A comprehensive study of floral organ characteristics and the breeding system in Malus halliana var. parkmanii

• Using a combination of macro- and micro-scale techniques, this research establishes that the reproductive system of Malus halliana var. parkmanii is mainly outcrossing, though partially self-compatible. • The study highlights a remarkable adaptability in the structure-function relationship of floral organs. • The study accurately elucidates the temporal changes in pollen viability and stigma receptivity. Malus halliana var. parkmanii Rehd is a valuable deciduous tree with ecological and ornamental significance, yet its flowering biology remains poorly understood. This study examined flowering phenology, floral traits, and breeding system of Malus halliana var. parkmanii using scanning electron microscopy (SEM), paraffin sectioning, TTC staining for pollen viability, aniline blue-peroxide staining for stigma receptivity, and artificial pollination experiments. Flowering was concentrated, with pink corollas enhancing visual appeal. Spatial separation of sexual organs promotes cross-pollination. The pollen P/E ratio averaged 2.28, and pollen grains exhibited a wavy exine surface. Flowers typically had 0-5 connate stigmas, a median placenta, and inverted ovules. Pollen viability peaked at 47.94 % during anthesis, while stigma receptivity was already high before flowering and reached maximum 1-2 days afterward. The mean P/O ratio was 2936.9, indicating obligate outcrossing. An outcrossing index of 4 suggests a predominantly cross-pollinated system with potential partial self-compatibility, though pollinators are likely required. Artificial pollination confirmed normal pollen germination and successful pollen tube growth through the style. These results provide essential insights into the reproductive characteristics of M. halliana var. parkmanii , supporting improved breeding programs and informed application in landscape design.