Assessment of the frost hardiness of shoots of silver birch (Betula pendula) seedlings with and without controlled exposure to freezing

Frost hardiness (FH) of the shoots of silver birch (Betula pendula Roth) container seedlings was assessed by electrolyte leakage (EL), visual scoring of damage to the stem (SB), and whole-plant viability tests after controlled exposure to freezing during frost hardening. FH was calculated as the temperature causing a 50% or 10% increase in electrolyte leakage (ELT50, ELT10), stem browning (SBT50, SBT10), proportion of damaged seedlings (DT50, DT10) or mortality (LT50, LT10). Heterogeneous variances in error are considered in the estimation of the above indices by nonlinear regression of the logistic function. A method for computing the standard error of the estimated temperature causing 10% damage is described. The FH estimates by the most reliable method of assessment SBT50 were then compared with electrical impedance parameters (EIS) and water contents (WC) measured without controlled exposure to freezing. Comparison of FH assessment methods showed that ELT50 estimated the DT10, SBT50, and DT50 well. The WC of the uppermost 10 cm of stem decreased in early autumn, when FH was above -10 degreesC. When the rapid increase in FH started, WC stabilized. Intracellular and extracellular resistance measured by EIS of unexposed stems correlated positively with FH.