Vanadium Phytotoxicity: vanadium uptake, translocation and interactions with nutrients in wheat and common bean

Vanadium has a widespread environmental distribution and originates from natural and anthropogenic sources. In recent years, concentrations of V in environmental matrices have increased due to an increased consumption of V in industrial activities. The bulk of anthropogenic V derives from the burning of fossil fuel products (petroleum, coal, and oil) and industrial wastes including fly ash, bottom ash, slag heaps, mining tailings, oil residuals, and municipal sewage sludge. Anthropogenic emissions of V to the atmosphere exceed natural sources by a factor of 1.7 at present and are destined to rise significantly. With the frequency of reporting of V contamination issues in soils and plant communities starting to rise, there is a pressing need to understand the effects of V effects on plants and the toxicity mechanisms involved.

Trace concentrations of V have been reported to be beneficial to plant growth while higher V concentrations are toxic. Vanadium toxicity in plants has been well demonstrated in solution culture. Close interactions between V and soil nutrients; esp. Ca, Fe, P, Mg, N have been reported during V uptake and redistribution in plants but with inconsistent results reported. In this thesis, I focussed on understanding V toxicity mechanisms in plants by investigating the impacts nutrients on V dynamics in plants as well as investigating the effect of V on the legume and rhizobia symbiosis. Six major experiments were designed to investigate V uptake, translocation and bioaccumulation processes in plants and to understand the impacts of P, Mg, Ca, Fe on V in Triticum aestivum L. cv. Spitfire (wheat) and Phaseolus vulgaris L. cv.  Bean Bush Blue Lake) (common bean) and V dynamics in common bean inoculated with Rhizobium leguminosarum biovar phaseoli (strain CC 511).

The initial rate trial experiment was aimed at understanding how vanadate [V (V)] supplied at concentration ranging from 8 to 512 µM affected tissue biomasses, and tissue concentrations of V, P, Ca and Fe in wheat and common beans using chelator buffered nutrient solutions. Ecotoxicological threshold values (effective concentrations for a 10% decrease in biomass, EC10) demonstrated that common bean was more sensitive to V in solution and tissue than wheat [EC10 in solution (µM) wheat: shoots 98 and roots 177; common bean: shoots 3 and shoots 6 and EC10 in tissue (mg/kg) wheat: shoots 36 and roots 3460; common bean: shoots 4.0 and shoots 2.5]. Statistically significant (P<0.05) interactions between solution V and tissue V concentrations and tissue nutrient concentrations were identified, mainly for Ca and Fe. Further investigations were conducted to understand V and Ca and V and Fe interactions using factorial experiments using 3 concentrations of V and 3 or 2 concentrations of Ca and Fe respectively. Vanadium effects on tissue Ca concentrations were not consistent and increasing Ca concentrations did not alleviate V toxicity.  Results showed that vanadate was associated with increased exudation of phytosiderophores (PS) under both Fe deficient and Fe sufficient conditions and that 100 µM V in solution, under Fe sufficient conditions, enhanced tissue Fe nutrition and tissue biomass production in wheat in comparison to 0 µM V control treatments. Fe deficiency induced phytosiderophores release was independent of V induced PS release in wheat. This is the first time in the published literature that V induced root exudation of phytosiderophore has been demonstrated. The physiological mechanisms of the higher Fe translocation induced by V remains to be confirmed. Iron acquisition by phytosiderophores appeared to contribute to V tolerance in wheat in comparison to common bean which makes sense given the different iron acquisition strategies of the two species.

A further experiment demonstrated that V had a significant effect on the common bean and rhizobia symbiosis.  For the first time in the literature, it was shown that V was detrimental to the legume-rhizobium symbiosis in root nodules. Vanadium increased the time to initiation of the first nodule, the total number of mature nodules and the % of active nodules. Vanadium significantly reduced rhizobia associated enhancement of root tissue N concentration in common bean and the damage increased as the solution V concentration increased from 25 µM to 100.  Rhizobial inoculation increased the toxicity of V to common bean by increasing V uptake in roots and translocation to the shoots causing significant biomass reductions in common bean which could also lead to biomagnification of V along food chains.

Finally, synchrotron-based X-ray Absorption Spectroscopy (XAS) was used to investigate how speciation of V(V) and V(IV) changed during uptake into roots, how the Ca concentration in the nutrient supply influenced V speciation and subsequent V accumulation in wheat and common bean in situ, which had not been examined previously. Results demonstrated that V(V) was preferentially taken up by plant roots compared to V(IV), with V(V) also having a greater toxicity to the plant roots. Vanadium speciation within root tissue of the two plant species differed considerably, with all V(V) being reduced to V(IV) in common bean while the majority of the V was also reduced to V(IV) in wheat although some remained as V(V). The majority of V (IV) occurred as uncompleted VO2+ while some V(IV) was bound to organic acids and antioxidants providing the first evidence for such compounds playing a role in V(V) detoxification.  Given the lower toxicity of V(IV) in comparison to V(V), reduction appeared to be an effective V(V) detoxification strategy used by both species. Changes in Ca concentration did not affect V speciation in the root tissue confirming the findings of Experiment 3. The formation of a precipitate with CaVO3 was precluded to be the dominant mechanism that enabled V(V) tolerance differences between species even when plants had greater access to Ca. The next stage for understanding changes in V speciation during uptake and translocation is to investigate subcellular localisation of V species.

The findings of this study have advanced the knowledge of V toxicity mechanisms in plants and shown future directions for V toxicity research in plants.