PHYSIOLOGICAL ATTRIBUTES RELATED TO THE GENERATION OF GRAIN-YIELD IN BREAD WHEAT CULTIVARS RELEASED AT DIFFERENT ERAS

Several authors have concluded that the main physiological changes produced by the genetic improvement of bread wheat grain yield were an increased harvest index associated with an increased number of grains. However, the underlying causes of these changes have been less considered, especially regarding crop-based measurements. Two irrigated field experiments were carried out to study the accumulation and partitioning of the dry matter and the associated generation of number of grains per m2 and grain yield of three bread wheat cultivars released in 1920 (Klein Favorito), 1940 (Eureka FCS) and 1980 (Buck Pucara) in Argentina. Grain yield of the modem cultivar was higher than that of the intermediate and old cultivar (755, 585 and 460 g m-2; mean of the two years, respectively). These yields were closely associated with the number of grains per m2 (30 150, 22 115 and 16 665; mean of the two years, respectively), confirming previous results. These differences in number of grains per m2 were more associated with differences in number of grains per spikelet than with differences in any other component. At anthesis the cultivars had accumulated similar above-ground biomass, but the proportion partitioned to spikes was higher as the year of release increased. The number of fertile florets produced by Buck Pucara was greater than those produced by other cultivars, which did not differ in this trait. The grain set percentage was the cause of the differences in number of grains per m2 between Eureka FCS and Klein Favorito. The number of fertile florets was related to spike dry weight at heading rather than at anthesis, emphasizing the period of floret primordia death as crucial for the generation of a higher number of fertile florets. The differences in the fate of florets among cultivars were related to spike growth during a short period (approximately 25 days) before anthesis. Spikes of Buck Pucara grew faster than those of the other cultivars from approximately 15 days before anthesis, while the spikes of Eureka FCS grew faster than those of Klein Favorito only in the final period of spike growth, between heading and anthesis. These differences will still more evident in the spike to stem dry weight ratio. The major reason for the increased ability of the modern cultivar to partitioning more biomass to spikes was that its spikes began their growth earlier when the stems were lighter than those of the old cultivar, leading to differences in their competitive ability for obtaining photoassimilates.