USE OF TEMPERATURE AND SHORT WAVE RADIATION TO PREDICT THE RATE OF SEEDLING EMERGENCE AND THE HARVEST DATE

Already in the nineteenth century great attention was given to temperature effects on germination and plant growth. So, for example, Sachs (1860) distinguished three cardinal points of vital activity, e. g. a minimum temperature below which no activity is present, an optimum temperature at which the highest activity occurs and a maximum temperature above which the activity is again zero. Haberlandt, as cited by Grafe (1914), gave cardinal temperature points for the germination of seeds of a large number of plants. Pfeffer (1903) early recognized, however, that 'the cardinal points can never be determined with more than the approximate accuracy, since their position is involved by the external conditions, by the duration of exposure, by the age of the plant and by its previous treatment'.

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

where S is a heat sum (constant heat sum) in degree days; T_mean resp. T_min 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 T_min.

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