Abstract
A field experiment and several controlled environmental experiments were conducted to study the thermal response during the phenological development of wheat. The varietal differences in the relationship between developmental rate (DVR) and mean temperature were calculated by using linear regression in each phase. The relationships between DVR and mean temperature were linear (r>0.96) in the S-E phase (from sowing to emergence) in both the field and the controlled environment, D-H phase (from double ridge to heading) in the field enviroment, and H-A (from heading to anthesis ) and A-M phases (from anthesis to maturity) in the controlled environment. However, in the E-D phase (from emergence to double ridge), the regression curves were parab olic. Particularly, photosensitive cultivars and cultivars with a high vernalization requirement expressed severe depression of DVR at and above 15°C mean temperature, In H-A and A-M phases of the field experiment, an accelerated DVR was observed above 22°C. The base temperature (at which DVR=0 in the linear equation) differed with the phases; it was lowest in the S-E phase (-0.4°C) and highest in the H-A phase (6.3°C). Since the differences of base temperature among cultivars were relatively small, it was possible to quantify genotypic trait as the thermal unit (reciprocal of the slope) by setting the same BT value. The DVRs of S-E, H-A, and A-M phases could be predicted from mean air temperature by using a simple linear equation. It was concluded that this equation is the most convenient method of evaluating the thermal response during phenological development of wheat within the normal temperature range.
