Systematic Analysis of Reproductive Barrier Types and Strengths in Interspecific Hybridization Involving Magnolia crassipes

<i>Magnolia crassipes</i> is a valuable species in <i>Magnolia</i> sect. <i>Michelia</i> known for its unique purple flowers, but interspecific reproductive barriers limit its use in breeding. Using <i>M. crassipes</i> as the maternal parent, we performed 13 pollination combinations (one selfed control and crosses with 12 taxa spanning five sections). We assessed reproductive processes from pollen-stigma interaction to seed and seedling performance, and verified hybrids using SSR markers. Reproductive barriers are strongly associated with phylogenetic distance, shifting from pollen-adhesion failure in crosses with donors from distant-section, to abnormal pollen-tube guidance in cross with <i>M. denudata</i>, and to fruit initiation in crosses with pollen donors from sect. <i>Michelia</i>. Among these <i>Michelia</i>-donor crosses, prezygotic barrier strength varied among combinations, as reflected by differences in stigma germination and ovule entry rates, which strongly influenced the potential for fruit set success. Postzygotic barriers further reduced reproductive success via seed abortion (peaking at 83.8%). However, all germinated hybrids exhibited normal early growth. Notably, backcrossing with the F1 hybrid <i>M.</i> 'Danxia' significantly improved reproductive compatibility (seed abortion rate 6.3% and germination rate 100%). This study clarifies the key barriers in <i>M. crassipes</i> hybridization and provides a basis and practical strategies for its genetic utilization.