Prediction of humic acids bioactivity using spectroscopy and multivariate analysis

Abstract

Humic substances (HS) represent the organic material most widespread in nature and have positive effects on plant physiology influencing nutrient uptake and root architecture. Understanding the structure–activity relationship has been considered an essential step to drive technological approaches to produce HS with enhanced plant growth promotion performance. In this study we used solid state nuclear magnetic resonance (CP-MAS 13C NMR) and infrared spectroscopy with diffuse reflectance (DRIFT), a collection of data from humic acid-like substances (HA) obtained from vermicompost produced with different sources of raw material and maturation stage. The bioactivity was evaluated using the ability of HA to induce lateral root emergence sites as well acidification of the liquid medium by treated-maize root seedlings. The structure–activity relationship was done using multivariate analysis including principal component analysis (PCA), principal component regression (PCR) and Spearman correlation joined with NMR and DRIFT results. The main variables (higher loadings) correlated with HA bioactivity were for NMR positive loadings for lignin (56ppm, 124ppm, 148 and 153ppm) and COOH groups (174ppm) and negative loadings for carbohydrates (64, 75 and 102ppm). For DRIFT the main positive loadings were aryl stretching and bending (1560, 1480, 860 and 780cm−1) probably from lignins; for 2926cm−1 and 2852cm−1 due to CH asymmetric and symmetric stretching, respectively, probably from long chain fat acids due to the presence of rocking absorption band of (CH2)n with n>4 at 720cm−1 and negative loadings due to carbohydrate moieties (around 1100cm−1, CO stretching) and carboxylic acids (1724cm−1, CO stretching and 1220cm−1, COH stretching and COH bending). The use of spectroscopy data to attempt to relate structure to bioactivity may be of practical interest in developing plant growth promoters using renewed source of humic materials.