CROP GROWTH AND RELATIVE GROWTH-RATES OF OLD AND MODERN WHEAT CULTIVARS

An analysis of the dynamics of green area index (GAI), dry matter (DM), relative growth rate (RGR) and crop growth rate (CGR) based on growing degree days (GDD) is presented for a historical series of wheats commercially released in Western Australia. Relative to the old cultivars, modern wheats were characterized by a greater RGR during the vegetative phase. This was achieved at a lower initial GAI, which persisted as the season progressed and was associated with a higher CGR at anthesis and greater grain yield at the end of the season. In the old cultivars, a greater GAI during the mid season declined rapidly as temperatures and soil moisture stress increased in spring, resulting in a lower GAI at anthesis. Together with lower CGR at anthesis this resulted in less dry matter and grain yield at final harvest. The higher grain yield of modern wheat cultivars was achieved with a high RGR during the vegetative phase and greater CGR from ear emergence to harvest.