The application of rare earth elements (REEs) to a wide range of plants has been reported by Chinese scientists to increase the growth and yield, and improve the quality of produce, but the nature of these effects is not understood. These REEs are applied through fertilisers the major components of which are the REEs lanthanum (La), cerium (Ce) and nitrate.
This thesis reports the effects of the foliar and root application of the REEs, La and Ce on the growth and mineral nutrition of com (Zea mays L)
and mungbean (Vigna radiata (L.) Wilczek). In the first experiment, foliar sprays from 0 to 1.0% of a commercial REE fertiliser obtained from China or equivalent concentrations of pure La plus Ce nitrate were applied to com and mungbean. Foliar application of REEs did not significantly increase the shoot dry weight of com or mungbean grown in a potting mix containing low concentrations of REEs (<1 mg kg-1 ). Both REE sources applied at 0.5 and 1.0% produced symptoms of foliar damage and reduced shoot dry weight in both plant species. Damage symptoms and growth reductions of plants sprayed with pure La and Ce solutions were similar to those of plants sprayed with commercial REE fertiliser.
Computer simulations using GEOCHEM-PC predicted that La and Ce solubility in solution would decrease with increasing pH and that phosphate precipitation of La and Ce would reduce the solubility of La, Ce and P. Chemical analyses of dilute nutrient solutions confirmed these predictions. At pH 4.5, over 97% of the La, Ce and P remained in solution when concentrations of up to 8 mM La or Ce were added in the presence of 5 mM P. However, substantial losses of these elements from solution occurred at pH 5.5; when 8 mM Ce was added in the presence of 5 mM P, only 2 mM Ce and 1
mM P remained in solution. All subsequent solution culture experiments were maintained at pH 4.5 to avoid such losses.
Dilute nutrient solutions were used to study the effects of La and Ce on root elongation. The root elongation of corn and mungbean decreased with increasing concentrations of La or Ce from 0 to 20
mM. Mungbean was more sensitive to La and Ce than corn. Examination of mungbean roots by electron microscopy showed that La at 1 mM reduced root hair production. At 5 and 10
mM severe structural damage to roots was observed. The presence of fulvic acid at 15 mg carbon L" in solution ameliorated the rhizotoxicity of 5 mM La to corn. The
root elongation experiments suggested that if La and Ce do have positive effects on plant growth, these effects are probable only below 1 mM.
In dilute, continuously flowing nutrient solutions, concentrations of La or Ce from 0 to 1.4 mM did not increase total dry weight of corn and mungbean, but increased the root growth of corn, and to a lesser extent, mungbean. Lanthanum at concentrations >0.4 mM or Ce at
>0.2 mM decreased the growth of mungbean. The highest concentration of La and Ce in the shoots of corn was 16 mg kg-1, whereas in the shoots of mungbean it was 51 mg kg-1. The corresponding La and Ce concentrations in the roots of corn were 2930 mg kg-1 and in those of mungbean 3970 mg kg-1 . The lower concentrations of La and Ce in the shoots and roots of corn than mungbean may have contributed to the higher tolerance of corn to both elements than mungbean. Roots of both plant
species contained La and Ce concentrations 25-200 times higher than measured in the shoots. Chemical analyses of the shoots revealed higher-than-adequate levels of P in both corn and mungbean. Foliar symptoms of Mn deficiency along with low shoot concentrations of Mn were found in mungbean exposed to Ce but not La.
Low concentrations of REEs were measured in soil solutions extracted from 19 Australian soils. In unamended soils, La concentrations ranged from 0 to 0.13 mM and Ce from 0 to 0.51 mM. The addition of lime to a subset of 10 acid soils increased soil solution pH and decreased the concentrations of REEs in the soil solution. Conversely, the application of gypsum decreased soil solution pH and increased the concentrations of REE in solution. Concentrations of La and Ce measured in soil solutions extracted from some soils were potentially
toxic to mungbean, on the basis of the flowing solution culture experiments. Solubility calculations suggested that Ce orthophosphate may control the solubility of Ce in soil solution.
In further flowing solution culture experiments, modifying the composition of the nutrient solution by decreasing the P and increasing the Mn concentration did not result in any beneficial effects of La or Ce on corn or mungbean. Lanthanum and Ce decreased the concentrations of Ca in the roots and shoots, and decreased Ca uptake of corn and mungbean, consistent with the known ability of REEs to block Ca channels. For corn, the uptake of Mg was unaffected and that of Zn and Cu much less affected than Ca. The uptake of Fe by corn was
unaffected, indicating that the mechanism of phytosiderophore mediated Fe uptake attributed to corn was not adversely influenced by La or Ce. The shoot concentrations and uptake rates of all nutrients by mungbean were significantly decreased by concentrations of La or Ce >0.2 mM, suggesting a general reduction in root function. It was concluded that, if beneficial effects of REEs on plant growth are to be found they will most likely occur at <0.2 mM REE in the root environment, or <0.5 mg kg-1 in the shoot tissue.
The claims for benefits from the application of REEs have not been substantiated; rather, the overwhelming majority of the evidence presented in this thesis demonstrates their detrimental effects on plant growth.