ASSESSMENT OF FROST DAMAGE TO LEAFLESS STEM TISSUES OF QUERCUS-PETRAEA - A REAPPRAISAL OF THE METHOD OF RELATIVE CONDUCTIVITY

Leafless Quercus petraea stems 1.4-5.0 mm in diameter were exposed to freezing temperatures before assessing frost damage visually and by two methods where electrolyte leakage from tissues was examined. Visual assessment could not discriminate lethal from non-lethal damage, whereas electrolyte leakage could. Assessments based on the rate of electrolyte leakage took almost twice as many man hours to obtain, but were more consistent than those produced using the method of relative conductivity. Full equilibration of electrolyte leakage following exposure to freezing temperatures required at least 5 days at 4 degrees C. Autoclaving the tissues at 121 degrees C for 15 min failed to release all diffusible electrolytes even after 10 days equilibration, but complete release was achieved 24 h after autoclaving for 120 min. Unless full equilibration is achieved, it is impossible to define a value of relative conductivity that is indicative of lethal damage. The use of predefined lethal values of relative conductivity is inadvisable because the relative conductivity of undamaged (unfrozen) shoots varies substantially with the physiological state of the tissues. Allowing 5-7 days after freezing to permit full electrolyte equilibration, followed by autoclaving for 90-120 min at 121 degrees C and allowing 24 h before measurement provides a reliable method for assessing freezing injury by the method of relative conductivity in small pieces of leafless Stem tissues. Differences between treatments can then be assessed using standard statistical procedures.