Selection of Birch Trees towards Resilience against High Salinity Based on Cells Heterogeneity in Callus Cultures of an In Vitro Collection

In the context of climate change and increasing soil salinization, research on adaptive breeding is becoming especially relevant. In vitro stress modeling is a promising biotechnological approach to plant selection for resistance to negative environmental factors. The effect of sodium chloride salinization (to simulate salt and osmotic stress) in vitro on the survival, growth, callusogenesis, and morphogenesis of 12 collection clones of Betula pendula L., B. pubescens Ehrh., B. pendula Roth var. carelica (Mercklin) Hämet-Ahti, and B. pendula ‘Dalecarlica’ (Lf) has been studied. A significant influence of genotype on the survival of explants and their ability to regenerate under salt exposure is demonstrated. It has been established that the use of a fourfold stepwise (from 0.2 to 1%) increase in NaCl exposure on a selective nutrient medium without hormones allows differentiating clones by salinity resistance. The highest adaptive capabilities under NaCl-induced salinity were shown by the clones of downy birch and the lowest by the clones of silver birch. The frequency of callus formation and the viability of callus cultures, which characterize the initial and final phases of callus formation and depend on the genotypic characteristics of the original plants, are informative and relatively simple indicators for differentiating clones. A selective in vitro system is presented, based on the genetic diversity of collection clones (species, varieties, polyploids, and mixoploids) and induced cellular heterogeneity of stem callus cultures obtained from the clones most resilient against salinity. The results of the studies confirm the potential of using the genetic variability of the initial material under conditions of simulated NaCl stress for the selection of birch clones (and lines) adapted to salinity. Three regenerated birch lines selected using selective nutrient media in vitro are promising initial material for adaptive selection.