Genetics of destemming in pepper: A step towards mechanical harvesting

Abstract

Introduction: The majority of peppers in the US for fresh market and processing are handpicked, and harvesting can account for 20–50% of production costs. Innovation in mechanical harvesting would increase availability; lower the costs of local, healthy vegetable products; and perhaps improve food safety and expand markets. Most processed peppers require removal of pedicels (stem and calyx) from the fruit, but lack of an efficient mechanical process for this operation has hindered adoption of mechanical harvest. In this paper, we present characterization and advancements in breeding green chile peppers for mechanical harvesting. Specifically, we describe inheritance and expression of an easy-destemming trait derived from the landrace UCD-14 that facilitates machine harvest of green chiles.Methods: A torque gauge was used for measuring bending forces similar to those of a harvester and applied to two biparental populations segregating for destemming force and rate. Genotyping by sequencing was used to generate genetic maps for quantitative trait locus (QTL) analyses.Results: A major destemming QTL was found on chromosome 10 across populations and environments. Eight additional population and/or environment-specific QTL were also identified. Chromosome 10 QTL markers were used to help introgress the destemming trait into jalapeño-type peppers. Low destemming force lines combined with improvements in transplant production enabled mechanical harvest of destemmed fruit at a rate of 41% versus 2% with a commercial jalapeńo hybrid. Staining for the presence of lignin at the pedicel/fruit boundary indicated the presence of an abscission zone and homologs of genes known to affect organ abscission were found under several QTL, suggesting that the easy-destemming trait may be due to the presence and activation of a pedicel/fruit abscission zone.Conclusion: Presented here are tools to measure the easy-destemming trait, its physiological basis, possible molecular pathways, and expression of the trait in various genetic backgrounds. Mechanical harvest of destemmed mature green chile fruits was achieved by combining easy-destemming with transplant management.

Identification of QTLs involved in destemming and fruit quality for mechanical harvesting of New Mexico pod–type green chile

“… Physical map of QTLs reported in this study from the three biparental populations ‘NuMex Iliad’ and MUC14 (red), ‘NuMex Odyssey’ and MUC14 (blue), and ‘NuMex Joe E. Parker’ and MUC14 (yellow). QTLs previously reported from Hill et al. (2023) (purple), and genes of interest related to fruit size or abscission (black) are also shown.…”

“…One other study has searched for destemming force QTLs in chile pepper. Studying populations that came from the easy destemming Mexican landrace UCD-14, Hill et al. (2023) reported destemming force QTLs on chromosomes 2, 3, 4, 9, 10, and 11.…”

“…A representation of overall fruit size, fruit size index, was recorded as fruit width times fruit length. To measure the destemming force, a Tohnichi torque gauge (Tohnichi America Corporation, Buffalo Grove, IL, United States) with a custom metal fork attachment that enables the removal of a pedicel with a twisting motion, meant to resemble the motion of the harvester, was used ( Hill et al., 2023 ). At the same time, a subjective rating of destemming efficiency, called “destemming score,” was taken, where the measurer rated the quality of the fruit after destemming.…”

“…(2011) , with some modifications. Instead of using Ape KI, the restriction enzyme Mly I was used ( Hill et al., 2023 ). Sequencing was done on an Illumina HS4000 in pools of 96 at the UC Davis Genome Center (Davis, California).…”

“…This is currently the only commercially available green chile New Mexico pod–type variety that has been developed with mechanical harvest in mind. With regard to the genetics of stem removal, a quantitative trait locus (QTL) study was recently conducted for two populations derived from a semi-domesticated pepper with a novel easy destemming trait called UCD-14, revealing several QTLs with a large effect ( Hill et al., 2023 ). Studies such as this can be a step toward breeding through marker-assisted selection, an expedited breeding method in which individuals of a population are screened and selected on the basis of the presence of genes that confer traits of interest.…”