USE OF TEMPERATURE AND SHORT WAVE RADIATION TO PREDICT THE RATE OF SEEDLING EMERGENCE AND THE HARVEST DATE
As a result of these researches numerous methods for evaluating the effect of temperature on germination and subsequent plant growth have been recommended in the past. An extensive review on the advantages and disadvantages of the various temperature efficiency indices such as length of the growth period, heat sum, exponential and physiological index has been given by Klages (1942). Growth expressed as germination, stem elongation often shows a linear response between minimum and optimum temperature. It follows that at a fixed temperature level, growth rate is proportional to time. The combined relationship for a definite growth stage can therefore be represented by
S = (Tmean - Tmin) x t
where S is a heat sum (constant heat sum) in degree days; Tmean resp. Tmin are the mean temperature and the minimum temperature for the growth activity in degrees centigrade; t is the time in days.
In experiments with various temperatures, a plot between T and the reciprocal value of t should give a linear relation, of which the slope gives a value for S and the intercept a value for Tmin.
The germination of seeds has been extensively studied under controlled temperature conditions in the laboratory and under field conditions with varying sowing-dates. In the field, however, a periodic variation in soil temperature occurs, the amplitude of which depends on available energy, soil depth, thermal conductivity and heat capacity of the soil. A