The effect of dwarfing genes in spring wheat in dry environments. I. Agronomic characteristics

Near-isogenic lines of wheat were used to investigate the possible yield penalty of dwarfing genes in rainfed environments. Field experiments were conducted over seven years in south-eastern Australia which were characterized by drought becoming progressively more intense as crop maturity approached. Isogenic sets for the Rht1 and Rht2 genes were chosen to cover a range of heights for the tall (rhtrht) lines, from 100 to 130 cm when grown in the absence of drought before anthesis. Rht1 or Rht2 reduced this height by about 23% whereas Rht1 and Rht2 together or Rht3 alone reduced this ceiling height by about 47%. Highest grain yields were achieved by lines with a height between 70 and 100 cm. Although major were usually required to achieve this optimum height, no specific advantage of the major dwarfing genes was found. Likewise, in tall and short isogenic populations, the short populations yielded more than the tall populations even though they all contained Rht1. The expression of the major dwarfing genes and their effect on agronomic characteristics varied depending on the genetic background but general trends were evident. Above-ground dry weight was reduced by 4.4% of the mean weight for each 10 cm reduction in height below 100 cm. Above 100 cm there was little change in above-ground dry weight. Harvest index decreased by 3.7% of the mean value for each 10 cm increase in height above 65 cm. When height was less than 65 cm, harvest index remained constant. Kernel number was more sensitive to variation in height than was kernel weight. In only one of the isogenic sets was there evidence for compensation between kernel number and kernel weight; in four sets, both kernel number and kernel weight matched variation in yield in a positive way.