Autotetraploid plants from callus cultures of Betula pendula Roth

Autotetraploid plants with 2n = 56 chromosomes were found in a population of plants derived from callus cultures of stem internodes of silver birch (Betula pendula Roth) with 2n = 28 chromosomes. These plants were readily identified by their twisted, pubescent foliage and slow rate of growth, and by cytological examination. The frequency of autotetraploidy varied with the auxin used in the culture medium. When indole-3-butyric acid was used, autotetraploids accounted for 3.8% of the total study population, whereas when naphthaleneacetic acid was used, the frequency of autotetraploidy increased to 11.0%. The remainder of the plants in the study population were morphologically normal with 2n = 28 chromosomes. The autotetraploids have potential value as breeding material for the production of triploid birches (2n = 42), which are known to be fast growing, through crosses with diploid silver birch.