Spectroscopic analysis reveals an opposite pattern between carnosic and rosmarinic acid concentrations in rosemary (Salvia rosmarinus)

Carnosic acid (CA) and rosmarinic acid (RA) are the principal secondary metabolites (SMs) of rosemary ( Salvia rosmarinus Spenn. – Lamiaceae). Previous studies have examined CA and RA separately. However, their opposing CA and RA concentration trends have not been examined. This study tested the hypothesis that CA (C_CA) and RA (C_RA) concentrations exhibit an opposite pattern that can be rapidly and non-destructively spectrally detected using visible-near infrared spectroscopy. Hyperspectral data were acquired using three methods: field-based canopy measurements with a bare-fiber spectrometer, unmanned aerial vehicle (UAV)-borne hyperspectral imagery of the canopy, and laboratory measurements of rosemary leaf powder using a spectrometer with bifurcated-fiber. The datasets were analyzed using one-band correlations and normalized difference spectral indices. Furthermore, to identify wavelengths contributing to the opposite pattern, partial least squares regression coefficients were examined exclusively in rosemary leaf powder, as water absorption bands masked the C_RA coefficients in the field-based canopy spectra. First derivatives of the standard CA and RA spectra demonstrated the opposite pattern at certain wavelengths that aligns with our spectral analysis. The opposite patterns between CA and RA are not only subjected to spectral analysis, but for nutrient–SM correlations as well, such as nitrogen correlating positively with C_CA but negatively with C_RA, and magnesium showing the reverse. Analyses of the hyperspectral data acquired from field grown plants, leaf powder and standard in the lab, together with nutrient–SM correlations,demonstrated the robustness of the opposite pattern. These opposite patterns demonstrated a reliable basis for non-destructive SM dynamics monitoring in rosemary.