Role of yield genetic progress on the biochemical determinants of maize kernel hardness

Abstract

Maize kernel hardness is a key attribute for the global maize processing industry. Breeding for on-farm yield has decreased kernel hardness. However, the underlying changes in zeins and starch composition, mechanistically related to kernel hardness, are unknown. We evaluated changes in zein and starch composition of 32 commercial hybrids released from 1965 to 2016 in Argentina. Kernel protein concentration decreased (∼25%), but zein concentration decreased at higher rates (45%). Zein fractions, Z1 and Z2, decreased throughout the study period. However, the concentration of specific Z2 components decreased (C1 and E zein), while others increased (C2)with hybrid release year. Hardness descriptors all were positively correlated with C1 and E zein concentration and negatively with C2 concentration. Kernel starch and amylose concentrations increased with hybrid release year. No clear changes in amylose to starch ratio were apparent over time. Significant genotype differences in amylose to starch ratio were evident (p < 0.001), but these differences were not correlated to hardness changes. Yield improvement has indirectly changed kernel composition, affecting hardness. Our results limit the relevance of starch components to hardness changes, reinforcing the role of specific zein fractions as observed throughout 50 years of breeding.