The oldest records of maize use are from the Tehuacan valley of Mexico, where the oldest harvested cobs are dated at 7200 years before present (bp). There is no convincing evidence of maize in the Old World before 1492. At this time cultivation of maize occurred throughout the Americas, except where it was too cold. Following introduction into the Old World, its cultivation spread widely between 50° N and 40° S. The bulk of the crop is grown in the warmer temperate regions and humid subtropics. It is not suited to hot semi-arid climates, or to tropical rainforest climates (Purseglove, 1972). The optimum temperature for germination is 18—21°C; germination is very slow below 13°C and fails at about 10°C. Selection of more low-temperature-tolerant lines has allowed viable cultivation of grain crops above 50° N in northern France and of silage crops to c. 55° N in Denmark, the Netherlands and England (Miedema et al., 1987; Long, 1999). Flowering is temperature and day-length dependent, and so cultivars and landraces from low latitudes show delayed flowering when moved northwards (Purseglove, 1972).
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