Combining dissected-leaf and purple-leaf traits in Betula pendula: development of new germplasm and selection of elite families

Objective Birch is an important ornamental and timber tree species in northern China. However, new germplasm combining dissected-leaf and purple-leaf traits is currently lacking, which restricts the breeding of ornamental-timber dual-purpose cultivars. This study aimed to create new dissected-leaf purple birch germplasm and to reveal the segregation patterns of dissected-leaf and purple-leaf traits in backcross progenies, as well as their relationships with growth traits.

Method The F₁ population was obtained by crossing the dissected-leaf green cultivar ‘Dalecarlica’ (Betula pendula ‘Dalecarlica’) as the female parent with the normal-leaf purple cultivar ‘Purple Rain’ (Betula pendula ‘Purple Rain’) as the male parent. Superior individuals with normal-leaf purple traits were then selected from the F₁ population and backcrossed with the female parent to establish the BC₁ segregating population. The progenies were evaluated for leaf shape, leaf color, and growth traits. Chi-square tests and linkage analyses were performed to detect segregation distortion and genetic linkage. The dynamic trade-off between purple-leaf traits and growth was analyzed using a Logistic growth model. Family heritability was estimated, and multi-trait comprehensive evaluation of superior germplasm was conducted using membership functions combined with the entropy weight method.

Result (1) The target phenotype combining dissected leaves and purple leaves was successfully obtained in the BC₁ population, with a frequency of 25.00%. Phenotypic segregation in 66.67% of the families deviated significantly from the expected Mendelian ratio of 1:1:1:1 (P < 0.05), and the green-leaf phenotype was completely absent in two families. Linkage analysis showed significant linkage signals between the leaf-shape and leaf-color genes in families f3, f4, f7, and f10, with recombination frequencies ranging from 37.11% to 44.73% and an average of 42.16%, indicating weak linkage. (2) The purple-leaf trait showed a negative correlation trend with plant height growth, suggesting a possible resource allocation trade-off between anthocyanin accumulation and growth. The family heritability of the purple-leaf trait, represented by anthocyanin content index, was as high as 97.91%–98.97%, while the heritability of plant height among different phenotypic groups ranged from 74.31% to 90.47%, indicating strong potential for efficient directional breeding. (3) Through multi-trait comprehensive evaluation, three types of superior germplasm were selected from the backcross population, namely fast-growing, highly ornamental, and growth-ornamental balanced types.

Conclusion Backcross breeding can effectively combine the dissected-leaf and purple-leaf traits in birch. In the BC₁ population, the two traits exhibited segregation distortion and weak linkage, and a dynamic trade-off was observed between the purple-leaf trait and growth. The comprehensive evaluation system established in this study, based on genetic segregation, growth performance, and ornamental traits, provides germplasm resources and an evaluation framework for the breeding of new birch cultivars with both ornamental and timber value.