Heat stress around flowering has negative effects on maize (Zea mays L.) grain yield. Most research on this topic focused on the response of pollen viability and pollination constraints, and little is known about the relative response to heat of plant grain yield (PGY) components [kernel number per plant (KNP) and individual kernel weight (KW)] and the physiological determinants of grain yield [light interception efficiency (ei), radiation use efficiency (RUE), and harvest index (HI)]. Field experiments were performed to study the response of physiological traits to contrasting air temperature regimes at ear level [nonheated control (TC) and heated (TH; with air temperature >35°C around noon)]. Heating was performed during periods of approximately 15 d at two growth stages [presilking (GS1) and postsilking (GS2)]. All silked ears received fresh pollen. Heating during GS1 caused (i) a larger delay in silking date than in anthesis date, (ii) an increase in male and female sterility, and (iii) a reduction in plant height and leaf area index, but not in ei Heating always caused a reduction in (i) plant and ear growth rates (EGR) around silking, (ii) RUE around silking, and (iii) HI and KNP. Final PGY was related to KNP (r2 = 0.89, p < 0.001) but not to KW. Variations in KNP were explained (r2 = 0.71, p < 0.0001) by variations in EGR postsilking (EGRPOST) and not presilking, evidence of long‐term effects of heat stress during GS1 Variations in EGRPOST depended on variations in RUE postsilking (RUEPOST) and not on biomass partitioning to the ear.

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