Phenotypic Responses to 75 Years of Recurrent Selection for Yield in the Iowa State Stiff Stalk Synthetic Maize Population.

Modern era hybrid maize yield increases and perhaps changing maize yield paradigms could be facilitated by understanding the physiological and genetic bases for phenotypic changes in response to recurrent selection for yield. Phenotypic changes in hybrids across decades include upright leaves, fewer tassel branches, and reduced silking-anthesis interval. These changes have also been observed in the Iowa Stiff Stalk Synthetic, but have not been studied as indirect responses to recurrent selection. Nor has it been determined which of these traits may result from changes in functionally related genes. This experiment’s objective was to identify and quantify indirect selection responses for several morphological phenotypes and their interactions with plant density in the Iowa Stiff Stalk synthetic population. Material from an unselected base population, Iowa State Stiff Stalk Synthetic (BSSS), was compared to the most advanced cycles of selection from three different recurrent selection programs initiated from the same base population. Selection was based on an index of root lodging, stalk lodging, grain moisture, and grain yield, with primary emphasis on grain yield. The advanced cycles (17 selection cycles) and the base population were compared at four densities: 40,000, 60,000, 80,000, and 100,000 plants/ha. Phenotypes measured included leaf angle, leaf length, leaf width, tassel branch number, plant height, leaf heights, total number of leaves, and silk delay. Treatments were replicated twice at each location and were arranged as a split plot design with density as the whole plot and pedigree as the sub plot. Population density significantly affected plant height and total number of leaves. Leaf angle, plant height, leaf length, tassel branch number, total number of leaves, and silk delay changed significantly from the base population to advanced cycles. These results demonstrate that selection for high yield led to significant changes in several key phenotypic responses of the plant.