Heat Stress during Late Vegetative Growth of Maize: Effects on Phenology and Assessment of Optimum Temperature

Prediction of phenology is based on thermal time (TT) computation, which requires the correct definition of base (T B ) and optimum (T O ) temperatures. Most information on these traits came from controlled environments using a wide range of mean air temperatures (T X ), including T X > T O and T X < T B These conditions are rarely found in field environments. We assessed the effect on development of day‐time temperatures above T O during late‐vegetative growth of maize ( Zea mays L), and established a model based on TT computation on a per hour (TT h , in °C h) rather than per day basis (TT d ) for T O estimation. Field experiments included two temperature regimes (T C : control; T H : heated) between V 11 and tasseling of T C We registered temperature at ear level, and dates of anthesis and silking. We computed developmental rates (DR), TT h above 8°C during treatment period (TT h1 ) and between V 11 and silking (TT h2 ), a stress index based on the quotient of differences in TT h (ΔTT h ) between T H and T C (SI = ΔTT h2 /ΔTT h1 ), and T O Heat stress caused a delay in flowering events, and a decline in DRs. Estimated T B was higher (12.7°C) than normally used in computations. Estimated T O was within the expected range (36°C > T O > 30°C), independently of T B Stressful temperatures promoted a delayed in silking, identified as an increase of at least 2.14°C h in TT h for each degree above T O Estimated T O differed between growing seasons ( P = 0.04), suggesting possible variation due to climatic effects.