Comparative assessment of floral bud morphological differences among three eucalyptus species and their potential relationship on hybrid pollination

As a globally significant economic timber species, Eucalyptus hybridization plays a crucial role in germplasm improvement. However, China’ s dominant cultivated timber species suffer from weak adaptability and poor wind resistance, necessitating urgent enhancement through hybridization. This study focused on Eucalyptus pellita, E. tereticornis, and E. urophylla, systematically analyzing eight quantitative traits across 240 floral bud sections. By integrating principal component analysis (PCA), correlation analysis, and cluster analysis, we revealed interspecific differences in floral bud morphology at the micro-physiological level and their associations with genetic diversity and hybridization efficacy. (1) The variation coefficients of floral traits in E. tereticornis were notably high, with stylar length, stylar width, ovule length, bud length, and bud width reaching 23.37%, 40.11%, 23.15%, 26.79%, and 19.03% respectively, indicating potentially high genetic diversity. (2) See Fig. 2, E. pellita exhibited the highest mean values across all eight quantitative traits, while E. tereticornis consistently ranked lowest in most traits (except style length). E. urophylla showed intermediate values, except for having the shortest style. (3) Principal Component Analysis revealed that the first two principal components cumulatively explained 78.40% of the total variance. Bud width, ovule length, stylar orifice width, and style length were identified as key discriminative traits effectively representing major phenotypic variations among these three Eucalyptus species. (4) There was a highly significant positive correlation between most of the variables (P < 0.001), such as bud width and average bud wall thickness (r = 0.84), stylar orifice width and style width (r = 0.77), ovule length and bud length (r = 0.74). (5) See Fig. 6, the cluster analysis revealed four distinct groups at a Euclidean distance threshold of 4. The study showed that most of the flower bud traits of the closely related E. urophylla and E. pellita did not differ significantly; however, the differences between them and the distantly related E. tereticornis were significant. This study systematically analyzes floral bud morphology in E. pellita, E. tereticornis, and E. urophylla, revealing significant interspecific variations in eight quantitative traits and elucidates interspecific kinship while providing data and theoretical support for Eucalyptus cross-pollination and breeding.