## 3 Results and Discussion
### Influence of Soil Characteristics on Plant Uptake  
There are few studies on tungsten uptake by plants and the reported tungsten concentrations in plants are highly variable due to the differences between plant species and experimental conditions [26; 30; 34]. Although in hydroponic experiments, the tungsten uptake can be directly linked to the concentration of metal in solution and to specific tolerance mechanisms, the interpretation of the phytoavailability in soil is more complex since it is essentially determined by tungsten species in the soil liquid phase. It is thus fundamental to evaluate the effects of soil properties on tungsten solubility and plant uptake, in particular of soil pH, which is considered the most important parameter that determines the amount and the forms of tungsten in soil solutions [9]. Considering the different soil pH, it appears that by increasing pH, plant uptake significantly increased (Figure 2). At the highest concentration (100 mg kg\({}^{-1}\)) of tungsten in soil, in plants grown in soil C with a pH value of 7.40, the amount of tungsten in the root portion (95.5 mg kg\({}^{-1}\)) was approximately thirteen times greater than that (7.1 mg kg\({}^{-1}\)) in the roots of plants grown in soil A. The same trend was found by comparing the concentration of tungsten in the roots of plants grown in soil B (18.6 mg kg\({}^{-1}\)) and soil A (7.1 mg kg\({}^{-1}\)). In this case, the concentration increased by more than twice as the soil pH increased from 4.50 to 5.80. Finally, the concentration of tungsten in the roots of plants grown in soil C increased by 5.1 times compared to that found in the roots of plants grown in soil B (from 18.6 to 95.5 mg kg\({}^{-1}\)). Figure 2: Mean values of tungsten concentrations in plants in relation to pH of the different soils. Bars indicate standard deviation. (**a**) shoots; (**b**) roots. The aboveground parts of the plant behaved similarly. As the pH increased, from soil A (4.5) to soil C (7.4), the concentration of tungsten in the aerial part of the plants increased at each concentration of tungsten in the soil. This increase was evident above all at the highest tungsten concentration (100 mg kg\({}^{-1}\)), from 1.8 to 21 mg kg\({}^{-1}\) of tungsten in the aerial part of the plants grown respectively in soils A and C, with an increase of about 12 times. In soil B, the concentration of tungsten in the aerial part of the plants was \(\sim\)5.43 mg kg\({}^{-1}\), with an increase with respect to soil A of approximately three times. In the aerial part of the plants grown in soil C, the amount of tungsten was about four times higher than that in plants from soil B. Increasing pH promotes the deprotonation of soil surfaces, thus reducing the retention of tungstate ions in the solid phase of soil [40]. The consequent increase in tungsten in the soil solution led to an increase in bioavailability and therefore a higher uptake by the plants. The greatest uptake occurred in the Fluvisol (soil C), which is characterized by alkaline pH values and a low content of organic matter, conditions that are very common in soils of the Mediterranean area. The organic matter is another parameter that seems to play a key role in the absorption of tungsten by plants. Figure 3 reports the uptake trend in relation to the different values of organic matter in the soils. The data reported in Figure 3 show that the different organic matter content in the three soils influences the concentration of tungsten in the aerial part and in the roots of the plants. As the amount of organic matter increases, the concentration of tungsten in plant tissues decreases. The action of organic matter can be ascribed to the adsorption reactions of tungstate on humic materials [41], with the formation of stable complexes [42] that reduce the tungsten mobility and bioavailability in  
Figure 3: Mean values of tungsten concentrations in plants in relation to organic matter (OM%) of the different soils. Bars indicate standard deviation. (**a**) shoots; (**b**) roots. soil.