Phenotypic Diversity and Ornamental Evaluation Between Introduced and Domestically Bred Crabapple Germplasm

Crabapples (Malus spp.) are important ornamental trees in northern temperate regions. However, their phenotypic diversity and ornamental values remain poorly characterized, due to a lack of systematic comparison between introduced and domestically bred cultivars/lines. This knowledge gap limits the effective utilization of their germplasm. In this study, 111 floral, foliar, fruit, and tree architectural traits were measured across 93 introduced (North American) and 118 domestically bred (Chinese) cultivars/lines. Comparative analyses using Shannon–Wiener (H′) and Pielou’s evenness (J) indices revealed that floral traits exhibited the highest phenotypic diversity, followed by fruits, leaves, and tree architecture. Among these, 51 key traits (e.g., budlet color, leaf area, and fruit shape) showed above-average diversity, while others (e.g., flower type, leaf cracking, and exocarp color) were less uniform, indicating rare phenotypes. Domestically bred cultivars showed significant improvements in flower color and type, mature leaf shape and size, and fruit characteristics, including novel budlet, bud and petal colors, increased stamen numbers, semi-double or double flowers, and diverse fruit colors. A multi-dimensional ornamental evaluation (Analytic Hierarchy Process) identified 26 superior genotypes and several organ-specific selections for flower- (26), fruit- (25), foliage- (21), and tree architecture-viewing (14) purposes. These findings provide a theoretical basis for updating Malus distinctness, uniformity, and stability (DUS) guidelines, targeted breeding, and strategic landscape applications, highlighting the potential of both introduced and domestic germplasm for ornamental improvement.