GENETIC-IMPROVEMENT IN WHEAT YIELD AND ASSOCIATED TRAITS - A REEXAMINATION OF PREVIOUS RESULTS AND THE LATEST TRENDS

Two field experiments were carried out with seven wheat cultivars (thr ee of them, including a commercial hybrid, released during the last 10 years) representing different eras of plant breeding, to evaluate gen etic improvement over the last century in grain yield, height. biomass , harvest index and grain yield components. Plots were fertilized and irrigated, and lodging and diseases were prevented. Main culm height w as negatively correlated with the year of release of the cultivars; pr obably as a consequence of selection for increased lodging resistance. There was no significant association between total above-ground bioma ss and year of release of the cultivars. On the other hand, grain yiel d increased as newer cultivars were released. Results indicate that du ring recent years harvest index has been kept as the main attribute re sponsible for increases in grain yield. In general, number of grains/m (2) was associated with increases in grain yield during the century. H owever, the newest cultivars showed an increased grain weight. In both growing seasons, cultivars released before 1980 showed a trend toward s reduced grain weight, but cultivars released after 1987 had a simila r number of grains per m(2) with a higher grain weight than their pred ecessors. This was probably because the most modern cultivars have a l onger grain-filling duration with a similar length of growth cycle.