Background and aims: Induction of dehydration tolerance is a key to achieving high survival rates in cryopreservation of plant specimens. It has been reported previously that two-step preculturing with sucrose effectively increased desiccation tolerance in axillary buds of gentian (Gentiana scabra), which allow the buds to survive cryopreservation. This study is aimed at characterizing each step of this preculturing and to elucidate physiological changes induced during this preculturing.
Methods: In standard two-step preculture, excised gentian axillary buds were incubated for 11 d on MS medium with 0.1 m sucrose at 25 degrees C (first step: mild osmotic stress was given) and the subsequent incubation on MS medium with 0.4 m and 0.7 m sucrose for 1 d each (second step). The levels of abscisic acid (ABA), proline and soluble sugars in gentian buds during the preculture were determined. Effects of various combinations of two-step preculturing and of exogenous ABA and proline were studied.
Key results: During the first preculture step, there was a transient increase in ABA content peaking on day 4, which declined to a background level at the end of the first and second step preculturing. Proline level increased steadily during the first preculture step and increased further in the second preculture step. Incubating buds with medium containing proline, instead of the two-step preculturing, did not allow them to survive desiccation. Incubating buds with ABA instead of 0.1 m sucrose-preculturing effectively increased desiccation tolerance only when it was followed by the second preculture step. Fluridone, an ABA synthesis inhibitor included in the two-step preculture medium, reduced desiccation tolerance of the buds. The normal first-step preculture increased the levels of soluble sugars 2.4-fold, especially sucrose and raffinose. Buds treated with the second preculture step had greatly increased sucrose levels.
Conclusions: These observations lead to the hypothesis that the first preculture step involves ABA-mediated cellular changes and the second step induces loading of sucrose in the gentian buds.