PREANTHESIS SHADING EFFECTS ON THE NUMBER OF GRAINS OF 3 BREAD WHEAT CULTIVARS OF DIFFERENT POTENTIAL NUMBER OF GRAINS

Past breeding programs for increased wheat grain yield have increased the number of grains per m2, associated with a higher spike dry weight at anthesis. Other workers have shown that shading during the preanthesis period strongly reduced the number of grains per m2 of the crop via reductions in the spike dry weight at anthesis. However, these effects have not been determined for cultivars of different potential number of grains per m2. A factorial field experiment was executed with three bread wheat cultivars released in 1920 (Klein Favorito), 1940 (Eureka FCS) and 1980 (Buck Pucara) in Argentina and two shading treatments (shading to 50% of the incoming solar radiation from the beginning of stem elongation to heading or no shading). Both cultivars and shading significantly affected the number of grains per m2 , but their interaction was not significant. Cultivars differed only in number of grains per spikelet, while shading reduced both the number of spikes per m2 and the number of grains per spike. The lack of a significant interaction indicates that the absolute reduction in grain number was similar for all cultivars, but the relative effect of shading was much greater for the old (low number of grains) cultivar than the modem one (62 and 42%, respectively). Preanthesis shading reduced number of grains per m2 on main shoots less (45%) than on tillers (65%). Therefore, the relative contributions of these shoot categories to total number of grains per m2 were strongly modified by shading. An evaluation of published data with conflicting findings on this shading effect suggests that the degree of modification observed has been related to the proportions of tiller and main shoot spikes: