Potential for improving nitrogen acquisition in pea

Field pea (Pisum sativum L.) fixes atmospheric nitrogen (N) by developing a symbiotic association with Rhizobia. Peas can satisfy most of their N requirements through the process of biological nitrogen fixation (BNF). Thus, incorporation of peas into cropping systems can increase farm profitability and long-term sustainability of Canadian agriculture by improving soil health and reducing fertilizer input costs. This dissertation focused on evaluating the BNF and agronomic performance of pea lines, previously generated by crossing pea nodulation mutants with pea cultivars, under Saskatchewan environments (Study I). Additionally, the BNF potential of 233 diverse pea accessions originating from various parts of the world was assessed and genome-wide association analysis was performed to identify marker-trait associations for N fixation and related traits (Study II). Finally, variation in seedling root system architecture (RSA) of diverse pea accessions was studied and its potential relationship with BNF and agronomic traits was explored (Study III). \nIn Study I, 20 pea breeding lines derived from crosses between two nodulation mutants (Rondo nod3 (super-nodulating mutant) and Frisson P88 Sym29 (hyper-nodulating mutant) and Saskatchewan adapted pea cultivars (CDC Meadow and CDC Dakota) were evaluated for BNF potential and agronomic performance at multiple locations in Saskatchewan. Two weeks after planting, 15N-labeled fertilizer was administered to determine %N derived from air (%Ndfa), using the 15N isotope dilution method. The %Ndfa of these lines ranged from 50 to 80%, which is greater than the previously reported range of 50-55% under Canadian environments. Several of the pea lines outperformed the parent cultivars CDC Meadow and CDC Dakota in terms of N fixing capacity, yield, and seed protein concentration. A positive correlation between %Ndfa and days to flowering (DTF), days to maturity (DTM) and seed protein concentration was observed.\nIn Study II, 233 diverse pea accessions were tested for their ability to fix N under Saskatchewan environments using the 15N natural abundance method. The shoot samples collected at two developmental stages, i.e., when the majority of the plots in the trial reached 50% flowering, and when the majority of the plots reached physiological maturity were used to measure %N and %Ndfa in the shoot samples through mass spectrometry. The phenotypic evaluation reflected wide genetic variation among the pea accessions for %Ndfa, %N and leaf chlorophyll content (SPAD). %Ndfa and %N in the pea accessions ranged widely from 1-75% and 1-4%, respectively, across the developmental stages and site-years. The SPAD values ranged from 3-58 CCI (chlorophyll content index) and showed positive correlation with yield (r = 0.46, p < 0.001). The accessions were genotyped using the genotyping-by-sequencing (GBS) method. A total of 28,102 high quality single nucleotide polymorphisms (SNPs) were selected to identify marker-trait associations. SNPs associated with %Ndfa, %N, and SPAD across four site-years were identified. Four SNPs and two SNPs for %Ndfa at the first and second developmental stages, respectively, along with four SNPs associated with SPAD were consistent at three out of four site-years. \nIn Study III, phenotypic variation for root system architecture (RSA) among 44 diverse pea genotypes, including breeding lines (from Study I) and germplasm accessions (from Study II), grown under controlled conditions for 14 days, was assessed using 2D hydroponic root imaging. Root image analysis revealed significant genotypic variability among the pea genotypes for all root traits namely, root length, root diameter, root volume, root surface area, number of root tips, root network width, root network depth, and root network convex area. Significant positive correlations ranging from 0.5 - 0.9 were observed among the evaluated root traits. Redundancy analysis (RDA) was performed to associate root traits with N fixation and field agronomic performance data collected in Study I and Study II. The quantified roots traits accounted for significant variation in the field traits (R2 = 0.3, p < 0.001). A significant association between field lodging susceptibility and seedling root system network width, and between field plant height and seedling root system network depth was observed. Seedling root diameter was positively associated with grain yield and N fixation in field trials. The findings from this study indicate that variation for seedling root traits in pea could aid selection for N fixation and other important agronomic traits. \nOverall, the findings of this research highlight the positive outcome of exploring pea germplasm diversity and pea breeding efforts for increased N fixation in field pea. Additionally, the evaluation of seedling RSA variation opens new avenues to understanding the prospective role of RSA in pea growth and adaptation under varying environments.

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