Field‑Based Dissection of Stomatal Anatomy and Conductance Reveals Stable QTL Under Drought and Heat in Wheat.

Stomatal traits balance carbon gain with water loss, yet their breeding potential in wheat remains underexploited. This study investigated physiological and anatomical stomatal responses across 2 years of large-scale field trials under water-limitation and warming treatments. Across both seasons, stomatal conductance (gs) declined under stress, reflecting strong environmental constraint on gas-exchange (water-limitation: -26.9%; warming-treatment: -13.8%). Partitioning responses by surface identified the adaxial surface as the dominant contributor to gs variation and the most stress responsive. Despite increases in theoretical anatomical gas-exchange capacity (gsmax), stomatal conductance efficiency (gse) declined, indicating decoupling between anatomical capacity and realised conductance. Drought reduced stomatal size while increasing density whereas warming increased size and density, yet both responses maintained gas-exchange capacity. Moderate-to-high heritability was observed for anatomical traits (water-limitation: 0.13-0.57; warming-treatment: 0.42-0.71), contrasting with lower, less stable heritability for gs (water-limitation: 0.13-0.41; warming-treatment: 0.13-0.50). Genome-wide-association-mapping identified 60 putative QTLs, predominantly for anatomical traits. Candidate loci on chromosomes 7B, 2B, 3B and 5 A emerged as pleiotropic genomic intervals influencing stomatal anatomy across environments. These findings indicate that wheat maintains anatomical gas-exchange capacity while dynamically regulating realised conductance under stress, highlighting anatomically stable stomatal traits as promising breeding targets for climate resilience and yield stability.