The economic value of Brassica oilseed crops, particularly Brassica napus (canola), is primarily attributed to their final grain yield, which is essential for various industries, including food, feed, and biofuel. The primary challenge in canola breeding is the requirement to select for quality and disease resistance traits alongside yield in a range of different target environments. However, direct selection for grain yield in canola breeding programs is challenging due to: first, its complex quantitative nature and the fact that yield evaluation occurs at the end of the growth cycle, and second, lack of enough seeds during early generations for conducting evaluation trials. Unpredictable factors such as environmental stress or improper management practices can adversely impact the assessment of yield. Therefore, enhancing the efficiency of selection and the rate of genetic gain for yield through indirect selection is critical.
Vegetative vigour is considered a potential proxy for yield evaluations in canola breeding programs, which can provide an opportunity to enhance genetic gain for yield. Vigour in canola is a foundational trait that supports co-selection for yield, quality, and disease resistance since it enhances growth, improving stress resilience facilitating the expression of traits across diverse environments. This makes vegetative vigour a valuable component in multi-trait breeding strategies. However, it is also a complex phenotype, and conventional methods of its evaluation in canola are limited and sometimes inaccurate. This thesis investigated the potential of quantifying vegetative vigour using new traits (vigour components) from as early as the sixth-leaf stage (early vigour) to the peak of the vegetative phase and before flowering in canola (stem elongation).
In this thesis, by conducting one glasshouse and three field trials, I measured a total of 21 diverse traits, including vegetative traits, flowering time and grain yield, encompassing the entirety of canola’s life cycle. These traits were measured on three phenotyping levels including seedling (six traits), a single leaf (i.e. the fourth rosette leaf; 11 traits), and agronomic plots (four traits). A diversity panel comprised of 300 canola lines from the Australian B. napus Homozygous Diversity Set (ABnHDS), which represents the global genetic diversity of canola, was used for my trials. This germplasm was genotyped previously with 12,413 markers from the Brassica 60K Illumina Infinium SNP array. The broad- and narrow-sense heritability was estimated for all the traits. The phenotypic and genetic correlations among the traits were investigated in a global analysis (for revealing their connections across the entire life cycle of canola) and also between field and glasshouse (for testing the usefulness of glasshouse measurements for predicting vigour, flowering time and grain yield in field). Additionally, I examined the genetic architecture of these traits using GWAS with one single-locus and two multi-locus models.
My results showed that several leaf and seedling characteristics demonstrated higher heritability estimates than the common methods of vigour assessment (i.e. measuring seedling total biomass and NDVI). These characteristics were also highly correlated with the common methods of vigour assessment, which demonstrates their usefulness as alternative methods for evaluating vegetative vigour in canola. Using global correlation analyses, it was shown that several components of early vigour can predict vigour throughout canola’s life cycle, such as specific leaf area and leaf dry matter content. Seeding components at stem elongation (leaf mass fraction and total leaf dry mass), as well as early vigour components (aspect ratio, and leaf roundness), were also highly correlated with flowering time. However, correlations between vigour components and grain yield were weaker, with the highest found for petiole length. I also demonstrated that measuring early vigour components in the glasshouse can predict vigour and flowering time under field conditions, though the prediction for grain yield was less strong. Additionally, several common SNPs were identified across different traits, with some consistently associated with the traits across multiple environments. By searching the flanking regions of the significant SNPs, several candidate genes were proposed for further downstream validation analyses in reverse genetics.
Overall, in this thesis, I provided a comprehensive data-driven evidence for the usefulness of leaf and seedling characteristics for quantifying vegetative vigour in canola. My results suggest that new potentials exist for quantifying vegetative vigour by its components in canola, which can be used to enhance the efficiency of selection for vigour and flowering time and to a lesser degree, for grain yield. My results create the opportunity for future studies to potentially enhance the rate of genetic gain.