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Mechanical threshing used when preparing maize seeds for planting subjects seed to damage and reduces seed quality. The purpose of this study was to assess the effect of mechanical threshing on the quality of maize seed threshed at different moisture contents (MCs). Seeds of dent maize JK968 and flint maize DD2 were threshed at 12, 15, 18, 21, and 24% MC. The damage degree was determined by iodine staining, and seed vigor was assessed by standard germination (SG), cold test germination (CTG), accelerated aging germination (AAG), seedling emergence rate, and seedling root and shoot length. The results showed that the damage percentage increased, and the seed vigor parameters decreased with increasing seed MC during threshing in both cultivars. For obtaining high seed quality, indicated by at least 90, 85, and 80% of SG, AAG, and CTG, respectively, JK 968 and DD2 should be threshed at MC lower than 15 and 18%, respectively. Furthermore, the damage mainly occurred in the apical part of seeds, irrespective of the threshing MC in both cultivars. When the embryo was damaged, seedling emergence rates were significantly reduced with great influence on shoot length. Damage to the endosperm resulted in little effect on seedling performance. Flint maize DD2 was more tolerant to mechanical threshing than dent maize JK968. These results provided technical reference for the production and processing of high vigor maize seeds.

Improving seed vigor in response to cold stress is an important breeding objective in maize that allows early sowing. Using two cold tolerant inbred lines 220 and P9-10 and two susceptible lines Y1518 and PH4CV, three connected F2:3 populations were generated for detecting quantitative trait locus (QTL) related to seed low-temperature germination ability. At 10°C, two germination traits (emergence rate and germination index) were collected from a sand bed and three seedling traits (seedling root length, shoot length, and total length) were extracted from paper rolls. Significant correlations were found among all traits in all populations. Via single-population analysis, 43 QTL were detected with explained phenotypic variance of 0.62%∼39.44%. Seventeen QTL explained more than 10% phenotypic variance; of them sixteen (94.12%) inherited favorable alleles from the tolerant lines. After constructing a consensus map, three meta-QTL (mQTL) were identified to include at least two initial QTL from different populations. mQTL1-1 included seven initial QTL for both germination and seedling traits; with three explaining more than 30% phenotypic variance. mQTL2-1 and mQTL9-1 covered two to three initial QTL. The favorable alleles of the QTL within these three mQTL regions were all inherited from the tolerant line 220 and P9-10. These results provided a basis for cloning of genes underlying the mQTL regions to uncover the molecular mechanisms of maize cold tolerance during germination.