Photoperiod and temperature influence the time from sowing to tassel initiation (TI) in maize (Zea mays L.), with appreciable genetic differences in relative sensitivity to these factors. Three experiments were run to examine these photothermal responses and genetic differences within 12 diverse cultivars chosen to represent germplasm adapted to temperate, tropical highland, tropical lowland, and subtropical environments. Photoperiod influenced the rate of progress toward TI in all cultivars (P < 0.05), with a threefold differences in sensitivity among them. In three of the more photoperiod-sensitive cultivars (all adapted to the tropics or subtropics), the value of the critical photoperiod, above which progress to TI is delayed by increase in photoperiod, was close to 13 h d-1. Temperature also influenced rate of progress toward TI. For many cultivars the optimum temperature (T(o)) for rate of progress towards TI was appreciably cooler than the range from 30 to 34-degrees-C that is generally assumed to apply throughout development from sowing to anthesis in maize; estimated values of T(o) in a 12-h photoperiod (Exp. 3) varied from 19 to 22-degrees-C in the tropical highland cultivar H-32 to 31-degrees-C in the tropical lowland cultivar Tuxpeno Crema I Cycle 18. In the five cultivars examined, inherent earliness (i.e., time taken to TI in optimum environments) was almost identical, despite considerable differences in T(o), while relations between rate of progress to TI and suboptimal temperatures were curvilinear, with similar base values (mean 9.4-degrees-C). Curvilinear relations were also detected at supraoptimal temperatures in three of the five cultivars. Curvilinearity in each of these relations (in contrast with the linear relations reported in many other crops) may be compatible with symptoms of damage to maize plants exposed continuously to extreme temperatures (e.g., 12 and 37-degrees-C).
