FROST HARDINESS OF NORWAY SPRUCE TREATED WITH ACID MIST EVALUATION OFTHE ELECTROLYTE LEAKAGE RATE TECHNIQUE

Solutions of acid mist at pH 2.5, 2.7 and 5.0 were applied at four mis ting frequencies to 2-year-old Norway spruce growing in pots in a glas shouse for 19 weeks, from July to November 1991. Frost hardiness asses sments were made on detached shoots. Three methods for analysing ion l eakage were compared: conductivity after 24 and 120 hr, and the rate o f leakage derived from measurements at 1,24 and 120 hr relative to the ion conductivity after autoclaving shoot segments in deionized water. The different methods of estimating the effects of acid mist on frost hardiness gave similar results, but the use of a single measurement a fter 24 hr was confounded by the inclusion of ions washed from the lea f surface. This bias was smaller for the single measurement after 120 hr, but only the electrolyte leakage rate determination could separate and restrict this potential source of bias. Estimates of leakage rate , which accommodates differences in hardening status and surface-adsor bed ions, were found to be the most robust, sensitive method for deter mining acid mist effects on frost hardiness. Measurements of relative conductivity based on 100% fewer measurements are unlikely to save tim e since they will require exploratory studies throughout the hardening period to establish the optimum times for making the single, critical measurement.