Division of Plant Industry, CSIRO. PO Box 1600 ACT 2601, Australia
This paper examines some advantages and problems associated with the use of thermal time or heat units to estimate heading time in wheat grown under high temperatures during the vegetative phase. It is shown with the assumption that rate of development increases linearly with increased temperature, as is intrinsic in the heat sum concept, is not correct at higher temperatures. Non-linearity at higher temperatures is particulary evident when development is delayed either by short photoperiod or by an unsatisfied vernalisation response. The trend to decelerated increases, in development rate at higher temperatures may be partially associated with carbon limitation as the provision of more light brings the response closer to the linear hypothesis. Slower rate of development (or longer time to heading) at high temperature can be associated with a slowing or cessation of primordia production on the growing apex. Again, this slowing can be partially reversed by increased light. This paper supports the use of curvilinear models to estimate development time as proposed by Angus et al., (1981).
