*3.1. Spectral Di*ff*erences Between Adaxial and Abaxial Surfaces*

The mean spectral reflectance of the adaxial and abaxial surfaces is shown in Figure 2a. The reflectance of the adaxial surface was much lower than that of the abaxial surface in the visible region (400 to 690 nm). This is because light incident from the adaxial side was more effectively absorbed than light incident from the abaxial leaf side [18]. In contrast, the adaxial reflectance was higher than that of the abaxial surface between 750 and 1400 nm. This was partly because the palisade structure at the adaxial side of the leaf contributed higher reflected radiation than the spongy structure at the abaxial side [17]. Spectral differences among the two leaf surfaces were small in the red-edge region (690 to 750 nm), especially between 718 and 732 nm, where differences in reflectance were less than 5% (Figure 2b). Differences were also less substantial at wavelengths longer than 1400 nm. Variations in the internal structure of the adaxial and the abaxial surface also contributed to these differences (Figure 3). The adaxial surface (Figure 3a) was characterized by increased waxes than the abaxial surface (Figure 3b). Rayleigh scattering by waxes is known to contribute to the higher reflectance of the adaxial surface at NIR wavelengths [41].

The correlation between LCC and the reflectance of both surfaces are plotted in Figure 4. The reflectance of both surfaces in the blue region (400 to 500 nm) and the main chlorophyll absorption region near 680 nm demonstrated the least sensitivity to LCC. High sensitivity to LCC was observed

at 570 nm and near 710 nm. The strongest correlation was at 710 nm (the correlation coefficient is −0.91) and 704 nm (the correlation coefficient is −0.85) for the adaxial and abaxial surfaces, respectively. The abaxial reflectance at wavelengths over 750 nm also demonstrated strong correlations. Overall, the correlations between LCC and abaxial reflectance were stronger than those between LCC and adaxial reflectance.

**Figure 2.** The spectral reflectance of the adaxial and abaxial surfaces (**a**) and the associated difference in reflectance among the two sides (**b**). (The solid line represents the mean of sampled reflectance and the shaded zone represents standard deviation).

**Figure 3.** Optical micrographs of the adaxial (**a**) and abaxial (**b**) surface of peanut leaves.

**Figure 4.** The correlation between LCC and the reflectance of the adaxial and abaxial surfaces from 350 to 2500 nm.
