YIELD, BIOMASS AND YIELD COMPONENTS IN DWARF, SEMIDWARF AND TALL ISOGENIC LINES OF SPRING WHEAT UNDER RECOMMENDED AND LATE SOWING DATES

Grain yield and some of its physiological determinants were analysed i n a field study conducted over two growing seasons with standard-heigh t (SH), semi-dwarf(SD) and dwarf (DD) isogenic lines of a spring wheat sown at recommended and delayed dates. The objective was to test whet her the action of Rht1 and Rht2 alleles in improving grain yield depen ded upon sowing date. As expected, the dwarfing genes significantly re duced plant height in both sowings and growing seasons. There was a te ndency for the SD line to produce more biomass than the SH and DD line s, although the differences were mostly not significant. Harvest index was linearly and negatively related to plant height. Consequently, th e SD and SH line showed the highest and lowest grain yields in all env ironments. The optimum height for grain yield was estimated to be c. 7 0 cm, and this value was not affected by sowing date. Lines carrying R ht1 and/or Rht2 alleles always showed more grains/m(2) (owing to an in creased number of grains per spike and spikes/m(2)) than the SH line. Conversely, average grain weight was negatively associated with the nu mber of grains/m(2). Because the slope of this negative relationship w as smaller(less negative) than that representing complete compensation , the relationship between grain yield and number of grains/m(2) was h yperbolic. Although these relationships are frequently regarded as a r eflection of increased competition among grains when the number of gra ins/m(2) is increased owing to the use of semi-dwarf genes, two altern ative hypotheses are analysed acid discussed.