Base temperature and heat unit requirement of 49 Mexican maize races

This work was developed in order to determine the base temperature (T-b) an d thermal requirement (TR) for 49 Mexican maize races. During 1982-1984, th ese races, represented by 148 accesions, were grown in several locations in Mexico. Ten field experiments were conducted representing diverse environm ental conditions. Among the characters measured were days to flowering (F) and daily mean temperature (T), which were used to determine Th and TR thro ugh a linear regression analysis between the developmental rate (1/F) of th e genetic materials and mean temperatures of the period F (Tm) in the locat ions. Results showed inter-racial differences in both T-b and TR values. In most cases, T-b differed from the 10 degrees C value typical in the United Scares Corn Belt for maize. Materials were roughly grouped into mid-elevat ion races, with average values of about 7 degrees C and 1100 heat units (HU ) for T-b and TR, respectively; highland races, with T-b of about 4 degrees C and TR of 1200 HU, and tropical races, with T-b of about 9 degrees C and TR of 1000 HU. Results suggest that T-b in most maize races differs from t hat for inbred genetic materials (fi-om U.S. Corn Belt or Mexico), typicall y 10 degrees C.