Effects of chilling on budburst of European ash in vitro and in vivo

The ongoing global climate change has aroused interest in the spring phenology of woody species in boreal and temperate regions that require cold temperatures to break bud dormancy. Most research on this topic has been concentrated on fruit species, rather than forest trees. We studied the effect of chilling duration in vitro and ex vitro on the timing of dormancy release in European ash plants. In vitro chilling (at 4–5ºC) of rooted ash plants delayed their subsequent growth when planted out in the greenhouse, and the length of delay correlated with chilling duration. The fastest growth was observed after 5–7 weeks of cold treatment, whereas 13 weeks reduced the plant viability. The impact of cold treatment for 300–3000 h was assessed on one-year-old ash plantlets. Subsequently, clonal micropropagation of European ash on a dihydroquercetin (taxifolin)-containing medium significantly reduced the requirement for chill temperatures: the plantlets came out of endodormancy after having accumulated 500 chill hours, whereas control (unchilled) plantlets required more than 1000 h. The budburst period was also significantly shorter in chilled plants. We suppose that the effect might have been due to the antioxidant activity of the flavonoid dihydroquercetin. The thermal time requirement for budburst in European ash was well described as an inverse exponential function of the number of chill hours at ≤ 5 °C. Late-flowering and photoperiod-insensitive European ash is a good object for studying the impact of external and internal factors on the mechanisms behind the induction and release of dormancy.