Sowing date restructures carbon partitioning between defence and nutritional susceptibility to modulate stem borer...

Sowing date acts as a deterministic regulator of plant-pest phenological alignment, however, its mechanistic role in restructuring host physiological and metabolic networks remains inadequately resolved. This study deciphers how sowing time restructures photosynthetic performance, structural composition, and biochemical metabolism to mediate stem borer (Chilo partellus) infestation in sweet sorghum. Four genotypes, categorised as susceptible (SUGARGRAZE, CSV 24SS) and tolerant (SPV 3078, SPV 3083), were evaluated under three different dates of sowing during the 2024 kharif season: 8^th June (D₁), 22^nd June (D₂), and 6^th July (D₃). Delayed sowing elevated mean infestation by 65.92%, from 41.67% (D₁) to 69.17% (D₃). Tolerant genotypes maintained photosynthetic stability, exhibiting higher net assimilation (23.45 µmol CO₂ m^-2 s^-1) and stomatal conductance (0.26 mol H₂O m^-2 s^-1) alongside reduced internal CO₂, concurrent with enhanced deposition of structural carbohydrates (cellulose, hemicellulose) and lignification. In contrast, susceptible genotypes displayed a metabolic shift toward nutritional enrichment, characterised by elevated accumulations of total soluble sugars, starch, protein, and free amino acids, traits strongly correlated with infestation severity (r = 0.97). Principal component analysis revealed two antagonistic trait assemblies; a defense module integrating photosynthetic performance, structural polymers, and secondary metabolites (phenolics, tannins, and saponins), and a susceptibility module comprising primary nutritional metabolites. The defense module was negatively correlated with infestation (r = -0.72 to -0.95). Early sowing promoted an integrated resistance phenotype through regulated carbon partitioning toward physical and biochemical defenses, whereas delayed sowing disrupted metabolic homeostasis, enhancing host nutritional quality and stem borer suitability. These results establish a physiological and metabolic framework in which sowing date directly modulates host plant resistance trajectories, providing a mechanistic basis for agronomic optimisation and trait-based selection in sweet sorghum.