Colchicine-induced chromosome doubling in Betula pendula 'Dalecarlica' and its effects on morphological characteristics and drought tolerance

Betula pendula 'Dalecarlica' is valued for its ornamental and timber properties, but its sensitivity to drought limits its widespread application. To improve drought tolerance, this study utilized colchicine-induced chromosome doubling to establish an effective tetraploid induction system, generate novel tetraploid germplasm, and evaluate its performance in terms of morphology, anatomy, and drought resistance. A colchicine-induced tetraploid induction system was established through an L₉(3 ³) orthogonal design that analyzed the effects of colchicine concentration, explant type (sterile stems, petioles, and leaves from diploid Betula pendula 'Dalecarlica'), and treatment time on induction efficiency. Of the 16 tetraploid clones obtained, variations in growth and stomatal traits were compared, and genetic parameters were evaluated. Additionally, tetraploid (4x-7) and diploid (2x-7) plants were subjected to comprehensive evaluations, including anatomical structure, physiological and biochemical parameters, photosynthetic characteristics, and growth responses under drought conditions. The results revealed that the optimal induction protocol involved treating young stems with 0.4% colchicine for 12 h, achieving a tetraploid induction rate of 67.20%, with explant type being the primary influencing factor. Tetraploid clones exhibited significant variations in growth and stomatal characteristics, with high genetic stability observed in seedling height and stomatal density. Anatomical analysis showed that tetraploid leaves had a thicker cuticle and a higher palisade-to-spongy mesophyll ratio, while young stems displayed a larger xylem-to-phloem ratio. Additionally, the relative water loss rate was significantly reduced in tetraploids. Under drought stress, tetraploids showed lower levels of MDA and H₂O₂, higher SOD activity, increased proline accumulation, and a faster ABA response. Stomatal size increased while stomatal density decreased, contributing to enhanced photosynthetic apparatus stability. The drought damage index in tetraploids was significantly lower than in diploids, although their relative growth rate was notably reduced, demonstrating a typical “growth-defense trade-off” strategy. This study successfully established an efficient tetraploid induction system for Betula pendula 'Dalecarlica'. Tetraploid plants exhibited enhanced drought tolerance through coordinated structural and physiological adjustments, offering significant potential for applications in water-efficient landscaping and stress-resistant timber breeding, while also providing valuable germplasm and theoretical insights for future genetic improvement. • Established efficient tetraploid induction for Betula pendula ‘Dalecarlica’ (67.2% rate) using 0.4% colchicine 12h on young stems. • Tetraploid clones varied in growth and stomatal traits; high heritability: height 88.16%, stomatal density 77.54%. • Tetraploids enhanced drought resistance via thicker cuticle, high xylem/phloem ratio, low MDA, high SOD synergy. • Under drought, tetraploids kept stable photosystems (high SPAD, low NPQ) but showed growth-defense trade-off (lower RGR). • Provided 16 tetraploid germplasms and theory basis for Betula drought-resistant breeding in water-saving landscapes and timber.