A comparison of analytical approaches for assessing freezing damage in black spruce using electrolyte leakage methods

Abstract

To compare different methods of quantifying shoot frost damage during controlled plant freezing tests, frost hardening of black spruce (Picea mariana (Mill.) BSP) seedlings exposed to three temperature hardening regimes over 16 weeks was assessed using electrolyte leakage and intact seedling methods. Electrolyte leakage was expressed as index of injury and was quantified either as the temperature needed to induce an index of injury of 5% (DT 5 ) or as the critical temperature (CT), the mildest temperature at which damage was first detected statistically. Damage to intact shoots was expressed as percent shoot browning and was quantified as the temperature at which 50% of needle tissue on the shoots was damaged (sLT 50 ) or as the temperature at which 50% of terminal buds were killed (bLT 50 ). Seedling response to hardening temperature varied, depending on the method used to quantify frost hardiness. When expressed as critical temperature, hardening continued over the 16 weeks at a constant rate with no differences detected between treatments. Intact seedling shoot damage, sLT 50 and bLT 50 , described a hardening process in which there was a large initial increase in hardening in the first 8 weeks, with less hardening occurring during the subsequent 8 weeks. Also, significant temperature effects were detected, with the greatest hardening occurring in a cool temperature (4 °C), the least in a warm temperature (20 °C), and an intermediate amount in a moderate temperature (10 °C). When quantified as DT 5 , the pattern of hardening was somewhat intermediate to the other two. Methods of determining frost hardiness were highly correlated, with the strongest correlation being between sLT 50 and bLT 50 (r 2 = 0.903). Both electrolyte leakage methods, DT 5 and CT, were linearly related to one another (r 2 = 0.666) and were more sensitive than the intact seedling methods, since they both detected damage at warmer temperatures. DT 5 was better correlated to intact measures of hardiness than was CT. Keywords: black spruce, index of injury, frost hardiness, critical temperature, damaging temperature, LT 50 .