Evaluation of Shoot-Growth Variation in Diverse Global Carrot (Daucus carota L.) Germplasm for Improvement of Stand Establishment

Carrot (Daucus carota L.) is a high value, nutritious, and colorful crop, but delivering carrots from seed to table can be a struggle for carrot growers. Weed competitive ability is a critical trait for crop success that carrot and its apiaceous relatives often lack owing to their characteristic slow shoot growth and erratic seedling emergence, even among genetically uniform lines. This study is the first field-based, multi-year experiment to evaluate shoot-growth trait variation over a 100-day growing season in a carrot diversity panel (N=695) that includes genetically diverse carrot accessions from the United States Department of Agriculture National Plant Germplasm System. This study provides the first broad-sense heritability estimates for early-season seedling vigor (0.68< H²< 0.80) and early-season canopy coverage (H² = 0.65) on a diverse collection. We report higher H² values for canopy coverage than in previous studies, suggesting identification of accessions with strong genetic potential for trait improvement. Late-season canopy height and harvest-day canopy height also demonstrated high broad-sense heritability (0.76< H²< 0.82). We developed a standardized vocabulary and protocol to describe shoot-growth and facilitate collaboration and communication across carrot research groups. In addition to developing methodology on a large collection of genetically representative germplasm, we report phenotypic variability for each trait in the carrot ontology, which we provide for breeders and researchers interested in evaluating shoot growth variation in their programs. Our study facilitates identification and utilization of carrot genetic resources, conservation of agrobiodiversity, and development of breeding stocks for weed-competitive ability, with the long-term goal of delivering improved carrot cultivars to breeders, growers, and consumers. Accessions with genetic attributes for high emergence and vigorous canopy growth are of immediate use to breeders targeting weed-tolerant or weed-suppressant carrots, which is of particular advantage to the organic carrot production sector, reducing the costs and labor associated with herbicide application and weeding. This study sets the stage for multi-environmental trialing of growth attributes in this carrot diversity panel, comparison of heritability among collections, and validation of accessions across environments. Accession selection can be further optimized for efficient breeding by combining shoot growth data with phenological data in this study’s companion paper to identify ideotypes based on global market needs.