Yield, biomass and yield components in dwarf, semi-dwarf and tall isogenic lines of spring wheat under recommended and late sowing dates

Grain yield and some of its physiological determinants were analysed in a field study conducted over two growing seasons with standard-height (SH), semi-dwarf(SD) and dwarf (DD) isogenic lines of a spring wheat sown at recommended and delayed dates. The objective was to test whether the action of Rht1 and Rht2 alleles in improving grain yield depended upon sowing date. As expected, the dwarfing genes significantly reduced plant height in both sowings and growing seasons. There was a tendency for the SD line to produce more biomass than the SH and DD lines, although the differences were mostly not significant. Harvest index was linearly and negatively related to plant height. Consequently, the SD and SH line showed the highest and lowest grain yields in all environments. The optimum height for grain yield was estimated to be c. 70 cm, and this value was not affected by sowing date. Lines carrying Rht1 and/or Rht2 alleles always showed more grains/m(2) (owing to an increased number of grains per spike and spikes/m(2)) than the SH line. Conversely, average grain weight was negatively associated with the number of grains/m(2). Because the slope of this negative relationship was smaller(less negative) than that representing complete compensation, the relationship between grain yield and number of grains/m(2) was hyperbolic. Although these relationships are frequently regarded as a reflection of increased competition among grains when the number of grains/m(2) is increased owing to the use of semi-dwarf genes, two alternative hypotheses are analysed acid discussed.