Entre la Oralidad y la Escritura: El Cuerpo.

Regarding interspecies variations in uptake of rare earths by plants it is self-evident that concen- trations of rare earth elements also differ within plant species. Indeed, several studies confirmed that rare earths contents in plants may vary extremely. On the one hand, mainly low contents have been reported in different parts of plants (Ichihashi et al.,1992), (Krafka, 1999) that is in the ng/g range or even smaller; on the other hand, comparatively high values were found in some plant species. Nevertheless, concentrations in plants are usually quite low (Tyler,2004) and plant soil ratios (transfer factors) of 0.04 - 0.05 have been described, while Krafka (1999) reported even lower transfer factors of 0.02 to 0.03.

In ordinary plants lanthanum concentrations determined were in the range of 3 to 5000 ng/g. With rare earth contents of less than 10 ng/g as presented in Table11.14(Wyttenbach et al.,1994), spruce needles were among the lowest values measured in plant leaves.Krafka(1999) determined similar values. Particularly low values were also reported in grains (Sun et al., 1994), (Li et al.,

1998a) and vegetables (Laul et al.,1979), (Bibak et al.,1999) with contents ranging from 104−to 102− µg/g. Mean concentrations are generally lower in plants compared to those in soil. A comparison between rare earth contents in wheat plants and in soil is shown in Table11.15. In wheat, low values of rare earth element contents were determined in both parts (root and tops). For lanthanum, a light rare earth element, values of 0.009 mg/kg in root and 0.006 mg/kg in stem and leaves were measured. Even lower values ranging from undetectable in roots to 0.002 mg/kg in the plant tops were found for yttrium (Zhimang et al.,2001).

Place La Ce Nd Sm Eu Tb Yb Lu Other RE I 43.4 35.6 23.9 5.2 1.26 1.35 3.97 0.417 0.477 II 54.0 56.4 43.5 10.1 2.33 2.70 7.06 0.838 0.840 III 96.1 65.3 58.9 14.5 3.14 3.30 6.82 0.857 1.057 IV 73.1 62.7 58.3 15.1 3.42 3.77 9.30 1.138 1.129 V 108.2 71.6 64.7 15.4 3.51 3.96 9.82 1.193 1.248 VI 94.3 67.3 68.0 18.1 4.11 4.26 8.50 1.062 1.248 MEAN 78.2 60.2 52.9 13.1 2.96 3.22 7.58 0.917 1.000

Table 11.14: Concentrations of rare earth elements (ng/g)in needles (Wyttenbach et al.,1996).

However, a few plants may accumulate large amounts of rare earths. With rare earth concentra- tions in the upperµg/g range they are considered as hyperaccumulators. Among these are certain ferns (Pteridophyta) (Ichihashi et al., 1992), (Ozaki et al., 2000), (Wei et al., 2001), pokeweeds (Phytolacca) (Ichihashi et al., 1992), hickory trees (Carya) (Robinson et al., 1958), (Robinson et al., 1960) or plants from the spurge family (Euphorbiaceae). Furthermore, higher values up to 106 ng/g were also determined in plants grown on highly mineralized soils (Miekeley et al.,

1994). In Japan, contents of about 100µg/g of lanthanum were reported in some ferns (Koyoma et al.,1987) that is about 104times more than those determined in spruce needles. Similar results were obtained for Dicranopteris linearis, a fern species, containing very high concentrations of rare earths (134 - 1754 µg/g in root, 107 - 632 µg/g in stem, 51 - 102 µg/g in petiole and 977 - 2272µg/g in lamina) (Wei et al., 2001), whereas lanthanum concentrations of up to 1 mg/g have

11.3 Rare Earths Uptake by Plants and Bioavailability to Plants

Soil Root Shoot

range mean range mean range mean

La 13.25 - 41.05 24.20 0.73 - 8.96 4.30 0.053 - 0.22 0.11 Ce 29.45 - 87.54 50.98 4.96 - 16.5 11.29 0.87 - 0.51 0.19

Pr 3.06 - 9.89 5.62 0.17 - 1.81 1.03 0.01 - 0.06 0.02

Nd 10.75 - 36.15 19.78 0.58 - 5.57 3.53 0.03 - 0.22 0.06

Table 11.15: Concentrations of rare earth elements (µg/g) in Chinese soils and shoots and roots of wheat (Triticum aestivum L.) from the same site (Shan et al.,2003b).

been reported earlier in ferns (Ichihashi et al., 1992). Thus ferns strongly absorb rare earth ele- ments from the soil even if they are present at relatively low concentrations (Wei et al.,2001). Yet a certain soil-plant barrier exists as the absorption weakens when rare earth concentrations become higher. On the one hand, this soil-plant barrier hampers the absorption of toxic metal ions, hence constitutes one of the main resistance mechanisms observed in plants, while on the other hand, it allows the maintenance of rare earths concentration in ferns within a certain range.

Due to the large variations of rare earths in plants even among individual plants of the same species, it is probably more reasonable to define a "normal range" comprising extreme values rather than a "normal concentration".

However, the determination of rare earths in plants entails some difficulties. Due to their extremely small concentration ratio plant/soil that is very small concentrations in plants a very sensitive, analytical technique is required (Henkelmann et al., 1997). Furthermore, already little contamination by adhering soil will fudge the results and concentrations measured will thus be higher than the endogenous plant concentration. For the complete removal, a special treatment using suitable solvents, such as CHCl3, is required since water cannot sufficiently dissolve the

epicuticular wax covering the leave surface under which contaminations are trapped. The mass of the terrigenous material may amount up to>100 mg/g plant. Yet the error committed by includ- ing surface contamination in the analysis of plant samples depends on the sample, the sampling situation as well as the element determined and thus has to be considered for each analysis sepa- rately. Most plant analysis are done for physiological purposes and completely wrong conclusions may be drawn if nonphysiological exogenous contribution is included. Careful interpretation of published data is therefore recommended as several investigations lack proper removal of contam- ination, and results from afflicted studies cannot be used in the discussion of plant uptake or of other plant physiological processes (Wyttenbach et al.,1998b), (Wyttenbach and Tobler,1998).

Two examples are given in order to emphasize the importance of careful removal of any aerosol or soil particle from the plant surface prior to analysis. A contamination of 5 mg soil per g cheat grass was determined byLaul and Weimer(1982). Compared with the results, it was concluded that rare earth concentrations measured in grass were almost totally due to contamination by soil, thus making these values completely invalid for plant physiological analysis. In contrast to that, all external material was excluded by washing the fresh samples with toluene/tetrahyrofuran (1:1) before the contents of rare earths in Norway spruce needles (Picea abies) were determined. Con- centration values found were therefore much lower than those reported in literature as shown in Table11.16. Lanthanum concentrations measured in spruce needles of age class 1 were 44 ng/g

11 APPLICATIONS OFRARE EARTH ELEMENTS TOAGRICULTURAL PLANTS

compared to 4.9 ng/g when exogenous material was carefully removed (Wyttenbach and Tobler,

2002).

Spruce Needles (ng/g) Earth Crust (µg/g)

Wyttenbach et al.(1994) Erdtmann(1991)b Goering et al.(1991) site 1 site 2 La 22.8 28.9 307 30 Ce 13.2 29.0 403 60 Sm 2.81 3.85 49.6 6.0 Eu 0.795 0.765 9.04 1.2 Tb 0.710 0.739 10.5 0.9 Yb 1.80 1.566 26.4 3.0 Lu 0.180 0.159 - 0.5

Table 11.16: Concentrations of rare earths in cornifer needles;bvalue include surface contamination.

For spruce needles, average values were 10.7 ng/g for La and 1.2 ng/g for Sc while the relative concentration of rare earth elements in the needles has been shown to be similar to that found in the earth crust.

Independently from the doses applied, rare earths accumulated strictly proportional to the nee- dle age class with concentrations in the current year’s needle (age class 1) being 5 times lower than those in age class five (Wyttenbach et al., 1996). Thus accumulation proceeds exactly with time and may therefore be characterized by the yearly increment, whereas there was no indication for a retranslocation of accumulated elements.