Calculating Heat Units via a Sine Function

The concept of heat units (degree days) has provided a basis for several studies to analyze the time between planting and maturity for plants. The model reported here utilizes a sine function to approximate the behavior of temperatures during the day. The available heat units for the day then are obtained by integrating the sine function given constraints of the plant growth process as reported by other authors. These constraints include: 1) a minimum temperature below which plant growth stops; 2) a high temperature above which plant growth remains unchanged for some interval up to a maximum temperature; and 3) a maximum high temperature above which plant growth is retarded. The model is developed explicitly for tomato ( Lycopersicon escutentum Mill.).