Assessment of Long-Term Cultivated In Vitro Willow Clones according to Chromosomal Analysis

Creation and long-term in vitro maintenance of valuable genotype collection is one of the modern approach to conservation of valuable gene pool of woody plants. However, during prolonged cultivation, genetic variability of cells and tissues may accumulate and lead to the loss of valuable characteristics of parental plants. It is therefore important to assess the genetic (including cytogenetic) stability of collection clones. The purpose of this work was to study the karyotype features (number, ploidy level and chromosome size) of various willow clones (S. dasyclados Wimm., S. viminalis L., S. purpurea L., S. caspica Pall., х S. palustris Host.) under conditions of long-term in vitro storage. No such research has been conducted on willow so far. Nevertheless, there is evidence of significant influence of ploidy level on growth, productivity, and composition of wood for willow plants. Our study was based on the plants of five micropropagated willow clones rated as promising for plantation forestry. The plants used in the study were maintained in vitro for a long period (14 years) by rare subculturing (once in 5 months) on a hormone-free 1/2 WPM nutrient medium under standard cultivation conditions (25 ± 2°C, 16 h light/8 h dark, 2.0 klx). Throughout the entire in vitro cultivation period, the plants showed proper growth and development, high regeneration activity, and no visible signs of somaclonal variation. Willow is one of the rather difficult objects for karyotype analysis. The authors improved the method of preparation and analysis of specimens. During the long-term cultivation, the clones showed cytogenetic stability, maintaining the ploidy (2n = 2х = 38 or 2n = 4х = 76) and the mixoploid nature of the original plants. Data obtained gives an update on the sizes of chromosomes of clones with different ploidy. The absolute chromosome length for diploid clones varies from 0.8 to 2.1 μm; tetraploid, from 0.9 to 2.5 μm. There were no statistically significant differences in the average chromosome length between diploid and tetraploid clones. All studied willow clones during long-term (for 14 years) in vitro cultivation on a hormone-free nutrient media retain the karyotype of the respective species.