Historia Lectora:

There is little information on the metabolic consequences after rare earths are assimilated by plants. It is still argued whether rare earths are deposited at extra or intracellular locations in plants. Regarding the similar linear time-depending accumulation behavior of the rare earths and of silicon in conifer needles (Marschner, 1986), (Wyttenbach et al.,1994), it has been suggested that rare earth elements provide similar dynamics, such as the uptake by the roots, and transport by the xylem (with the transpiration stream). Thus, it might also be possible that rare earths are deposited as hydroxides together with amorphous silica. However, a study performed on hyperac- cumulating ferns showed that about 40 % of the rare earths were bound to high molecular proteins (Guo et al., 1996). Zhimang et al. (2001) demonstrated that in wheat bioaccumulation of rare earth elements was much higher in the root than on the top parts. Similarities in bioaccumulation patterns observed in roots and tops suggested that rare earths in root could be transported to the above ground parts of the plant. However, as most rare earths were assumed to be absorbed and deposited on the cell wall, penetration into the cell membrane was considered difficult (Brown et al.,1990), (May et al.,1997).

Generally, roots have been reported as the main accumulation sites for rare earths in several plant species. Consistent with that, Hong et al. (1996) reported that absorbed rare earths were predominantly accumulated in the roots (88 - 90 %) while only 10 to 12 % entered the cortex

11.3 Rare Earths Uptake by Plants and Bioavailability to Plants

and peduncle of plants. The average ratio in root to shoot of about 10 also emphasized the much greater accumulation potential of roots for lanthanum (Zhang and Shan, 2001). Higher accumu- lation values of rare earths in roots compared to leaves and therefore higher concentration ratios were also reported by Wahid et al. (2003). It was further reported that high doses lanthanum efficiently enhanced the permeability of the cell membrane thus disturbing their physiological function. This consequently leads to higher accumulation of rare earth elements in the roots as further transportation to the shoots is hampered (Brown et al.,1990), (Chang,1991).

Nevertheless, accumulation of rare earths in plant roots may also reflect the general behavior of plants responding to environmental stress. Plants tend to limit the translocation of heavy metals to their aboveground parts in order to minimize or overcome toxic effects caused by metals in soil (Foy, 1983). Limited transport of absorbed lanthanum from the roots to the plant tops was also observed byDiatloff et al.(1995b), who similarly put this down to a plant-protection function from adverse effects of lanthanum. Accordingly, Zhou and Liu(1996) demonstrated that after being absorbed, rare earths were fixed or precipitated on the cell wall of plant roots which hampered further transportation to the plant tops. In contrast to that, easy uptake of lanthanum from the root to the shoot was reported in wheat plants (Wang et al.,2001c).

However, Chen et al. (1995b) suggested that the soil pH could change the structures of the cell wall and plasmolemma of the plant as well as the osmotic pressure of the plasma inside the plant, thereby inhibiting the entrance of rare earth ions along with the fluid into the cell. The uptake of rare earths by the above-soil parts of the plant (shoot) is thus inhibited which results in lower accumulation. This is consistent with the general expectation of greater bioavailability of rare earths at lower soil pH and, hence, with the observation of a significant negative correlation between rare earth contents in roots and soil pH (Zhang and Shan, 2001). Li et al.(1998a) and

Yang et al.(1999) attributed higher accumulation values of rare earths in roots than in other parts (stems, leaves and grains) observed in corn, rice and wheat to sequential distribution from the soil solution to the roots and then further to the tops. Five steps are thought to be required for the transportation of rare earths from soil to the plant system (Wei et al., 2001). First total rare earths to soluble rare earths in soil, second soluble rare earths to root, third root to stem and then further to petiole and at last from petiole to lamina. Nevertheless, accumulation of rare earths has also been shown to be influenced by the application rate. A correlation between the accumulation behavior of La, Ce, Pr and Nd and the concentration of applied rare earth-containing fertilizer was demonstrated in winter wheat (Zhang and Shan, 2001). At low fertilization rates, accumulation increased with higher concentrations, whereas the inverse was observed at high fertilization rates and remained constant over the medium rare earth range. In accordance with that,

Maheswaran et al.(2001) also found higher concentrations of lanthanum in barley and wheat with higher rates of lanthanum application, particularly when applied as foliar. However, when applied at lower rates no significant differences were observed, especially when lanthanum was applied to soil. A recent study reported that there was no significant accumulation with the soil-dressing method. Furthermore, for both soil and foliar dressing methods, no distinct residues of rare earths were found in plant grains (Liang et al., 2005). As the relationship between accumulation and fertilization rates is of high importance regarding environmental effects of rare earths, it is further discussed in Chapter7.

11 APPLICATIONS OFRARE EARTH ELEMENTS TOAGRICULTURAL PLANTS