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 preanth esis period strongly reduced the number of grains per m2 of the crop v ia reductions in the spike dry weight at anthesis. However, these effe cts have not been determined for cultivars of different potential numb er of grains per m2. A factorial field experiment was executed with th ree bread wheat cultivars released in 1920 (Klein Favorito), 1940 (Eur eka FCS) and 1980 (Buck Pucara) in Argentina and two shading treatment s (shading to 50% of the incoming solar radiation from the beginning o f stem elongation to heading or no shading). Both cultivars and shadin g significantly affected the number of grains per m2 , but their inter action was not significant. Cultivars differed only in number of grain s per spikelet, while shading reduced both the number of spikes per m2 and the number of grains per spike. The lack of a significant interac tion indicates that the absolute reduction in grain number was similar for all cultivars, but the relative effect of shading was much greate r for the old (low number of grains) cultivar than the modem one (62 a nd 42%, respectively). Preanthesis shading reduced number of grains pe r m2 on main shoots less (45%) than on tillers (65%). Therefore, the r elative contributions of these shoot categories to total number of gra ins per m2 were strongly modified by shading. An evaluation of publish ed data with conflicting findings on this shading effect suggests that the degree of modification observed has been related to the proportio ns of tiller and main shoot spikes: