Heat stress frequently limits grain yield of summer crops. Most research on maize (Zea mays L.) has analyzed heat stress effects on crop physiology and kernel set but little attention has been given to kernel weight and its composition. The objectives of this work were (i) to assess the response of maize oil yield components (kernel number and weight, embryo oil concentration, and embryo/kernel ratio) to postflowering heat stress and (ii) to explore changes in the sensitivity to this constraint across developmental stages and genotypes. Hybrids with different kernel types (flint, popcorn, and semident) were exposed to contrasting temperature regimes (nonheated and heated: air temperature at ear level > 35°C) during early or late stages of the effective grain filling. Oil yield was affected by early (up to −60%), and to a lesser extent, late (up to −40%) heat‐stress episodes. These effects were mediated by lighter kernels (r2 = 0.94; P < 0.001) and lower kernel oil concentrations (r2 = 0.80; P < 0.001). Reductions of kernel weight were related to shorter grain‐filling periods (r2 = 0.48; P < 0.01), and those of kernel oil concentration were mainly associated with lower embryo oil concentrations (r2 = 0.64; P < 0.001). Kernels of semident hybrids were the most prone to alter their weights and oil concentrations in the face of a wide range of heat stress intensities. Variations in kernel oil concentration due to high temperature became notorious when kernel weight reductions were larger than 20%.

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Episodes of extremely high temperatures (>35°C) may cause a premature cessation of maize kernel growth (i.e., heat stress), depressing crop grain yield. However, little is known about the influence of this constraint on chemical composition of maize kernels, a key trait for end‐use related attributes. Four maize genotypes (flint, popcorn, temperate semi‐dent, and temperate × tropical semi‐dent) with distinctive endosperm types were grown at heated and non‐heated temperature regimes during the early or late stages of the effective grain‐filling period. Heat stress during early stages decreased both protein and starch contents of kernels, but the impact on the former was lower (up to −42%) than on the latter (up to −50%), resulting in increases of kernel protein concentration (up to +14%). The flint and popcorn hybrids, with hard endosperm type, tended to be less vulnerable to such effects due to an enhanced capacity to sustain kernel growth. For all hybrids, heat stress during late stages of kernel growth, reduced similarly protein and starch contents (up to −38%), without affecting kernel protein concentration. Heat stress always altered endosperm protein composition by increasing the relative abundance of glutelins, and β‐ plus γ‐zeins, at the expense of that of α‐zeins. The significant environment × endosperm‐type interaction for kernel chemical compounds should be linked to quality parameters for different maize end‐uses.