Improvement of Iranian Wheat Cultivars Bred During 1956-1995 in Relation to Wild Oat Competition

To evaluate the genetic improvement in yield of wheat (triticum aestivum) bred between 1956 and 1995, a field experiment was carried out in arak, iran, during the 2001-2002 growing season. six wheat cultivars, from different years of release (omid 1956, bezostaya 1969, azadi 1979, ghods 1989, alamot 1995 and alvand 1995) were sown with and without competition from wild oat {aven fatua). the results showed a trend to increasing yield at later release dates with a rate of 2.2 % yr1 and 3.8% yr1 under weed free and weedy conditions, respectively. the greater yield increase obtained in weedy conditions indicates an improvement in the potential competitive abilities of new cultivars. harvest index (hi) and biomass were highest in new cultivars. increased hi could explain 74% of the gain in yield, whereas, the remaining 26% of yield improvement was attributed to increases in biomass. there were no differences between cultivars in extinction coefficients (k) and radiation use efficiency (rue). however, rue was reduced significantly in weedy conditions. rue increased with the year of release by 0.77% yr1 and 0.14% yr1 under weed free and weedy conditions, respectively. average rue values of of 1.3 g mj1 (in terms of absorbed photosynthetically active radiation, apar) and 0.83 g mj-1 were estimated in weed free and weedy conditions, respectively. leaf area index (lai) was the only component that decreased as year of release increased.