DAYTIME AND NIGHTTIME THERMAL THRESHOLDS ASSOCIATED WITH WHEAT YIELD LOSS IN THE YAQUI VALLEY, SONORA, MEXICO

Rising temperatures in southern Sonora are a major factor affecting wheat yield; however, the thermal thresholds that reduce yield and the specific growth stages during which these effects occur have not been defined. The objective was to identify the day and night temperature thresholds associated with yield loss and to determine the phenological stage most affected. During the 2025–2026 crop season, hourly temperatures were recorded in 17 commercial fields in the Yaqui Valley and at the nearest weather station from the Sonora automated weather station network. Daytime hours ≥30 °C and nighttime hours ≥14 °C were recorded for each phenological stage and related to yield using linear regression, impact analysis by 1 °C interval with bootstrap resampling, and multiple regression. Hours at ≥30 °C between spike emergence and physiological maturity explained 91 % of the yield variation (−23.3 kg ha⁻¹ h⁻¹; p < 0.001), and the impact per hour increased above 33 °C and 36 °C. At night, the impact was significant starting at 18 °C, but its effect could not be separated from the daytime effect (r = 0.90). Digital sensors placed within each wheat field, recorded an average of 252 more daytime hours at ≥30 °C and 140 fewer nighttime hours at ≥14 °C than the weather stations; consequently, the thresholds derived from station data are not equivalent to those experienced by the crop. Daytime heat during grain filling was the thermal factor most strongly associated with wheat performance in the region.