Genetic changes in durum wheat yield components and associated traits in Italian and Spanish varieties during the 20th century

Twelve field experiments comparing 24 durum wheat varieties from three periods-old (< 1945), intermediate (1950-1985) and modern (1988-2000)-were carried out in order to ascertain the advances made in durum wheat yield components and related traits in Italian and Spanish germplasm. Grain yield improvements were based on linear increases in the number of grains per m(2) and harvest index, while grain weight and biomass remained unchanged. Yield per plant increased at a rate of 0.36 and 0.44% y(-1) and the number of grains per m(2) improved by 39% and 55% in Italian and Spanish varieties, respectively. The mean rate of increase in the number of grains per m(2) was 0.55% y(-1). Plants per m(2), spikes per plant and grains per spike contributed 20%, 29% and 51%, respectively, to the increase in the number of grains per m(2). The enhance of the number of grains per m(2) was due to the greater grain set in the modern varieties, since the number of spikelets per spike remained unchanged. Harvest index increased overall by 0.48% y(-1) (0.40 and 0.53% y(-1) in Italian and Spanish varieties, respectively). Plant height was the trait that suffered the most dramatic changes (it decreased at a rate of -0.81% y(-1), with little difference between the varieties of the two countries), as consequence of the presence of the Rht-B1 dwarfing gene. Harvest index and plant height, which were the traits that most contributed to discriminating between periods, remained unchanged from 1980 to 2000. The higher rates of improvement in Spain are discussed in the context of the contrasting strategies followed to improve durum wheat yield in the two countries.