Premature Dehardening in Vaccinium myrtillus During a Mild Winter: A Cause for Winter Dieback?

During a 5 °C warmer-than-average winter in north-eastern Sweden, Vaccinium myrtillus suffered lethal injuries. During the following, equally mild winter a retrospective investigation was carried out to elucidate the underlying mechanisms. Rehydrated shoots were most cold tolerant when tested in early winter, intermediate tolerant in mid-winter and least tolerant in late winter. Solute content of shoots decreased considerably over winter. It is suggested that the damage is caused by the progressive respiratory loss of cryo-protective sugars. Based on gas exchange it was estimated that initial carbohydrate reserves would have lasted 4 months only if tissue water content had remained high. As snow cover was thin and weather often clear, shoots were dehydrated which improved their cold tolerance 5-10°C. This may explain why dieback did not occur during this winter, but during the preceding winter when misty and rainy days were twice as frequent. In addition to long-term dehardening, a short-term dehardening was described following transition to constant non-freezing temperatures, which was similar to a phenomenon observed in Picea abies. Chlorophyll fluorescence as a tool for probing freezing damage was evaluated. This revealed a high capacity for recovery from initial damage.