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 tw o methods where electrolyte leakage from tissues was examined. Visual assessment could not discriminate lethal from non-lethal damage, where as electrolyte leakage could. Assessments based on the rate of electro lyte leakage took almost twice as many man hours to obtain, but were m ore consistent than those produced using the method of relative conduc tivity. Full equilibration of electrolyte leakage following exposure t o freezing temperatures required at least 5 days at 4 degrees C.Autocl aving the tissues at 121 degrees C for 15 min failed to release all di ffusible electrolytes even after 10 days equilibration, but complete r elease was achieved 24 h after autoclaving for 120 min. Unless full eq uilibration is achieved, it is impossible to define a value of relativ e conductivity that is indicative of lethal damage. The use of predefi ned lethal values of relative conductivity is inadvisable because the relative conductivity of undamaged (unfrozen) shoots varies substantia lly with the physiological state of the tissues. Allowing 5-7 days aft er freezing to permit full electrolyte equilibration, followed by auto claving for 90-120 min at 121 degrees C and allowing 24 h before measu rement provides a reliable method for assessing freezing injury by the method of relative conductivity in small pieces of leafless Stem tiss ues. Differences between treatments can then be assessed using standar d statistical procedures.