in Fig. 2, which presents the K/Cs discrimination ratios for each compartment j defined as: (K/Cs)j =
CR[K]j CR[Cs]j
(2)
where CR[K]j and CR[Cs]j are the 40 K and 137 Cs concentration ratios, respectively, in a given plant compartment j (main root, main stem, bark, branch, twig, new leaf, old leaf, green fruit or mature fruit). This figure shows that the K/Cs values were approximately equal to unity for the different compartments of each individual plant studied in this work. Thus, the relationship between 40 K and 137 Cs in the plant suggests that these two elements behaved alike. In particular, previous measurements performed in chili pepper trees showed that the green fruits had higher 137 Cs concentration than the red ones, but, unlike caesium, the 40 K concentration increased as the pepper fruits turned red (mature). However, if 137 Cs was not available in the soil, the green fruits had higher 40 K concentration than the red ones (Carvalho et al., 2006). Such behaviour suggested the occurrence of a direct competition between Cs+ and K+ ions during the accumulation process throughout the plant, indicating that the two elements have a common accumulation mechanism. In this work the competition between Cs+ and K+ ions in chili pepper tree was investigated. Our measurements showed that this inversion between their concentrations only occurred for the fruit compartment. In the other compartments, the K/Cs discrimination ratio values were approximately equal to unity. On the other hand, the comparison between values of 137 Cs concentrations from different organs of the chili peppers trees contaminated by caesium with the respective 40 K concentrations from the uncontaminated chili pepper trees produces K/Cs approximately equal to unity for all compartments of the chili pepper trees. This effect is shown in Table 4 and Fig. 2d, since the 40 K concentrations from uncontaminated chili peppers trees were used in their elaboration.
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Fig. 1. Stem-to-fruit concentration ratios for caesium and ammonium as function of the fruit diameter for (a) lemon; (b) orange; (c) guava; (d) chili pepper trees.
These findings suggested, therefore, the possibility of using the caesium to predict the behaviour of monovalent cations in tropical woody fruit plants. In addition, some plants are able to replace high proportion of potassium by sodium without an effect on growth and others can have an additional growth stimulation which cannot be achieved by increasing the K content of the plants. Growth stimulation by Na is caused mainly by its effect on cell expansion and on the water balance of plants (Marschner, 1995). Such observations would indicate that Na+ , K+ and Cs+ could compete during the uptake by plants. Then, this result suggests that caesium could also be used to trace the Na nutrition of plants. An analysis about the correlations between Na and Cs behaviour in tropical plants will be performed by our group. 3.3. NH4 + /Cs discrimination ratio Analogously to Eq. (2), the NH4 + /Cs discrimination ratios for each compartment j are defined as: (NH4 /Cs)j =
CR[NH4 ]j CR[Cs]j
(3)
where CR[NH4 ]j and CR[Cs]j are the NH4 + and 137 Cs concentration ratios, respectively. From Fig. 2, the data showed that the ratio NH4 /Cs was <1 for the youngest compartments of each individual plant studied in this work, confirming the result observed in Fig. 1 where the time dependence of CR[Cs] and CR[NH4 ] values in the fruits of lemon, orange, guava and chili pepper trees did not
provide evidence of a correlation between ammonium and caesium behaviour. Thus, these results suggest an unclear relationship between 137 Cs and free NH4 + in the different plant organs, indicating that caesium would not be as good a tracer for NH4 + as it was for K for these plant species. However, it is important to remember that the phenol-hypochlorite colorimetric method allows evaluating only of the concentration of free NH4 + ions present in the plant tissues. Thus, this observation leads to the need for further evaluation of the effects of nitrogen metabolism on caesium uptake, total amounts of Cs take up and root–shoot ratios of Cs, in order to verify the existence of a correlation between caesium and total nitrogen (the sum of free ammonium, amino acids, proteins and other Ncompounds) behaviour. Furthermore, K is a nutrient that remains in ionic form and it is never assimilated in any organic compound (the same as Cs behaviour). In contrast, plant cells avoid NH4 + toxicity by converting NH4 + generated from root-to-shoot transport, or from other metabolic processes such as photorespiration in the leaves, into amino acids.
3.4. Radioecology and soil to plant transfer factor for 137 Cs Since some tropical plants had been cultivated in the garden of one of the sites where the Goiania radiological accident occurred, it is important to evaluate if there are any potential radiological implications on the consumption of their contaminated fruits. Table 5 shows the mean values of activity concentrations of the 137 Cs available in soil within the rooting zone for each plant and its
R.M. Anjos et al. / Environmental and Experimental Botany 65 (2009) 111–118
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Fig. 2. K/Cs and NH4 /Cs discrimination ratios as a function of compartment j for (a) lemon; (b) orange; (c) guava; (d) chili pepper trees. j = 1 represents the main root; j = 2 the main stem; j = 3 bark; j = 4 branch; j = 5 twig; j = 6 old leaf; j = 7 new leaf; j = 8 mature fruit and; j = 9 green fruit. Table 5 Mean values of 137 Cs activity concentrations (Bq kg−1 dry weight) in soil and mature fruits, and their soil-to-fruit transfer factors (TFs) from the lemon, orange, guava and chili pepper trees Mature fruit a
Lemon Orangeb Guavac Chili pepperd
137
Cs activity concentration in soil (kBq kg−1 )
14 (5) 38 (10) 88 (25) 40 (6)
137
Cs activity concentration in fruit (Bq kg−1 )
301 (24) 600 (46) 1933 (162) 2099 (198)
TF 0.022 (0.008) 0.016 (0.004) 0.022 (0.007) 0.052 (0.008)
Values in parentheses represent one standard deviation from the mean from 30 measurements. Average water content of a 86%; b 84%; c 82%; d 85%.
respective soil to fruit transfer factor. The results indicate that the soil contamination was not the same for all the plants, showing a higher transfer of caesium to chili pepper when compared to lemon, orange and guava, since the soil-to-plant transfer factors for 137 Cs were about 5% for chili pepper and 2% for lemon, orange and guava. However, these TF values are in agreement with the range of soil to fruit transfer factors for tropical or temperate fruits observed by Carini (2001). In terms of radioecology, the dose criteria for the implementation of restrictions on the consumption of radionuclide contaminated food products and drinking water refer to estimated doses of internal radiation by radionuclides in food and water consumed within a year (Anjos, 2006). For instance, the limits in foods of general consumption, milk and infant food, and drinking water for 134 Cs, 137 Cs or 103 Ru are set at 1000 Bq kg−1 (IAEA, 1988). Since the limits for fruits are referred to fresh weight, while the fruit data
reported in Tables 1–5 are related to dry weight, before to apply this criterion of restrictions on the consumption the 137 Cs activity concentrations in fruits have been converted into fresh weight according to the water content. Assuming the water content of about 85% in these tropical fruits, the 137 Cs activity concentrations in fresh fruits are far below the IAEA recommendation and so these fruits could be consumed. 4. Conclusions The comparative behaviour of Cs+ , K+ and NH4 + ions in tropical woody fruit trees was evaluated through the use of their concentration ratios in different parts or compartments of the plants. Despite the 137 Cs, 40 K and NH4 + concentration ratios exhibiting considerable dependence on plant species, our results disclosed that Cs+ , K+ and free NH4 + ions have similar behaviour, since the highest
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values of concentrations of these ions were observed in the growing parts of the plants, such as fruits, new leaves, twigs and barks. Thus, 137 Cs could be used to approximate the transport or accumulation of ammonium and potassium ions in the plants. However, the results indicated Cs+ was a better tracer for K+ than it was for NH4 + . A significant correlation between 137 Cs and 40 K concentrations was observed in these tropical plants. The K/Cs discrimination ratios were approximately equal to unity in different compartments of each individual plant. Significant temporal changes in the NH4 + concentrations were observed during the development of fruits, while the 137 Cs concentration alterations were not of great importance, disclosing, therefore, that Cs+ and free NH4 + ions could have distinct accumulation rates in a few organs of plants. Acknowledgements The authors would like to thank the Brazilian funding agencies CNPq, CAPES, FAPERJ and FAPESP for their financial support. References Anjos, R.M., Facure, A., Lima, E.L.N., Gomes, P.R.S., Santos, M.S., Brage, J.A.P., Okuno, E., Yoshimura, E.M., Umisedo, N.K., 2001. Radioactivity teaching: environmental consequences of the radiological accident in Goiania (Brazil). Am. J. Phys. 69, 377–381. Anjos, R.M., Umisedo, N.K., Facure, A., Yoshimura, E.M., Gomes, P.R.S., Okuno, E., 2002. Goiˆania: 12 years after the 137 Cs radiological accident. Radiat. Prot. Dosim. 101 (1–4), 201–204. Anjos, R.M., 2006. Radioecology teaching: response to a nuclear or radiological emergency. Eur. J. Phys. 27, 243–255. Antonopoulos-Domis, M., Clouvas, A., Gagianas, A., 1990. Compartment model for long-term contamination prediction in deciduous fruit trees after a nuclear accident. Health Phys. 58, 737–741. Antonopoulos-Domis, M., Clouvas, A., Gagianas, A., 1991. Radiocesium dynamics in fruit trees following the Chernobyl accident. Health Phys. 61, 837–842. Antonopoulos-Domis, M., Clouvas, A., Gagianas, A., 1996. Long term radiocesium contamination of fruit trees following the Chernobyl accident. Health Phys. 71, 910–914. Barci-Funel, G., Dalmasso, J., Barci, V.L., Ardisson, G., 1995. Study of the transfer of radionuclides in trees at a forest site. Sci. Total Environ. 173–174, 369–373. Bell, J.N.B., Shaw, G., 2005. Ecological lessons from the Chernobyl accident. Environ. Int. 31, 771–777. Carini, F., 2001. Radionuclide transfer from soil to fruit. J. Environ. Radioact. 52, 237–279. Carvalho, C., Anjos, R.M., Mosquera, B., Macario, K., Veiga, R., 2006. Radiocesium contamination behavior and its effect on potassium absorption in tropical or subtropical plants. J. Environ. Radioact. 86, 241–250.
Endres, L., Mercier, H., 2001. Influence of nitrogen forms on the growth and nitrogen metabolism of bromeliads. J. Plant Nutr. 24, 29–42. Facure, A., Umisedo, N.K., Okuno, E., Yoshimura, E.M., Gomes, P.R.S., Anjos, R.M., 2001. Remains of 137 Cs contamination in the city of Goiˆania, Brazil. Radiat. Prot. Dosim. 95, 165–171. Fogh, C.L., Anderson, K.G., 2001. Dynamic behaviour of 137 Cs contamination in tree of the Briansk region, Russia. Sci. Total Environ. 269, 105–115. Husted, S., Hebbern, C.A., Mattsson, M., Schjoerring, J.K., 2000. A critical experimental evaluation of methods for determination of NH4 + in plant tissue, xylem sap and apoplastic fluid. Physiol. Plant. 109, 167–179. IAEA, 1988. The Radiological Accident in Goiˆania. International Atomic Energy Agency, Vienna. Marschner, H., 1995. Mineral Nutrition of Higher Plants, 2nd ed. Academic Press Inc., San Diego, CA, USA. McGee, E.J., Synnott, H.J., Johanson, K.J., Fawaris, B.H., Nielsen, S.P., Horrill, A.D., Kennedy, V.H., Barbayiannis, N., Veresoglou, D.S., Dawson, D.E., Colgan, P.A., McGarry, A.T., 2000. Chernobyl fallout in a Swedish spruce forest ecosystem. J. Environ. Radioact. 48, 59–78. Mosquera, B., Carvalho, C., Veiga, R., Mangia, L., Anjos, R.M., 2006. 137 Cs distribution in tropical fruit trees after soil contamination. Environ. Exp. Bot. 55, 273– 281. Myttenaere, C., Schell, W.R., Thiry, Y., Sombre, L., Ronneau, C., van der Stegen de Schrieck, J., 1993. Modeling of Cs-137 cycling in forests: recent developments and research needed. Sci. Total Environ. 136, 77–91. Nisbet, A.F., Konoplev, A.V., Shaw, G., Lembrechts, J.F., Merckx, R., Smolders, E., Vandecasteele, C.M., Lonsjo, H., Carini, F., Burton, O., 1993. Application of fertilizers and ameliorants to reduce soil to plant transfer of radiocaesium and radiostrontium in the medium to long term—a summary. Sci. Total Environ. 137, 173– 182. Sanches, N., Anjos, R.M., Mosquera, B., in press. 40 K/137 Cs discrimination ratios to the aboveground organs of tropical plants. J. Environ. Radioact. doi:10.1016/ j.jenvrad.2008.01.003. Schell, W.R., Linkov, I., Myttenaere, C., Morel, B., 1996. A dynamic model for evaluating radionuclide distribution in forests from nuclear accidents. Health Phys. 70, 318–335. Shaw, G., Bell, J.N.B., 1991. Competitive effects of potassium and ammonium on caesium uptake kinetics in wheat. J. Environ. Radioact. 13, 283– 296. Siebrecht, S., Tischner, R., 1999. Changes in the xylem exudate composition of poplar Populus tremula × P. alba)-dependent on the nitrogen. J. Exp. Bot. 50, 1797– 1806. Weatherburn, M.W., 1967. Phenol-hypochlorite reaction for determination of ammonia. Anal. Chem. 39, 971–973. Willey, N., Tang, S., 2006. Some effects of nitrogen nutrition on caesium uptake and translocation by species in the Poaceae, Asteraceae and Caryophyllidae. Environ. Exp. Bot. 58, 114–122. Yoshida, S., Muramatsu, M., 1998. Concentrations of alkali and alkaline earth elements in mushrooms and plants collected in a Japanese pine forest, and their relationship with 137 Cs. J. Environ. Radioact. 41, 183–205. Zhu, Y.G., Shaw, G., Nisbet, A.F., Wilkins, B.T., 2002. Effect of external potassium supply and plant age on the uptake of radiocaesium (137 Cs) by broad bean (Vicia faba): interpretation of results from a large-scale hydroponic study. Environ. Exp. Bot. 47, 173–187.
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Environmental and Experimental Botany journal homepage: www.elsevier.com/locate/envexpbot
Caesium, potassium and ammonium distributions in different organs of tropical plants R.M. Anjos a,∗ , B. Mosquera a , N. Sanches a , C.A. Cambu´ı b , H. Mercier b a b
Instituto de F´ısica, Universidade Federal Fluminense, Av. Gal. Milton Tavares de Souza s/n, Gragoat´ a, Niter´ oi Cep 24210-346, RJ, Brazil Instituto de Biociˆencias, Universidade de S˜ ao Paulo, Caixa-Postal 11461, S˜ ao Paulo Cep 05422-970, SP, Brazil
a r t i c l e
i n f o
Article history: Received 29 February 2008 Accepted 2 April 2008 Keywords: Tropical woody fruit trees 137 Cs, 40 K and NH4 concentration ratios K/Cs and NH4 /Cs discrimination ratios
a b s t r a c t In the present work the distribution of ions in aboveground plant parts was studied in order to establish the suitability of using radiocaesium as a tracer for the plant absorption of nutrients, such as potassium (K+ ) and ammonium (NH4 + ). We present the results for the distributions of 137 Cs, 40 K and NH4 + from four tropical plant species: lemon (Citrus aurantifolia), orange (Citrus sinensis), guava (Psidium guajava) and chili pepper (Capsicum frutescens). Activity concentrations of 137 Cs and 40 K were measured by gamma spectrometry and concentrations of free NH4 + ions by a colorimetric method. Similarly to potassium and ammonium, caesium showed a high mobility within the plants, exhibiting the highest values of concentration in the growing parts of the tree (fruits, new leaves, twigs, and barks). A significant correlation between activity concentrations of 137 Cs and 40 K was observed in these tropical plants. The K/Cs discrimination ratios were approximately equal to unity in different compartments of each individual plant, suggesting that caesium could be a good tracer for 40 K in tropical woody fruit species. Despite the similarity observed for the behaviour of caesium and ammonium in the newly grown plant compartments, 137 Cs was not well correlated with NH4 + . Significant temporal changes in the NH4 + concentrations were observed during the development of fruits, while the 137 Cs activity concentration alterations were not of great importance, indicating, therefore, that Cs+ and free NH4 + ions could have distinct concentration ratios for each particular plant organ. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Radiocaesium has been introduced into the environment via nuclear weapons testing, nuclear waste disposal, nuclear or radiological accidents, and authorized discharges from nuclear installations. Caesium tends to be retained in the soil through fixation and adsorption to clay minerals and organic matter, persisting in the plant rooting zone for a long period due to its long physical half-life of 30.2 years. Cs+ can be readily absorbed by roots and can be translocated to the aboveground plant parts. For this reason, 137 Cs is a radioactive pollutant of great concern which can rapidly integrate into biological cycles and accumulate in terrestrial ecosystems. Since the removal of radiocaesium from contaminated forest or agricultural ecosystems is difficult, studies on the distribution and transfer of radiocaesium in these ecosystems are important in order to predict the future contamination of vegetal products (Zhu et al., 2002; Anjos, 2006).
∗ Corresponding author. Tel.: +55 21 26295770; fax: +55 21 26295887. E-mail address: [email protected] (R.M. Anjos). 0098-8472/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2008.04.001
Many studies on biogeochemical cycles of radiocaesium have been carried out, mostly in European forests, after the Chernobyl accident (Antonopoulos-Domis et al., 1990, 1991, 1996; Shaw and Bell, 1991; Myttenaere et al., 1993; Barci-Funel et al., 1995; Schell et al., 1996; Fogh and Anderson, 2001; Carini, 2001; Zhu et al., 2002; Bell and Shaw, 2005). Experimental results for temperate species have provided information for the development of models that explain radiocaesium transport through several compartments in a forest ecosystem. Different types of models were developed, as summarized by Schell et al. (1996). However, these models were unable to provide information significant enough to enable the prediction of radiocaesium behaviour in tropical ecosystems. For this reason, results for tropical plants are also of great concern, since these species do not necessarily follow the same pattern as temperate plants. Chemical behaviour of radiocaesium is expected to be similar to that of other monovalent inorganic cations, such as ammonium and the stable alkali (Na, K, Rb and Cs) elements (Shaw and Bell, 1991; Yoshida and Muramatsu, 1998; Zhu et al., 2002; Sanches et al., in press). Ammonium is a central intermediate in the nitrogen metabolism of plants. An important function of potassium is to adjust the osmotic pressure of cells, being, then, directly related
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to the plant growth. After completion of cell extension, for maintenance of the cell turgor K can be fairly readily replaced in the vacuoles by others solutes such as sodium (Marschner, 1995). However, information on the relationship between radionuclides and the major nutrient elements in forest or agricultural ecosystems is still limited. In addition, the quantitative importance of NH4 + transporting N from root to shoot and the capability of plants to store NH4 + in the leaves are still subjects of substantial controversy. Some of them are related to the use of inadequate analytical procedures for extraction and quantification of NH4 + in plants (Husted et al., 2000). The data for trace stable alkali elements such as Na, Rb and Cs in forest ecosystems are limited because of the lack of simple analytical methods. Even using the gamma spectrometry method, in order to perform the 40 K measurements, it is necessary to use at least 50 g dry weight of plant sample, due to its low activity concentration. Only exiguous amounts of sample (about 3 g) allow performing 137 Cs measurements from samples contaminated by radiocaesium. Therefore, the study of 137 Cs is potentially more useful than just for radioecology, since it is interesting to evaluate the potential role of 137 Cs as a tracer for plant nutrition. In order to deepen the understanding on the behaviour of monovalent cations, the purpose of this work is to examine simultaneously the activity concentrations of 137 Cs and 40 K, and the concentrations of NH4 + in several compartments of tropical plants, verifying the hypothesis that these elements have similar concentration ratios for each set of plant organs.
per samples were collected during the spring of 2005. Lemon and orange samples were collected during the spring–summer of 2006. At that time, the guava trees were 7 years old and around 5.0 m high. The chili pepper plants were 1-year-old and around 1.0 m high. The lemon and orange trees were 4 years old and around 2.5 m high. The ground was composed of a red soil containing construction remains with sandy–clay–loam texture. Among its main features, the total potassium content ranged from 0.5 to 2.9%, the sand content from 70 to 81%, the silt content from 13 to 20%, and the clay content from 5.9 to 6.3%. The pH value ranged from 5.8 to 6.3. Soil pH was measured in 1 M KCl in a solid–liquid ratio of 1:2.5 after 1 h equilibrium. The organic matter content ranged from 1.5 to 1.7%. The organic C content was measured with a wet combustion technique using K2 Cr2 O7 and H2 SO4 followed by CO2 analysis. The cation exchange capacity (CEC) ranged from 5.0 to 5.5 cmol kg−1 soil. CEC was measured using silver thiourea as the index cation in a 0.1-M NH4 OAc buffer. Soils from different depths were also collected. The sampling was performed in layers of 5 cm, reaching around 50 cm depth. Since for each plant species, three plants were sampled, then, 12 soil profiles were measured (one for each plant), each one formed by 10 sub-samples. Thus, for each plant species, 30 soil samples were collected. This procedure allowed obtaining a representative sampling of the caesium present within the rooting zone for each plant species studied. 2.2. Determination of 137 Cs and 40 K
2. Materials and methods 2.1. Sampling One of the worst radiological accidents ever reported occurred at the urban area of Goiania, Brazil, in 1987. A piece of a teletherapy machine, containing a 137 Cs source with an activity of 50.9 TBq, was removed from a derelict clinic and sold as scrap. Attracted by the blue glow emitted by the 137 CsCl salt, children and adults handled and distributed the material, in powder form, among relatives and friends, after destroying the shielding. As a consequence, around 1000 persons were irradiated, 250 contaminated either internally and/or externally and 49 were taken into the hospital. Of these, 28 had serious radiation burn problems, one had his forearm amputated and four died. The decontamination process required the demolition of seven residences and some other buildings, and the removal of the topsoil from large areas. In total, around 3500 m3 of radioactive waste were generated (IAEA, 1988; Anjos et al., 2001). Since 1999, a study on the environmental consequences of this radiological accident has been performed by our group (Facure et al., 2001; Anjos et al., 2002). In particular, at one of the sites involved in the accident and has then been inhabited since 1990. Thereafter, their residents have planted several tropical plants in the garden. Thus, as a development of previous works, measurements of the activity concentrations of 137 Cs and 40 K, and NH4 + concentrations from four tropical plant species were performed: lemon (Citrus aurantifolia), orange (Citrus sinensis), guava (Psidium guajava) and chili pepper (Capsicum frutescens). For each specie studied, samples were taken from three plants, separated into nine plant compartments (root, stem, bark, branch, twig, new leaf, old leaf, green fruit and old fruit), and for each compartment 10 sub-samples were analysed. Thus, for each plant compartment measured 30 samples were collected. In order to evaluate the 137 Cs activity concentration and NH4 + concentration as function of the fruit diameter, the green and mature fruits were also rearranged according to their sizes. Guava samples were collected during the spring of 2003. Chili pep-
Sample preparation and analysis were carried out at the Laboratory of Radioecology (LARA) of the Physics Institute of Federal Fluminense University. The root, main trunk, twig, leaf and fruit samples were first washed with distilled water, while the soil samples were sieved through a 1-mm screen. Then they were submitted to a drying process, at 110 ◦ C. Additionally, the vegetable samples were ground to powder. Afterwards, all samples were packed into cylindrical plastic containers, dry-weighed and sealed. The dry weights of each vegetable sample were about 30–50 g and about 200 g for soil samples. The samples were analysed by the conventional technique of gamma spectrometry, using one NaI detector. Radionuclide activities in plants were calculated from the net fullenergy peaks (661.7 keV gamma-ray line for 137 Cs and 1460.8 keV for 40 K) and the measured efficiency of the detector. The 40 K calibration was performed using a reference material (RGK-1) obtained from the International Atomic Energy Agency. Standards of radiocaesium were made by mixing uncontaminated sawdust with the liquid 137 Cs solution prepared by the Brazilian National Nuclear Energy Commission (CNEN). Their results were inter-compared with two laboratories in Brazil. Technical details of sample preparation and analysis can be obtained from Mosquera et al. (2006) and Carvalho et al. (2006). Hence, using the dry weight of the samples, their respective activity concentrations of 137 Cs and 40 K could be expressed in activity per unit mass (Bq kg−1 ). The uncertainties of the specific 137 Cs activity from each individual measurement of plant samples ranged from 5 to 7% and around 10% for the specific 40 K activity. The detection limit was estimated to be 7.0 Bq kg−1 for 137 Cs and 75 Bq kg−1 for 40 K. 2.3. Determination of NH4 + Concentrations of free NH4 + ions were estimated using the phenol-hypochlorite colorimetric method (Weatherburn, 1967). Sample preparation and NH4 + analysis were carried out at the Laboratory of Plant Physiology of the Biosciences Institute of Sao Paulo University. Frozen samples (1 g fresh mass) were ground in liquid
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Table 1 Mean values of activity concentrations (Bq kg−1 dry weight) of 137 Cs and 40 K and NH4 + concentrations and their stem-to-compartment j concentration ratios (CRs) of lemon trees contaminated by caesium j
Compartmenta
Concentrations 137
Lemon tree (Citrus aurantifolia) 1 Main root 2 Main stem 3 Bark 4 Branch 5 Twig 6 Old leaf 7 New leaf 8 Mature fruitb 9 Green fruit
Cs (Bq kg
−1
)
97 (10) 90 (7) 245 (31) 183 (20) 281 (17) 420 (26) 581 (47) 301 (24) 395 (22)
Stem-to-compartment j concentration ratios 40
K (Bq kg
−1
)
236 (20) 230 (20) 600 (48) 432 (37) 723 (54) 1216 (91) 1249 (106) 759 (76) 1049 (55)
+
NH4 (ppm)
CR[Cs]
CR[K]
CR[NH4 ]
1.5 (0.2) 1.5 (0.1) 2.6 (0.1) 1.7 (0.1) 1.6 (0.1) 3.4 (0.2) 2.4 (0.2) 1.9 (0.1) 3.6 (0.2)
1.1 (0.1) 1.0 (0.1) 2.7 (0.4) 2.0 (0.3) 3.1 (0.3) 4.7 (0.5) 6.5 (0.7) 3.3 (0.4) 4.4 (0.4)
1.0 (0.1) 1.0 (0.1) 2.6 (0.3) 1.9 (0.2) 3.1 (0.4) 5.3 (0.6) 5.4 (0.7) 3.3 (0.4) 4.6 (0.6)
1.0 (0.2) 1.0 (0.1) 1.7 (0.2) 1.1 (0.1) 1.1 (0.1) 2.3 (0.2) 1.6 (0.2) 1.3 (0.1) 2.4 (0.2)
Values in parentheses represent one standard deviation from the mean from 30 measurements. a The main stem samples had diameters of about 7 cm; the branches between 2 and 3 cm; the twigs between 0.5 and 1 cm; 6 cm for mature fruits and between 1 and 4 cm for green fruits. b Average water content of 86%.
nitrogen to a fine powder and homogenized with 4 mL of distilled water. The homogenate was centrifuged and the supernatant collected. 300 L aliquots were used in ammonium quantification. Concentrations of free NH4 + ion were expressed in parts per million (ppm). The colorimetric analysis of NH4 + is based on the classical Berthelot reactions: NH4 + is converted to monochloramine, which subsequently reacts with a phenolic compound having an unsubstituted paraposition. All reactions occur under alkaline conditions in the presence of nitroprusside as a catalyst. Ammonia, hypochlorite and the phenolic compound react and an intensely blue-green chromophore (an indophenol complex) is formed as reaction product, quantified by colorimetry at 640–655 nm (Husted et al., 2000; Endres and Mercier, 2001). The uncertainties of the NH4 + concentrations ranged from 3 to 6%. The detection limit was estimated to be 25 M.
2.4. Soil to plant transfer factor Transfer factor (TF) has been used for many years to predict concentrations of radionuclides that could be expected in food crops after accidental releases of radionuclides into the environment. TF is expressed as the ratio between the activity concentration of the radionuclide in the dried edible part of the plant and that in the oven dried soil (both in Bq kg−1 ). TF values are very useful to help estimate radiation doses to people, which might result from
contaminated food supplies after a release of radionuclides to the environment. 2.5. Stem-to-compartment concentration ratio The capability of plants to store ions can be estimated through the comparison between their respective concentrations in different parts or organs of the plant. In this way, it is possible to compare the concentration of a given radionuclide or stable element between younger and older parts of the plant, using stemto-compartment concentration ratios, defined as: CRj =
Cj
(1)
Cstem
where Cj is the concentration of the element in a given plant compartment j (main root, main stem, bark, branch, twig, new leaf, old leaf, green fruit or mature fruit). For a radionuclide, Cj is given in activity per unit mass (Bq kg−1 ). If the element is stable, Cj can be expressed, for example, in parts per million (ppm). Cstem represents the concentration of the element in the main stem. According to Tables 1–4, the values of the NH4 + concentrations are nearly identical for the main root and the main stem, respectively, which constitute the oldest parts of the plant. The same occurs for the activity concentrations of 137 Cs and 40 K, respectively. Thus, in this work, the main stem was assumed to be the oldest compartment of the plant in the evaluation of the concentra-
Table 2 Mean values of activity concentrations (Bq kg−1 dry weight) of 137 Cs and 40 K and NH4 + concentrations and their stem-to-compartment j concentration ratios (CRs) of orange trees contaminated by caesium j
Compartmenta
Concentrations 137
Orange tree (Citrus sinensis) 1 Main root 2 Main stem 3 Bark 4 Branch 5 Twig 6 Old leaf 7 New leaf 8 Mature fruitb 9 Green fruit
Cs (Bq kg
169 (12) 174 (10) 774 (68) 289 (25) 691 (56) 1178 (88) 1290 (109) 600 (46) 658 (64)
−1
)
Stem-to-compartment j concentration ratios 40
K (Bq kg
100 (10) 102 (9) 400 (32) 177 (23) 331 (26) 675 (60) 711 (69) 336 (36) 353 (38)
−1
)
+
NH4 (ppm)
CR[Cs]
CR[K]
CR[NH4 ]
1.5 (0.1) 1.5 (0.1) 2.5 (0.3) 1.8 (0.1) 1.6 (0.1) 3.2 (0.3) 4.7 (0.2) 0.70 (0.07) 1.6 (0.3)
1.0 (0.1) 1.0 (0.1) 4.4 (0.5) 1.7 (0.2) 4.0 (0.4) 6.8 (0.6) 7.4 (0.8) 3.4 (0.3) 3.8 (0.4)
1.0 (0.1) 1.0 (0.1) 3.9 (0.5) 1.7 (0.3) 3.2 (0.4) 6.6 (0.8) 7.0 (0.9) 3.3 (0.5) 3.5 (0.5)
1.0 (0.1) 1.0 (0.1) 1.7 (0.2) 1.2 (0.1) 1.1 (0.1) 2.1 (0.2) 3.1 (0.2) 0.47 (0.07) 1.1 (0.2)
Values in parentheses represent one standard deviation from the mean from 30 measurements. a The main stem samples had diameters of about 8 cm; the branches between 2 and 3 cm; the twigs between 0.5 and 1 cm; green fruits between 3 and 5 cm and; mature fruits between 7 and 8 cm. b Average water content of 84%.
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Table 3 Mean values of activity concentrations (Bq kg−1 dry weight) of 137 Cs and 40 K and NH4 + concentrations and their stem-to-compartment j concentration ratios (CRs) of guava trees contaminated by caesium j
Compartmenta
Concentrations 137
Guava tree (Psidium guajava) 1 Main root 2 Main stem 3 Bark 4 Branch 5 Twig 6 Old leaf 7 New leaf 8 Mature fruitb 9 Green fruit
Cs (Bq kg
−1
)
860 (70) 835 (65) 2425 (230) 1275 (125) 1216 (102) 1493 (104) 2144 (262) 1933 (162) 2157 (265)
Stem-to-compartment j concentration ratios 40
K (Bq kg
−1
)
195 (17) 174 (16) 436 (28) 244 (33) 277 (19) 265 (19) 394 (36) 413 (33) 466 (52)
+
NH4 (ppm)
CR[Cs]
CR[K]
CR[NH4 ]
15 (1) 13 (1) 23 (2) 15 (1) 14 (1) 16 (1) 12 (1) 1.7 (0.1) 25 (7)
1.0 (0.1) 1.0 (0.1) 2.9 (0.4) 1.5 (0.2) 1.5 (0.2) 1.7 (0.2) 2.6 (0.4) 2.3 (0.3) 2.6 (0.4)
1.1 (0.1) 1.0 (0.1) 2.5 (0.3) 1.4 (0.2) 1.6 (0.2) 1.5 (0.2) 2.3 (0.3) 2.4 (0.3) 2.7 (0.4)
1.1 (0.1) 1.0 (0.1) 1.8 (0.2) 1.2 (0.1) 1.1 (0.1) 1.2 (0.1) 0.9 (0.1) 0.13 (0.01) 1.9 (0.6)
Values in parentheses represent one standard deviation from the mean from 30 measurements. a The main stem samples had diameters of about 15 cm; the branches between 3 and 5 cm; the twigs between 0.5 and 1 cm; green fruits between 3 and 5 cm and; mature fruits between 7 and 8 cm. b Average water content of 82%.
tion ratios of caesium (CR[Cs]j ), potassium (CR[K]j ) and ammonium (CR[NH4 ]j ). 3. Results and discussion 3.1. 137 Cs, 40 K and NH4 + distributions in different plant compartments Concentrations of NH4 + and activity concentrations of 137 Cs and from root to shoot for four woody fruit species were evaluated as well as the capability of such tropical plants to store these ions. Analyses of these values reveal the distributions of nutrients among the different parts of each plant. Given the homogeneity of the results, two clusters were identified of younger (fruits, newly leaves, twigs, branches and bark) and older parts (main stems and main roots) and their respective average and standard deviation are reported in Tables 1–4. These data refers to the mean of 30 samples from each plant compartment and allow correlating potassium, ammonium and caesium distributions to justify these remarks statistically. The results suggest that tissues with low Cs activity concentration (on a dry weight basis) had low K activity concentration as well, in such a way that both 137 Cs and 40 K had simultaneously higher activity concentrations in the new parts than in the older parts. In addition, 137 Cs activity concentration in the different organs of these trees decreased according to foliage > fruits > twigs > branches > main stem. However, the caesium concentration ratio in a specific compartment depended on the plant species. For instance, in the orange, lemon and chili pep40 K
per trees (see Tables 1, 2 and 4, respectively) the 137 Cs concentration ratios (CR[Cs]) in the foliage were 5–7 times higher than in the main stem of the plants, while in the leaves of the guava trees (Table 3) it was only 2.5 times higher. Different behaviours were also observed in caesium accumulation in the barks and fruits of these trees. For instance, in guava (Table 3) bark > foliage > fruits, while in orange (Table 2) foliage > bark > fruits. These results suggest that the 137 Cs distribution can show small variations in different species of tropical fruit trees. Barci-Funel et al. (1995) analysed three conifer trees from different species: a pine tree (Pinus silvestris), a spruce (Pinus picea) and a larch (Larix). The 137 Cs activity concentration in the extremities of these trees (needles, shoots apex, twigs, bark) was found to be larger than those measured in the inner cylinder of the main branches, indicating an accumulation towards the ends, where the exchanges with the ambient air are stronger (driving force of transpiration). The concentrations of 137 Cs in the barks were found to be one or two orders of magnitude higher than those of the inner parts of the plant, whereas the 137 Cs concentrations in the needle and twig were from 3 to 17 times higher. In another study on pine and spruce trees, McGee et al. (2000) found similar results for the 137 Cs concentrations in the needle and twig, but the bark activities were only five times higher. Fogh and Anderson (2001) obtained similar results for pine, birch and oak trees. They found that growing parts (leaves, needles, twigs and barks) had specific 137 Cs activities up to one order of magnitude higher than those of the older parts of the plant. Nevertheless, from Tables 1–4, our present results show that for tropical fruit trees, the extremity part activities were only around two to seven times higher than those of
Table 4 Mean values of activity concentrations (Bq kg−1 dry weight) of 137 Cs and 40 K and NH4 + concentrations and their stem-to-compartment j concentration ratios (CRs) of chili pepper trees contaminated by caesium j
Compartmenta
Concentrations 137
Chili pepper tree (Capsicum frutescens) 1 Main root 2 Main stem 3 Branch 4 Twig 5 Leaf 6 Mature fruitb (red) 7 Green fruitb
Cs (Bq kg−1 )
961 (98) 955 (100) 950 (102) 2158 (211) 5832 (518) 2099 (198) 2548 (239)
Stem-to-compartment j concentration ratios 40
K (Bq kg−1 )
448 (51) 450 (40) 495 (62) 1075 (143) 2200 (221) 1001 (184) 1328 (157)
NH4 + (ppm)
CR[Cs]
CR[K]
CR[NH4 ]
5.5 (0.2) 5.6 (0.2) 5.4 (0.6) 6.2 (0.6) 5.4 (1.8) 3.1 (0.6) 12 (5)
1.0 (0.2) 1.0 (0.2) 1.0 (0.2) 2.3 (0.3) 6.1 (0.8) 2.2 (0.3) 2.7 (0.4)
1.0 (0.2) 1.0 (0.2) 1.1 (0.2) 2.4 (0.3) 4.9 (0.6) 2.2 (0.3) 2.9 (0.4)
1.0 (0.1) 1.0 (0.1) 1.0 (0.1) 1.1 (0.1) 1.0 (0.3) 0.6 (0.1) 2.1 (0.9)
Values in parentheses represent one standard deviation from the mean from 30 measurements. a The main stem samples had diameters of about 3 cm; the branches between 1 and 2 cm; the twigs between 0.2 and 0.5 cm; green fruits between 1.8 and 2.5 cm and; above 2.8 cm for red fruits. b Average water content of 85%.
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the inner parts of the plant, suggesting that the 137 Cs distribution in tropical fruit trees shows small alterations when compared with conifers or temperate forest trees. Similarly to caesium, potassium showed a high mobility within the plants, exhibiting the highest values of activity concentration in the growing parts of the tree (fruits, new leaves, twigs, and barks). In addition, the distributions of caesium and potassium have been shown to be weakly dependent on the age of specific plant parts. The values of the 137 Cs and 40 K concentration ratios did not exhibit any statically significant difference between the newly grown plant parts, such as new leaves and green fruits, and the respective older parts, such as old leaves and mature fruits (Tables 1–4). For instance, Table 2 shows that CR[Cs] and CR[K] for new leaves from orange trees were 7.4 ± 0.8 and 7.0 ± 0.9, respectively, while their old leaves had CR[Cs] and CR[K] values equal to 6.8 ± 0.6 and 6.6 ± 0.8. Green fruits from orange trees had CR[Cs] and CR[K] values equal to 3.8 ± 0.4 and 3.5 ± 0.5, respectively, while their mature fruits had CR[Cs] and CR[K] values equal to 3.4 ± 0.3 and 3.3 ± 0.5, respectively. Similar effects can be observed from leaves and fruits from lemon, guava and chili pepper trees, as shown in Tables 1, 3 and 4, respectively. These results suggest that 137 Cs and 40 K have similar behaviour in the studied tropical plants and thus, 137 Cs could trace the transport or accumulation of potassium in such plants. This evidence is in agreement with Zhu et al. (2002), according to whom potassium, used in soils in relatively large quantities as fertilizer, is believed to be effective in inhibiting the uptake of radiocaesium, due to the ability of their ions to block radiocaesium uptake by plant roots. K is not the only soil-based chemical treatment that increases the plant growth rate and also reduces radiocaesium uptake by plants. In contrast, there are some fertilizing treatments that produce the opposite effect. The application of acid or ammonium-based fertilizers, for instance, has been found to increase the availability and hence the transfer of radiocaesium to plants, showing that the presence of NH4 + ions in the soil could affect caesium transfer from soil to plants (Shaw and Bell, 1991; Nisbet et al., 1993; Willey and Tang, 2006). NH4 + is taken up by roots and can be produced in several biochemical processes including nitrate (NO3 − ) assimilation, photorespiration, phenyl propanoid metabolism, degradation of transport amides or protein catabolism. It is generally assumed that most of the NH4 + absorbed by or generated in roots becomes assimilated in the roots and that only minor amounts are translocated to the shoot (Siebrecht and Tischner, 1999; Husted et al., 2000). Similarly, it is generally believed that the NH4 + generated in the leaves is rapidly and efficiently assimilated and that NH4 + concentration in the leaf tissue, therefore, always remains very low. However, several experiments have shown the occurrence of high NH4 + concentrations in plant tissues (Husted et al., 2000). Thus, it is interesting to investigate the existence of a correlation between NH4 + and 137 Cs concentrations in aboveground plant parts and so to evaluate whether caesium would be an efficient element to trace ammonium. From Tables 1–4, it is possible to observe that, just like Cs+ , free NH4 + ions were possibly absorbed and translocated to the aboveground plant parts, exhibiting the highest values of concentration in their growing parts. This result discloses that Cs+ and free NH4 + ions have similar behaviour and thus, 137 Cs could also trace the transport or accumulation of ammonium ions in the plants. Moreover, the NH4 + concentration ratios (CR[NH4 ]) for these plant compartments exhibited a considerable dependence on the plant species. For instance, for the lemon (Table 1) and orange (Table 2) trees, the NH4 + probably absorbed in the roots and presented in the leaves was about twice higher than that accumulated in the guava or chili pepper trees (Tables 3 and 4, respectively). On the other hand, lemon tree fruits showed twice higher NH4 + accumulation than the orange tree fruits. According to Husted et al. (2000), this behaviour
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could be explained by the fact that plant species exhibit considerable difference in their capability to reduce NO3 − and to assimilate NH4 + in the roots. Thus, plants with limited assimilatory capacity may translocate NH4 + to the shoot and may differ both in their capacity to assimilate NH4 + in the leaf tissue and its tolerance to NH4 + accumulation in the leaves. In contrast to 137 Cs and 40 K behaviours, NH4 + concentration ratios have been shown to be significantly dependent on the age of specific plant parts. Fig. 1 shows the average values of the 137 Cs and NH4 + concentration ratios during the maturing process of lemon, orange, guava and chili pepper fruits. In all these cases there was a higher NH4 + concentration in the fruit formation phase, followed by a decrease in its level as the fruits became mature, suggesting that ammonium could have been assimilated in amino acids during the maturation process. A similar process could also occur in the leaves. Thus, Tables 1–4 and Fig. 1 suggest that the assumption of a physiological equivalence between the Cs+ and NH4 + ions does not hold, since they had distinct concentration ratios in different organs of trees. This result is reinforced by comparison between Cs and K concentrations ratios and Cs and NH4 + concentration ratios. 3.2. K/Cs discrimination ratio Although the caesium concentration ratios depend on the plant species, Tables 1–4 indicate that both caesium and potassium have similar concentration ratios in different organs of same trees, since the CR[Cs] and CR[K] values are practically identical for each particular plant compartment. This result can be better observed in Fig. 2, which presents the K/Cs discrimination ratios for each compartment j defined as: (K/Cs)j =
CR[K]j CR[Cs]j
(2)
where CR[K]j and CR[Cs]j are the 40 K and 137 Cs concentration ratios, respectively, in a given plant compartment j (main root, main stem, bark, branch, twig, new leaf, old leaf, green fruit or mature fruit). This figure shows that the K/Cs values were approximately equal to unity for the different compartments of each individual plant studied in this work. Thus, the relationship between 40 K and 137 Cs in the plant suggests that these two elements behaved alike. In particular, previous measurements performed in chili pepper trees showed that the green fruits had higher 137 Cs concentration than the red ones, but, unlike caesium, the 40 K concentration increased as the pepper fruits turned red (mature). However, if 137 Cs was not available in the soil, the green fruits had higher 40 K concentration than the red ones (Carvalho et al., 2006). Such behaviour suggested the occurrence of a direct competition between Cs+ and K+ ions during the accumulation process throughout the plant, indicating that the two elements have a common accumulation mechanism. In this work the competition between Cs+ and K+ ions in chili pepper tree was investigated. Our measurements showed that this inversion between their concentrations only occurred for the fruit compartment. In the other compartments, the K/Cs discrimination ratio values were approximately equal to unity. On the other hand, the comparison between values of 137 Cs concentrations from different organs of the chili peppers trees contaminated by caesium with the respective 40 K concentrations from the uncontaminated chili pepper trees produces K/Cs approximately equal to unity for all compartments of the chili pepper trees. This effect is shown in Table 4 and Fig. 2d, since the 40 K concentrations from uncontaminated chili peppers trees were used in their elaboration.
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Fig. 1. Stem-to-fruit concentration ratios for caesium and ammonium as function of the fruit diameter for (a) lemon; (b) orange; (c) guava; (d) chili pepper trees.
These findings suggested, therefore, the possibility of using the caesium to predict the behaviour of monovalent cations in tropical woody fruit plants. In addition, some plants are able to replace high proportion of potassium by sodium without an effect on growth and others can have an additional growth stimulation which cannot be achieved by increasing the K content of the plants. Growth stimulation by Na is caused mainly by its effect on cell expansion and on the water balance of plants (Marschner, 1995). Such observations would indicate that Na+ , K+ and Cs+ could compete during the uptake by plants. Then, this result suggests that caesium could also be used to trace the Na nutrition of plants. An analysis about the correlations between Na and Cs behaviour in tropical plants will be performed by our group. 3.3. NH4 + /Cs discrimination ratio Analogously to Eq. (2), the NH4 + /Cs discrimination ratios for each compartment j are defined as: (NH4 /Cs)j =
CR[NH4 ]j CR[Cs]j
(3)
where CR[NH4 ]j and CR[Cs]j are the NH4 + and 137 Cs concentration ratios, respectively. From Fig. 2, the data showed that the ratio NH4 /Cs was <1 for the youngest compartments of each individual plant studied in this work, confirming the result observed in Fig. 1 where the time dependence of CR[Cs] and CR[NH4 ] values in the fruits of lemon, orange, guava and chili pepper trees did not
provide evidence of a correlation between ammonium and caesium behaviour. Thus, these results suggest an unclear relationship between 137 Cs and free NH4 + in the different plant organs, indicating that caesium would not be as good a tracer for NH4 + as it was for K for these plant species. However, it is important to remember that the phenol-hypochlorite colorimetric method allows evaluating only of the concentration of free NH4 + ions present in the plant tissues. Thus, this observation leads to the need for further evaluation of the effects of nitrogen metabolism on caesium uptake, total amounts of Cs take up and root–shoot ratios of Cs, in order to verify the existence of a correlation between caesium and total nitrogen (the sum of free ammonium, amino acids, proteins and other Ncompounds) behaviour. Furthermore, K is a nutrient that remains in ionic form and it is never assimilated in any organic compound (the same as Cs behaviour). In contrast, plant cells avoid NH4 + toxicity by converting NH4 + generated from root-to-shoot transport, or from other metabolic processes such as photorespiration in the leaves, into amino acids.
3.4. Radioecology and soil to plant transfer factor for 137 Cs Since some tropical plants had been cultivated in the garden of one of the sites where the Goiania radiological accident occurred, it is important to evaluate if there are any potential radiological implications on the consumption of their contaminated fruits. Table 5 shows the mean values of activity concentrations of the 137 Cs available in soil within the rooting zone for each plant and its
R.M. Anjos et al. / Environmental and Experimental Botany 65 (2009) 111–118
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Fig. 2. K/Cs and NH4 /Cs discrimination ratios as a function of compartment j for (a) lemon; (b) orange; (c) guava; (d) chili pepper trees. j = 1 represents the main root; j = 2 the main stem; j = 3 bark; j = 4 branch; j = 5 twig; j = 6 old leaf; j = 7 new leaf; j = 8 mature fruit and; j = 9 green fruit. Table 5 Mean values of 137 Cs activity concentrations (Bq kg−1 dry weight) in soil and mature fruits, and their soil-to-fruit transfer factors (TFs) from the lemon, orange, guava and chili pepper trees Mature fruit a
Lemon Orangeb Guavac Chili pepperd
137
Cs activity concentration in soil (kBq kg−1 )
14 (5) 38 (10) 88 (25) 40 (6)
137
Cs activity concentration in fruit (Bq kg−1 )
301 (24) 600 (46) 1933 (162) 2099 (198)
TF 0.022 (0.008) 0.016 (0.004) 0.022 (0.007) 0.052 (0.008)
Values in parentheses represent one standard deviation from the mean from 30 measurements. Average water content of a 86%; b 84%; c 82%; d 85%.
respective soil to fruit transfer factor. The results indicate that the soil contamination was not the same for all the plants, showing a higher transfer of caesium to chili pepper when compared to lemon, orange and guava, since the soil-to-plant transfer factors for 137 Cs were about 5% for chili pepper and 2% for lemon, orange and guava. However, these TF values are in agreement with the range of soil to fruit transfer factors for tropical or temperate fruits observed by Carini (2001). In terms of radioecology, the dose criteria for the implementation of restrictions on the consumption of radionuclide contaminated food products and drinking water refer to estimated doses of internal radiation by radionuclides in food and water consumed within a year (Anjos, 2006). For instance, the limits in foods of general consumption, milk and infant food, and drinking water for 134 Cs, 137 Cs or 103 Ru are set at 1000 Bq kg−1 (IAEA, 1988). Since the limits for fruits are referred to fresh weight, while the fruit data
reported in Tables 1–5 are related to dry weight, before to apply this criterion of restrictions on the consumption the 137 Cs activity concentrations in fruits have been converted into fresh weight according to the water content. Assuming the water content of about 85% in these tropical fruits, the 137 Cs activity concentrations in fresh fruits are far below the IAEA recommendation and so these fruits could be consumed. 4. Conclusions The comparative behaviour of Cs+ , K+ and NH4 + ions in tropical woody fruit trees was evaluated through the use of their concentration ratios in different parts or compartments of the plants. Despite the 137 Cs, 40 K and NH4 + concentration ratios exhibiting considerable dependence on plant species, our results disclosed that Cs+ , K+ and free NH4 + ions have similar behaviour, since the highest
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values of concentrations of these ions were observed in the growing parts of the plants, such as fruits, new leaves, twigs and barks. Thus, 137 Cs could be used to approximate the transport or accumulation of ammonium and potassium ions in the plants. However, the results indicated Cs+ was a better tracer for K+ than it was for NH4 + . A significant correlation between 137 Cs and 40 K concentrations was observed in these tropical plants. The K/Cs discrimination ratios were approximately equal to unity in different compartments of each individual plant. Significant temporal changes in the NH4 + concentrations were observed during the development of fruits, while the 137 Cs concentration alterations were not of great importance, disclosing, therefore, that Cs+ and free NH4 + ions could have distinct accumulation rates in a few organs of plants. Acknowledgements The authors would like to thank the Brazilian funding agencies CNPq, CAPES, FAPERJ and FAPESP for their financial support. References Anjos, R.M., Facure, A., Lima, E.L.N., Gomes, P.R.S., Santos, M.S., Brage, J.A.P., Okuno, E., Yoshimura, E.M., Umisedo, N.K., 2001. Radioactivity teaching: environmental consequences of the radiological accident in Goiania (Brazil). Am. J. Phys. 69, 377–381. Anjos, R.M., Umisedo, N.K., Facure, A., Yoshimura, E.M., Gomes, P.R.S., Okuno, E., 2002. Goiˆania: 12 years after the 137 Cs radiological accident. Radiat. Prot. Dosim. 101 (1–4), 201–204. Anjos, R.M., 2006. Radioecology teaching: response to a nuclear or radiological emergency. Eur. J. Phys. 27, 243–255. Antonopoulos-Domis, M., Clouvas, A., Gagianas, A., 1990. Compartment model for long-term contamination prediction in deciduous fruit trees after a nuclear accident. Health Phys. 58, 737–741. Antonopoulos-Domis, M., Clouvas, A., Gagianas, A., 1991. Radiocesium dynamics in fruit trees following the Chernobyl accident. Health Phys. 61, 837–842. Antonopoulos-Domis, M., Clouvas, A., Gagianas, A., 1996. Long term radiocesium contamination of fruit trees following the Chernobyl accident. Health Phys. 71, 910–914. Barci-Funel, G., Dalmasso, J., Barci, V.L., Ardisson, G., 1995. Study of the transfer of radionuclides in trees at a forest site. Sci. Total Environ. 173–174, 369–373. Bell, J.N.B., Shaw, G., 2005. Ecological lessons from the Chernobyl accident. Environ. Int. 31, 771–777. Carini, F., 2001. Radionuclide transfer from soil to fruit. J. Environ. Radioact. 52, 237–279. Carvalho, C., Anjos, R.M., Mosquera, B., Macario, K., Veiga, R., 2006. Radiocesium contamination behavior and its effect on potassium absorption in tropical or subtropical plants. J. Environ. Radioact. 86, 241–250.
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