Capturing Early-Stage Chilling Response in Field-Grown Sorghum and Maize by Integrating Aerial Phenotyping and...

Early-season chilling limits sorghum establishment in temperate environments, yet the metabolic basis of this sensitivity remains poorly understood. We combined temporal spectral reflectance with untargeted metabolomics to characterize species and genotype-level responses to early planting conditions. Normalized Difference Vegetative Index (NDVI) and Normalized Difference Red-Edge (NDRE) index revealed consistently faster seedling development and greater early-season greenness in maize compared with sorghum, with chilling-tolerant sorghum genotypes exhibiting intermediate reflectance patterns. Principal component analysis of metabolite profiles separated maize and sorghum along a dominant species axis, while planting date effects and genotype-specific metabolic stability were captured along secondary axes. Untargeted metabolomics identified 144 differentially accumulated metabolites (DAMs) between maize and sorghum and 56 DAMs between early- and optimum-planted sorghum. Early planting induced marked metabolic reorganization in sorghum, characterized by the accumulation of phenylpropanoid intermediates (chlorogenic acid, 4-hydroxybenzoic acid), osmoprotective sugars and polyols (galactinol, mannitol), amino acid derivatives (pipecolic acid, O-acetylserine), and aromatic amino acids, accompanied by depletion of central carbon metabolites including sucrose, fructose, sorbitol, and 3-phosphoglycerate. Pathway enrichment analysis indicated chilling-induced suppression of starch and sucrose metabolism, fructose-mannose pathways, and amino sugar metabolism, alongside activation of phenylpropanoid, terpenoid, aromatic amino acid, glucosinolate, and cyanoamino acid biosynthesis in sorghum compared with maize. The results of the current study demonstrate that early-season chilling constrains carbon turnover in sorghum and promotes diversion of metabolic flux toward antioxidative and osmoprotective pathways. The spectral metabolomic framework presented here identifies biochemical signatures associated with chilling sensitivity and provides targets for improving early-season vigor in sorghum.