Climate-change-driven shifts in secondary metabolism of oat ( Avena sativa L.): toward candidate metabolite markers for drought and waterlogging stress

Climate change is increasing the frequency and intensity of drought, waterlogging, and other environmental extremes that threaten oat (Avena sativa L.) production and quality.This review examines oat secondary metabolites, including avenanthramides, phenolic acids, flavonoids, avenacins, and avenacosides, in relation to primary metabolism and physiological responses to contrasting water stresses.Oat drought research links salicylate signaling with the modulation of carbon, antioxidant, and photo-oxidative metabolism, whereas oat-specific evidence for waterloggingresponsive secondary metabolites is much more limited.Stressresponsive metabolites are therefore distinguished from candidate metabolite markers and from independently validated markers.An evidence matrix separates four levels: validated oat markers, oat stress-associated candidates, oat biochemical candidates lacking water-stress-specific validation, and cross-species or mechanistic hypotheses.The matrix also identifies gaps in tissue, developmental stage, analytical identification, response magnitude, and association with tolerance phenotypes.Neither metabolite accumulation nor antioxidant activity alone establishes predictive value for stress tolerance.A mechanistic framework connects osmotic adjustment and carbon allocation under drought with oxygen-limited energy metabolism and reoxygenation-related redox responses under waterlogging.Targeted identification, genotype-resolved physiological phenotyping, independent validation, and field evaluation are required before candidate metabolite markers can be used in oat breeding.Stress resilience and functional grain quality should be evaluated as related but distinct outcomes.