Polyploidy Shapes Functional Trait Diversity in Mulberry Across Cytotype Ranges

Mulberry exhibits chromosome variation from 2n = 14 to 2n = 308. This remarkable cytotypic diversity provides a unique opportunity to investigate the functional consequences of genome multiplication and expansion across ploidy levels. Polyploidy and genome expansion are key drivers of plant functional trait diversity, yet their consequences across cytotype ranges remain poorly understood in woody perennials. A field-based comparative analysis was performed across mulberry cytotypes ranging from diploid (2x) to 22-ploid (22x). Morphological, anatomical, and physiological traits associated with cell size, cell division, and biomass accumulation were quantified, and multivariate relationships were evaluated using principal component analysis (PCA). Trait plasticity was assessed relative to the diploid baseline using a plasticity index. Functional traits exhibited a nonlinear response to increasing ploidy. Tetraploids (4x) showed the most favorable trait integration, combining enhanced leaf size and biomass with balanced cellular organization, while triploids (3x) displayed moderate performance. Principal component analysis further revealed distinct cytotype-specific trait clustering, with tetraploids (4x) representing the most advantageous cytotype through positive associations with leaf length (LL), leaf width (LW), and leaf area (LA), while triploids (3x) showed positive correlations with petiole length (PL) and photosynthetic rate (Pn). In contrast, higher cytotypes, particularly 22x, showed reduced growth efficiency despite increased cell size and stomatal measurements. Trait downsizing beyond tetraploidy and reduced plasticity at higher ploidy levels indicate rate-limited growth. These patterns reveal a trade-off between cellular expansion and developmental efficiency, supporting a large-genome constraint in mulberry. Cytotype variation strongly structures trait expression in mulberry, with tetraploidy representing an apparent functional optimum. The decline in performance at extreme ploidy highlights genome-size constraints as a key mechanism shaping plant functional diversity and provides a mechanistic basis for optimizing cytotype selection to improve growth efficiency and sustainability in mulberry systems.