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.

Prediction of phenology is based on thermal time (TT) computation, which requires the correct definition of base (TB) and optimum (TO) temperatures. Most information on these traits came from controlled environments using a wide range of mean air temperatures (TX), including TX > TO and TX < TB These conditions are rarely found in field environments. We assessed the effect on development of day‐time temperatures above TO during late‐vegetative growth of maize (Zea mays L), and established a model based on TT computation on a per hour (TTh, in °C h) rather than per day basis (TTd) for TO estimation. Field experiments included two temperature regimes (TC: control; TH: heated) between V11 and tasseling of TC We registered temperature at ear level, and dates of anthesis and silking. We computed developmental rates (DR), TTh above 8°C during treatment period (TTh1) and between V11 and silking (TTh2), a stress index based on the quotient of differences in TTh (ΔTTh) between TH and TC (SI = ΔTTh2/ΔTTh1), and TO Heat stress caused a delay in flowering events, and a decline in DRs. Estimated TB was higher (12.7°C) than normally used in computations. Estimated TO was within the expected range (36°C > TO > 30°C), independently of TB Stressful temperatures promoted a delayed in silking, identified as an increase of at least 2.14°C h in TTh for each degree above TO Estimated TO differed between growing seasons (P = 0.04), suggesting possible variation due to climatic effects.

Hormonal plant growth regulators (HPGRs) have been evaluated in field grown maize (Zea mays L.), but never as a tool for prevention or mitigation of heat stress. We analyzed grain yield determination of maize crops exposed to contrasting temperature regimes (nonheated control plots [TC]; heated plots [TH]) and the application of HPGRs associated with ethylene metabolism (ethephon [ETH]; MCP [1‐MCP]). Heating extended over daytime hours between V11 and tasseling (VT), and products were sprayed immediately before (V11) and/or during (V16) heating. Plants treated with ETH always had reduced height (10–21%) and leaf area (3–10%), but these trends usually had no effect on light interception during treatment period. Biomass production was markedly affected by heating, but a significant interaction effect (P < 0.01) indicated that HPGRs caused (i) no effect among TH plots, and (ii) a decrease (13–19% for ETH and 3.8–9.4% for MCP) among TC plots. The interaction effect computed for grain yield highlighted that ETH had mild negative effects (≤ 18%) among TC plots and large positive effects among TH plots (up to 73%), whereas MCP had no effect among the former and mild positive (V16) or negative (V11) effects among the latter. Variations in grain yield were due to variations in kernel numbers (r2 ≥ 0.92), which were explained by ear growth rate around flowering (r2 ≥ 0.97). Timely application of HPGRs was critical for improving biomass allocation to the ear (ETH) and having adequate blockage of ethylene receptors (MCP).