Final Selection of Quality Protein Popcorn Hybrids

Abstract

Quality Protein Popcorn (QPP) BC₂F₅ inbred lines were produced through an interpopulation breeding system between Quality Protein Maize dent (QPM) and elite popcorn germplasm. In 2019, five QPP F₁ hybrids were selected for further evaluation due to superior agronomics, endosperm protein quality, and popping quality traits. Though these BC₂F₅ QPP hybrids were phenotypically similar to their popcorn parents, the QPP cultivars conveyed slightly inferior popping characteristics when compared to the original popcorn germplasm. The objective of this study was twofold. First, BC₂F₅ inbred lines were crossed to their popcorn parents and BC₃F₄ inbred lines were produced for hybridization to test the agronomic, protein, and popping trait effects from an additional QPP by popcorn backcross. Second, BC₂- and BC₃-hybrids were simultaneously evaluated alongside ConAgra Brands^® elite cultivars and ranked for potential commercialization in the spring of 2020. These 10 QPP hybrids were grown alongside five ConAgra Brands^® elite popcorn cultivars in three locations and agronomic, protein quality, and popping quality traits were evaluated. Significant improvements in popcorn quality traits were observed in the QPP BC₃ cultivars compared to their BC₂ counterparts, and yield averages were significantly lower in BC₃-derived QPP hybrids compared to the BC₂ population. Protein quality traits were not significantly different between QPP backcrossing populations and significantly superior to ConAgra elite popcorn varieties. Utilizing a previously published ranking system, six QPP hybrids, three from the BC₂F₅ population and three from the BC₃F₄ population, were evaluated as candidates for final selection. The successful evaluation and ranking system methodology employed is transferable to other hybrid production and testing programs. Incorporating this analysis with concurrent sensory studies, two QPP hybrids were chosen as premier cultivars for potential commercialization.

Keywords: hybrid-analysis; index-selection; maize-breeding; opaque-2; popcorn.

FIGURE 1

Breeding scheme to produce BC₂F₅ and BC₃F₄ QPP F₁ hybrids. Overall breeding scheme from 2018 to 2020. (A) In the summer of 2018, BC₂F₅ QPP inbreds were crossed in full diallel to produce F₁ hybrids. BC₂F₅ inbreds were also selectively crossed to their respective original popcorn parents to produce heterozygous O₂o₂ BC₃F₁ offspring. (B) Heterozygotes were self-pollinated to produce segregating BC₃F₂ offspring which was selected at the seed based on opaque-2 phenotyping of vitreousness, protein-profiling, and later marker-assisted selection. (C) Homozygous F₂ seed was grown and self-pollinated prior to 2019 summer. (D) Homozygous mutant o₂o₂ BC₃F₃ seed was harvested and grown to produce BC₃F₄ QPP seed in the summer of 2019. All inbred lines were identified as o2-carrying predominantly through protein-profiling. (E) BC₂F₅ and BC₃F₄ QPP inbred lines were grown in the spring of 2020 and selectively crossed to produce similar QPP hybrids of differing backcross generations. (F) BC₂F₅- and BC₃F₄-derived F₁ hybrids were grown in three locations and evaluated alongside ConAgra elite varieties for selection.

FIGURE 2

Protein profiling of QPP and ConAgra elite lines. (A) Principle component analysis (PCA) of protein-bound amino acids in raw flour of ConAgra elite lines, B73 for reference, and QPP hybrids revealed a distinct segregation between QPP and original popcorn-derived cultivars. B73 grouped with ConAgra lines H12, H13, and H14, while H11 and H15 independently segregated with QPP H2. QPP H2 had a distinct proteome compared to all other QPP hybrids and similar lysine levels compared to ConAgra lines. (B) Protein-bound lysine in raw flour of all genotypes revealed significantly higher lysine levels in all QPP lines except H2. (C) Zein extraction and SDS-PAGE analysis of randomly selected kernels revealed a significant reduction in 22-kD-alpha zein, varying production of 19-kD-alpha zein, and increased expression of the 27-kD γ-zein in QPP lines compared to ConAgra lines, consistent with a homozygous opaque-2 profile. Compositely, these results verified the successful introgression and stabilization of the homozygous mutation in the BC₂F₅ and BC₃F₄ QPP populations.

FIGURE 3

Flake morphology assessment of QPP and ConAgra elite lines. Random samples of QPP and ConAgra lines from each experimental plot were given a description of butterfly (B—blue), mushroom (M—red), or mixed flake morphology (MX—white). Each cultivar was assigned a total of nine descriptions (three from each of the three locations). All ConAgra varieties were assigned butterfly morphology except H15, which was assigned three MX morphologies. QPP BC₂-derived hybrids are displayed on the first row, respective QPP BC₃-derived hybrids are on the second row, and original popcorn hybrids from the respective pedigrees are arranged on the third row to enable column comparison between similar QPP and ConAgra pedigrees. Commercial lines H14 and H15 are positioned on the fourth row.

FIGURE 4

2020 Ranking System Selection Index Results. Utilization of the 2020 Ranking System enabled a visual display of overall cultivar ranking from best to worst, left to right, respectively. Color by variable identified individual hybrid pitfalls (the longer the stacked column, the farther from the best hybrid) and high trait values. H13, PP1 x PP3, ranked highest out of all hybrids, scoring relatively lower only because of its lack of protein-bound lysine content. QPP H10 ranked the best compared to all other QPP lines and ranked higher than H14, or Commercial Line 1. QPP BC₂-derived hybrids H3, H4, and H1 ranked, respectively, higher than the rest, while BC₃-derived hybrids H6, H9, and H7 all ranked higher than original popcorn hybrid line H12. H8, H5, and H2 ranked lowest out of all hybrids. Economic weights for each trait were determined to reflect consumer and producer interests in a ready-to-eat Quality Protein Popcorn product. Lysine levels, yield, and expansion volume were considered equally important traits while ranking all hybrids.