Browsing by Author "Komahan, Dona Harshani Shanika"

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    Towards better New Zealand adapted industrial hemp (Cannabis sativa L.) cultivars : a quantitative genetic analysis of key traits and evaluation of genetic diversity : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Manawatu, New Zealand
    (Massey Library, 2023-09-12) Komahan, Dona Harshani Shanika
    Hemp (Cannabis sativa L.) is emerging as a promising commercial crop in New Zealand. Breeding programmes developing material specifically adapted to the country’s target population of environments are at their early stages of establishment. With 20 approved hemp cultivars, 15 of which are imported, it is vital to prioritize research and development focused on the genetic improvement of hemp to generate cultivars that are well adapted to New Zealand’s crop production environments, offering both economic and agronomic benefits. The primary aim of this thesis is to lay the foundation for future hemp breeding in New Zealand by focusing on three critical initial steps: (1) conducting multi-site trials to evaluate the performance of commercial hemp cultivars based on key traits and characterize the effects of Genotype × Environment interaction (G × E interaction), (2) estimating quantitative genetic parameters and predicting genetic gain for key traits to identify breeding strategies that would improve the efficiency of maximizing genetic gain, and (3) assessing genetic diversity using molecular markers and characterizing available genetic resources. These steps are crucial for developing new cultivars that are both climate-resilient and beneficial to New Zealand’s hemp industry. Multi-site trials were conducted for two years over the 2019/2020 and 2020/2021 growing seasons using six hemp cultivars approved to be grown in New Zealand: CFX-2, CRS-1, Ferimon 12, Katani, Futura 75, and Finola. The 2019/2020 trials were conducted in Palmerston North and Wairarapa, while the 2020/2021 trials were conducted at two nearby sites in Palmerston North with different soil characteristics. Across both multi-site trials, biomass yields extend over a broad range from 1.43 to 28.41 t/ha, and seed yields ranged between 0.018 and 3.78 t/ha. The study’s findings on G × E interactions showed a complex scenario that mostly differed from prior reported research. Although significant (P < 0.05) genotypic variation was observed in most traits, only a few, specifically stand establishment in the 2019/2020 season, along with plant height and stem diameter in the 2020/2021 growing season and biomass yield across both years, showed significant (P < 0.05) G × E interactions, suggesting consistent performance across sites. The traits plant height, stem diameter, biomass yield, seed yield, and thousand seed weight have all revealed high cultivar mean broad sense heritability, indicating potential exploitable underlaying genetic variation that could be used to generate breeding pools for cultivar development. A quantitative genetic analysis was conducted using 50 half-sib families (HS families) focused on six key traits: stem diameter, plant height, number of internodes, seed yield, thousand seed weight, and biomass yield to investigate the magnitude of additive genetic variation within a breeding population. The study also aimed to predict the rate of genetic gain for these traits. Significant (P < 0.05) additive genetic variation was observed among the 50 HS families for traits: seed yield, biomass yield, stem diameter, plant height, and the number of internodes, highlighting each trait’s additive genetic variation available for breeding. Deterministic simulation of the breeding strategies, among HS family selection (AF-HS) and among- and within-family selection (AWF-HS), was conducted to predict genetic gain for these traits. AWF-HS yielded the highest genetic gains compared to AF-HS. The study estimated moderate to strong positive genetic correlations among all traits, generally higher than the phenotypic estimates. The genetic correlation coefficients suggest that genotypes with higher seed and biomass yields can be developed. There was significant (P < 0.05) differences between some HS families and the two commercial check cultivars, Fasamo and Férimon 12. HS families outperforming these commercial checks were identified. A laboratory experiment examined the genetic diversity and the degree of variability within and among six New Zealand-approved hemp cultivars using seven previously developed microsatellite (SSR) markers. STRUCTURE analysis identified two distinct genetic clusters that align with the plant’s reproductive biology (monoecious and dioecious), that further subdivided based on end-use (fibre, seed, fibre and seed, and CBD cultivars). The genetic diversity metrics revealed moderate genetic diversity among the six hemp cultivars. The average observed heterozygosity (Ho) and the expected heterozygosity (He) were 0.44 and 0.50, respectively. The Wright’s fixation index (FIS) varied from 0.26 (Futura 75) to -0.02 (Finola). The Analysis of Molecular Variance (AMOVA) revealed that a large portion of the total genetic diversity was found within individual cultivars. While only 11% of the molecular variation was attributed to differences among the cultivars, 19% was attributed to variations among individuals within each cultivar. Notably, the genetic variation within individual plants (69%) exceeded the variation observed both among individuals and among different cultivars. Additionally, considerable molecular variation was observed between male and female individuals of dioecious hemp cultivars. The genetic diversity metrics of female and male groups indicated that female individuals possess greater variation than their male counterparts. Collectively, results from this Ph.D. study lay the groundwork for future hemp breeding research and development in New Zealand. They provide an initial resource, offering insights into genetic diversity, genotypic variation, and the impact of G × E interactions on key traits evaluated among six introduced offshore cultivars. Furthermore, the estimates of quantitative genetic parameters add value to the existing knowledge base, enhancing the development of hemp cultivars suited to New Zealand crop production environments.