CHANGES IN PHYSIOLOGICAL TRAITS ASSOCIATED WITH GRAIN YIELD IMPROVEMENT IN SINGLE-CROSS MAIZE HYBRIDS FROM 1930 TO 1970

Four single-cross maize hybrids were selected to represent the germplasm used during each decade from the 1930s to the 1970s. These hybrids were grown at low, medium, and high plant densities in four environments. The patterns of change in machine-harvestable and total grain yield across decades of maize breeding were compared with changes in morphological and physiological traits to determine a basis for increased grain yield.;Significant linear increases in dry matter production across decades were observed at a seedling stage, 50% silk, and harvest. Increases in dry matter production were associated with complementary changes in leaf area and leaf orientation resulting in an increase in effective photosynthetic area across decades and with an increase in the photosynthetic period. Decades of hybrids did not differ significantly for carbon dioxide exchange rate.;Significant increases in total amount of dry matter partitioned to the grain were associated with faster rates of grain filling. Faster rates of grain filling were associated with a larger number of kernels per plant. Reductions in harvest index in association with increases in dry matter production accounted for observed decreases in lodging.;Ear-sink strength as characterized by kernel depth, weight, and number increased across decades. An increase in the number of plants with machine-harvestable ears was associated with increases in machine-harvestable grain yield.;The linear increase in machine-harvestable grain yield across decades was explained by complementary changes in total grain yield, lodging resistance and number of machine-harvestable ears. Significant increases in total grain yield occurred only between the 1930 and 1940 and the 1960 and 1970 decades. The increase in density tolerance and across decades observed in machine-harvestable grain yield was associated with similar changes in total grain yield, machine-harvestable and total ears and stalk lodging.;Increased density tolerance for both machine-harvestable and total grain yield across decades was associated with the development of hybrids which maintain greater ear-sink strength at high plant density.