Morphological, biochemical and molecular insights into heat stress tolerance in wheat (Triticum aestivum)

Wheat ( Triticum aestivum ) productivity is increasingly constrained by heat stress, particularly when high temperature coincides with reproductive development and grain filling. This review integrates morphological, physiological, biochemical, and molecular responses of wheat to heat stress and links them to breeding-relevant traits such as canopy temperature depression, stay-green behaviour, photosynthetic stability, membrane thermostability, antioxidant capacity, source-sink remobilization, and grain filling. At the molecular level, the review emphasizes the heat shock factor-heat shock element-heat shock protein (HSF-HSE-HSP) cascade, hormonal and epigenetic regulation, non-coding RNAs, QTL/GWAS evidence, and multi-omics approaches. A central focus is the strength of evidence: expression-responsive transcripts, miRNAs, lncRNAs, proteins, and metabolites are distinguished from genetically supported or functionally validated heat-tolerance genes, while QTLs and associations are considered in terms of effect size, genotype × environment interaction, repeatability, and breeding utility. Emerging pan-genomic resources, genome editing, high-throughput phenotyping, and artificial intelligence are discussed as tools for connecting molecular discovery with selection. The synthesis indicates that durable improvement of wheat thermotolerance will require multi-environment phenotyping, validation of causal alleles and genes in wheat backgrounds, and integration of physiological selection with genomic prediction and precision breeding for stable yield under future heat scenarios.