Old and modern durum wheat varieties from Italy and Spain differ in main spike components

Four field experiments comparing 24 durum wheat varieties grown at different periods during the 20th century in Italy and Spain were carried out to assess the changes caused by breeding activities on the number of grains per main spike and its determinants: number of spikelets per spike, number of grains per spikelet, fertile flowering and grain setting. Increases of 0.14grainsspike-¹ year-¹ (0.43%year-¹ in relative terms) and 0.08grainsspike-¹ year-¹ (0.22%year-¹) were observed in Italian and Spanish varieties, respectively. The overall change in the number of grains per spike in Italian germplasm (29.5%) was due to increases in both, the number of spikelets per spike (7.5%) and the number of grains per spikelet (20.3%), while in Spanish varieties the increase in the number of grains per spike (19.5%) was only attributed to the improvement of the number of grains per spikelet. The increase in the number of fertile florets per spike (about 12%) was similar in both countries, but while it explained more than 70% of the changes in the number of grains per spike in Spanish varieties, grain setting was responsible for most of the improvement in the number of grains per spike in the Italian germplasm. The percentage of florets setting grains was 68 and 64% in modern Italian and Spanish varieties, respectively. Most of the changes in the number of grains per spikelet were found in the upper part of the spike on Italian varieties, whilst they were more evenly distributed in the Spanish ones. The main achievement derived from the introduction of the Rht-B1 dwarfing gene was an increase in the number of grains per spikelet, but it did not have any effect on the number of spikelets on the main spike. The lack of genetic associations between grain setting and both the number of spikelets per spike and the number of fertile florets per spike suggests that future yield gains may be obtained through increases in the three components independently.