From Light Harvesting to Grain Filling: Chlorophyll Fluorescence, Pigment Composition, and Oxidative Status as Discrete Yield Determinants in Rye

Improving rye (<i>Secale cereale</i>) yield under increasing climatic stress remains a major challenge for sustainable cereal production. We examined whether early-vegetative physiological, biochemical, and molecular traits can predict final grain yield in hybrid-breeding components. Across three consecutive seasons, 14 genotypes were evaluated under controlled cold-greenhouse conditions for chlorophyll fluorescence, pigment content, hydrogen peroxide (H₂O₂), salicylic acid (SA) levels, and the expression of selected antioxidant and defence-related genes, and these traits were related to yield components. Across years, photosynthetic efficiency (<i>F</i>_v/<i>F</i>_m, Rfd), chlorophyll content, and foliar H₂O₂ emerged as the most consistent predictors of kernel mass, spike number, and kernel number. In contrast, non-photochemical quenching, SA, and carotenoid contents showed weak or inconsistent relationships with yield. These findings indicate that light-harvesting capacity, PSII performance, and oxidative balance are central to reproductive success in rye. The stability of these trait-yield correlations across three seasons provides the basis for a physiological robustness index for hybrid rye, with predictive models achieving accuracies up to R = 0.51. This work demonstrates the potential of using a compact set of early-stage, high-throughput physiological traits to accelerate selection for stress-resilient, high-yielding rye cultivars.