Recovery of cryopreserved silver birch shoot tips is affected by the pre-freezing age of the cultures and ammonium substitution

The aim of the present study was to improve the cryopreservation protocol for silver birch (Betula pendula Roth) in vitro shoot tips. The recovery of shoot tips derived from young cultures (shoot tips excised from less than 20-month-old cultures) was significantly better than that from old cultures (shoot tips excised from cultures at least 55 months old) when the standard protocol, i.e. the slow-cooling procedure with PGD as cryoprotectant, was used. The recovery of shoot tips of both ages was, however, improved by substituting KNO₃ for ammonium during cold hardening and post-thaw cultivation. The substitution of ammonium with KNO₃ improved recovery in most of the genotypes, indicating the possibility of developing a cryopreservation protocol applicable to several genotypes.