01543naa a2200193 a 450000100080000000500110000800800410001910000160006024500680007626000090014452010640015365000160121765000160123365000170124965000110126665000160127765300100129377300460130317856942010-07-09       1960    bl ---       0--  u    #d1 aWANG, J. Y.  aA critique of the heat unit approach to plant response studies.  c1960  aThe "heat unit approach" in use for over two centuries, is a scheme for studying plant-temperature relationships by the accumulation of daily mean temperatures above a certain threshold temperature during the growing season. If a plant has a base temperature of 40 F and the mean temperature on a given day is 55 F, the difference in degrees for that day is termed "degree-days" or "heat units". In this example, we would have 15 degree-days. If 1200 degree-days are required for maturity, the plant should reach maturity by the time 1200 degree-days have been accumulated. This sum required for a particular crop variety has been assumed by heat unit workers to be a constant value and is termed the varietal constant. By knowing the varietal constant, users predict the date on which their crops should be harvested. This linear relationship, known as the "remainder index method," gave rise to a number of expressions, such as "degree-days," "heat units," "growing degree-days," "growth units," and others relating plant responses to season thermal levels.  aclimatology  atemperature  aClimatologia  aPlanta  aTemperatura  aPlant  tEcologygv.41, n.4, p.785-790, Oct. 1960.