14.3.2 Thermal time

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In practical terms the time taken for organ initiation and development or between one developmental stage and another is dependent on temperature and can be related to cumulative heat (thermal time). ‘Heat sums’ used in this way often have a considerable advantage over the use of normal calendar time, particularly in analysing field data where the temperature varies from season to season or from one planting to another. However, it is important to take into account the possibility that heat sums can be influenced by other environmental factors, such as light. The use of thermal time depends on the linearity of the response to temperature and a knowledge of the base temperature below which growth or development does not occur (Figure 14.13). Thermal time is simply a summation of cumulative differences between daily mean temperature and a specified base temperature, and has units of degree-days (°C d). Linearity of crop response to temperature is lost as the optimum temperature is approached, but the use of thermal time may be extended into the higher temperature range through the use of a curvilinear model. Base temperature may show genetic variation and differ between one developmental stage and another. The base temperature for bud break in grapevines is approximately 4°C, but is closer to 7°C for leaf appearance (Moncur et al. 1989). In wheat, base temperature increases with successive stages of development (Angus et al. 1981).



Figure 14.13 Phenology (timing of cardinal points in growth and development) can be predicted with greater certainty via thermal units than via calendar time. The units used, degree-days (°C d), are a summation of mean daily temperature X time. Useful application of thermal time is limited to the range where temperature responses are linear. For this reason a base temperature has to be defined, below which growth or development approaches zero, and an optimum temperature, above which the rate of growth no longer increases or may decrease with temperature. Cereal grains illustrate some of the advantages and difficulties of using thermal units for analysing growth data. (a) The duration of grain filling in response to temperature in wheat when plotted as the reciprocal of time (1/ duration in days) against temperature should be a straight line. This relationship obviously breaks down at the high end of the temperature scale. (b) The effect of temperature on the pattern of kernel filling in sorghum can be presented either using calendar time or thermal time (°C d). The shift to thermal time suggests that temperatures of` 26°C and higher are above the optimum for kernel filling and this is most evident when a more realistic base temperature for sorghum of 9.7°C is used instead of 0°C (Based on Chowdhury and Wardlaw 1978)

Heat sums have been used to predict the stages of crop development, knowledge that can be important for timing the application of herbicides, insecticides and fertiliser.

Photothermal time

This takes account of the interaction between daylength and temperature in controlling development. Integration of temperature with time is now restricted to the duration of the light period. Thus ‘photothermal time’ = Σ L (TLTB), where L is day length as a fraction of 24 hours, TL is the average temperature during daylight hours, and TB is base temperature.