Diurnal and Phenological Modulation of Canopy Temperature in Wheat Breeding Under Mediterranean Conditions

< 0.001). Variance-component analyses showed a strong genetic signal within each Stage × Environment combination, with 87.6-97.7% of total variance attributable to genotypic effects pooled across hours. Third, the optimal phenotyping window was context dependent: under rainfed conditions, genotypic discrimination consistently peaked around mid-afternoon (~15:00 h), whereas under irrigation, the optimal window shifted with stage (13:30-15:00 h). Genotype rankings were also markedly less stable across hours under rainfed conditions, indicating substantial within-day re-ranking as atmospheric demand increased. Finally, thermal exposure analyses showed that exceeding a physiologically relevant threshold (CT > 32 °C) depended strongly on time of day and stage; maximum CT captured short heat events missed by daily means. Clustering and alluvial analyses revealed frequent reclassification across stages, with only a small subset remaining consistently cooler, particularly under stress. Random regression of CT on vapor pressure deficit (VPD) indicated that CT-VPD sensitivity was largely environment-dependent and showed weak cross-environment correspondence (Spearman ρ = -0.166). Overall, single-time-point CT phenotyping provides an incomplete view of thermal status, underscoring the need for multi-temporal protocols and context-specific measurement windows for breeding and physiological interpretation under drought and heat.