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An Enzyme Kinetic Equation to Estimate Maize Development Rates¹

While use of thermal units to predict development rate is very old, nearly all thermal unit equations are empirical and have no theoretical basis. The poikilotherm equation is unique in that it has a theoretical basis and accounts for both high and low temperature nonlinear development rate responses; however, this equation had never been applied to field grown plants. In this study, the equation, along with more traditional thermal unit equations, was applied to field grown maize (Zea mays L.).

Three hybrids of maize were grown in two plantings at two locations in each of 2 years. The coefficient of variation across mean unit sums for each planting was used as the criterion for comparing six other equations with the poikilotherm equation. Other equations used were the growing-degree-day (GDD) 40, the GDD 50, the 50 to 86 cut-off, the heat-stress, the Ontario corn heat unit equations, and day count.

The GDD 40 equation best described the planting to tassel initiation interval. Silking appeared to be delayed by high temperatures, thus the tassel initiation to silking interval was best described by the heatstress equation. There appeared to be no difference between different thermal unit equations for the silking to black layer formation and planting to black layer formation intervals. Even though the poikilotherm equation was based on physiology of the plant, it was no better than the other methods of accumulating heat units.

Key Words: Growing-degree-day • Models • Thermal units • Temperature effects • Zea mays L.

² Research agronomist, USDA, SEA-AR, Temple, TX 76501 (formerly graduate student, Univ. of Missouri, Columbia) and assistant professor, Univ. of Missouri, Columbia, MO 65211, respectively.

Received for publication June 16, 1980.