Light signaling as a central component of crop-weed competition

Abstract Early-season crop yield loss frequently occurs even when resources are abundant, challenging traditional resource-based models of crop–weed competition. Drawing on decades of research on the critical period for weed control, this review highlights evidence that brief exposure of crop seedlings to neighboring weeds can trigger rapid and irreversible reductions in yield potential through resource-independent mechanisms. Central to these processes are weed-induced changes in light spectral quality, particularly reduced red:far-red (R:FR) ratios, which activate the phytochrome-mediated shade avoidance syndrome (SAS). These responses alter morphology, biomass allocation, canopy architecture, photosynthetic capacity, redox homeostasis, defense signaling, and nitrogen metabolism. Low R:FR light induces persistent photosynthetic and metabolic constraints, increases reactive oxygen species (ROS) signaling, suppresses jasmonic acid- and salicylic acid-mediated defenses, and modifies nitrate assimilation and root traits in species- and genotype-dependent manners. Collectively, weed-derived signals during early crop development can lead to lasting physiological reprogramming. Integrating light-mediated signaling with metabolic, defense, epigenetic, and lncRNA-mediated pathways provides a mechanistic framework for understanding yield loss and identifies potential targets for enhancing crop competitiveness and resilience in weed-infested agroecosystems.