3.4.2. Modelling of Zn Concentration

The SVM and MLR approaches based on wavelengths were able to predict the Zn contents with accuracies of 42–47% and based on spectral indices with accuracies of 70–85% in the testing set, respectively (Tables 3 and 4). As shown in Figure 6, the predicted values overestimated the observed values in most cases of wavelength-based models. However, a more uniform distribution was found between the observed and predicted values in spectral indices-based models. Therefore, spectral indices-based models tend to be preferred for predicting Zn contents in the grapevine leaves.

**Figure 6.** Standardized values (between 0 and1) of the observed (horizontal axis) and the predicted (vertical axis) concentration of Zn based on wavelengths (top) and spectral indices (bottom) in the testing sets of the SVM and MLR methods.
