Effect of Abscisic Acid, Cold Hardening, and Photoperiod on Recovery of Cryopreservedin VitroShoot Tips of Silver Birch

Recovery of cryopreservedin vitroshoot tips of silver birch (Betula pendulaRoth) was doubled by incorporating abscisic acid (ABA) in the culture medium during cold hardening of the mother shoots. The average recovery of shoot tips was over 40% after cold hardening for 28 days at +5°C under an 8/16 light/dark photoperiod on medium containing 10−4M ABA. ABA was effective in combination with low temperature and short daylength only, although large genotypical differences were noted. ABA had two different effects: it enhanced cold hardening and increased callus formation during regeneration of cryopreserved shoot tips.