**5. Statistical modeling of experimental data on the accumulation of heavy metals over time in fresh lettuce**

By interpolating the experimental data, functions were obtained that indicate the variation over time of the zinc concentrations from the soil into the plants. **Figure 8** showed linear interpolation, the results being similar to the increase in the concentration of metals in the soil-plant system. It has been observed that in soils with high concentrations of zinc there has been a large and rapid accumulation.

The interpolated functions obtained can help calculate other important parameters that influence the growth and development of plants on soils contaminated with zinc.

In **Figure 9** graphically represents the variations of Zn concentrations in lettuce grown on the soils of the three infestation categories, in the form interpolated by second-degree polynomials and in the form of experimental data. The same order is

**Figure 8.** *Linear interpolation of the increase of Zn concentration in fresh lettuce.*

**Figure 9.** *Quadratic interpolation of the increase in heavy metal concentration in fresh lettuce.*

easily visible between the heavy metal concentrations in the plants of the three categories of soil infestation, as in the case of linear interpolation.

From **Figure 9** it is observed that in plants the arrangement of curves of variation of heavy metal concentrations and their increasing monotony is permanent both for the third degree polynomial and for the separation of curves, without any common points between the three curves being observed.

The interpolation curves by third degree polynomials, for increasing the concentration of heavy metals in plants, are represented in **Figure 10** and for fourth degree polynomials, are represented in **Figure 11**.

**Figure 10.** *Cubic interpolation of the increase in the concentration of heavy metal (Zn) in fresh lettuce.*

### **Figure 11.**

*Polynomial interpolation of the fourth degree for the increase of heavy metal (Zn) concentrations in fresh lettuce crops.*

It is observed from **Figures 10** and **11** that, generally, the order and monotony of interpolation curves of the variation in the concentration of heavy metals in plants is observed by polynomial curves, with a few exceptions, the curve corresponding to the soil contaminated by 3% Zn slightly reaches the curve corresponding to the soil contaminated by 4.5% and it is very almost, toward the end of the study period, to the curve corresponding to plants grown in soil contaminated with 1.5% Zn.
