The concept of heat units (HUs, also known as growing degree days, GDDs) is based on the fact that plant development stages depend on the accumulated heat experienced prior to a critical phenological milestone. In general, it holds that the lower the temperature and thus accumulated heat, the slower the rate of growth and development of plants and their organs. Also, the amount of accumulated heat required to complete a given developmental stage does not vary substantially from year to year. For this study, a lower temperature threshold (base) of 10.0°C was used.
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