Effects of defoliation on the frost hardiness and the concentrations of soluble sugars and cyclitols in the bark tissue of pedunculate oak (Quercus robur L.)

As a measure of frost hardiness, we determined an index of injury (I -30) in living bark tissue of 20-year-old pedunculate oaks (Quercus robur L.) that had been manually and almost completely defoliated in the first half of May of one or two years, and of non-defoliated control trees. I -30 was calculated as a percentage value on the basis of electrolyte leakage from samples artificially frozen at a temperature of -30°C, and from unfrozen control samples. In parallel, the bark's concentrations of soluble sugars, of nitrogen and of quercitol, a cyclic polyol, were measured. Repeated defoliation significantly reduced the frost hardiness of the bark as well as its concentrations of raffinose, stachyose, nitrogen and quercitol. The I -30 values were correlated with the total concentration of soluble sugars and with the concentrations of the individual sugar compounds, but not with the quercitol concentration. Less tight, yet significant correlations were obtained between I-30 and nitrogen concentrations. We conclude that repeated defoliation decreases the bark's capability to acclimatize to winter frost due to a reduction in the concentrations of soluble sugars, particularly those of raffinose and stachyose.