Date of Graduation


Document Type


Degree Name

Master of Science in Horticulture (MS)

Degree Level





Margaret Worthington

Committee Member

John Clark

Second Committee Member

Jacquelyn Lee

Third Committee Member

Ryan Dickson

Fourth Committee Member

Martin Egan


Blackberry, Plant Breeding, Rubus


The fresh-market blackberry (Rubus L. subgenus Rubus Watson) industry has been expanding for the past two decades. Blackberry market growth can be attributed to many factors including expanded production regions, improved production systems, and release of superior cultivars. Blackberry breeding is a time- and labor-intensive process which would benefit from implementation of molecular markers for highly heritable, important traits. Next generation sequencing, a high-quality reference genome, and software capable of analyzing this complex genome were applied in a genome-wide association study (GWAS) to reveal maker-trait associations related to prickles and plant height variation in fresh-market blackberry. In 2020 and 2021, 374 and 267 blackberry breeding selections and released cultivars of blackberry were surveyed for prickles and plant height, respectively. A single locus was found for prickles on chromosome four at 30.48 to 34.31 Mb. Two QTL associated with plant height were discovered on chromosomes four and six and physical locations were at 25.95 to 26.53 Mb and 13.94 to 22.40 Mb, correspondingly. Possible candidate genes were determined using locus-specific linkage disequilibrium calculations 5 Mb around the peak marker and searches for genes in pathways related to the traits of interest. Five possible candidate genes were found for prickles: a squamosa promoter-binding protein-like domain 6 (SPL6), a MYB domain protein 16 (MYB16), an agamous-like MADS-box protein 30 (AGL30), a homeobox-leucine zipper protein IV (HOX3), and a trichome birefringence-like 27 protein (TBL27). Three possible candidate genes were found for plant height near the QTL on chromosome four which were a gibberellin 2-oxidase 6 (GA2ox6) and two DELLA protein repressor-of-GA-like 1 (RGL1). On chromosome six, two 2-oxoglutarate-dependent dioxygenase and Fe(II)-dependent oxygenase (GA20ox1) genes were found within the QTL region. A project on vegetative responses of three blackberry cultivars with unique architecture attributes to exogenous gibberellic acid (GA3) and prohexadione calcium (P-Ca) applications was conducted to complement the plant height portion of the GWAS. The plant growth regulators (PGRs) were applied to ‘Natchez’ (typical height, typical internodes), ‘Sweet-Ark® Ponca’ (typical height, shortened internodes), and Baby Cakes® (reduced-stature and shortened internodes). The experiment was conducted in parallel at two locations (Arkansas and North Carolina) in the summer of 2021. Results of this study were complex. Cultivar main effects were common, and supported the varying architectural attributes of cultivars used in the experiment. The most consistent effect of GA3 observed in this study was an increase in the number and size of lateral branches. In Arkansas all cultivars responded similarly to GA3 treatments with increased number, length, and biomass of laterals as well as increased total stem biomass and total biomass of all tissue types. In North Carolina the lateral biomass of all cultivars was increased by GA3 treatments, but only ‘Natchez’ had increased lateral number and only ‘Natchez’ and ‘Ponca’ had longer laterals on GA3 treated plants. P-Ca applications reduced internode length and stunted primocane height at both locations and on all cultivars. However, in instances where cultivar by P-Ca rate interactions were significant, the effect of P-Ca was strongest in ‘Natchez’ and weakest in ‘Baby Cakes’. Exogenous GA3 and P-Ca may have applications on managing plant height in dwarf and semi-dwarf blackberries, but these results suggest that their responses to PGRs will be subtler than in standard height cultivars. Results indicated that P-Ca is indeed effective for managing vigor of blackberry plants, although effects on yield are unclear and must be researched in-depth to determine viability. Exogenous GA3 applications indicated that UA dwarf blackberry plants are likely somewhat GA-insensitive because applications did not fully recover height in the dwarf cultivar tested. The results of the PGR study also suggest that the positive effect of GA3 on lateral growth and development may be useful in mitigating unintended effects of P-Ca that previous research has indicated could be limiting.