EFFECT OF NITROGEN SUPPLY ON FROST HARDINESS IN CALLUNA-VULGARIS (L) HULL

Frost hardiness in Calluna vulgaris (L.) Hull. which had received ammo nium nitrate applications in the held for 30 months, was assessed usin g scores of visible injury and measurements of the rate of total elect rolyte leakage from excised shoots following controlled freezing treat ments in the laboratory. There was good overall correlation between th e two methods (Spearman correlation coefficient 0.77), but leakage mea surements were more sensitive than injury scores to the effects of nit rogen. Visible injury was not significantly altered by nitrogen supply (Kruskal-Wallis non-parametric test). Ion leakage was analyzed in dif ferent ways, using either calculations of the first-order rate coeffic ients or expressions of relative conductivity. These analyses produced similar results with respect to the effect of frost and nitrogen. Sho ots of nitrogen-fertilized (40, 80 and 120 kg ha(-1) yr(-1)) C. vulgar is sampled in October 1991 showed significantly (P < 0.05) less leakag e after overnight frosts of -15 and -20 degrees C than did the water-t reated controls. In October the temperature which killed 50 % of the s hoots (LT(50)), derived from the leakage data, was raised by at least 4 degrees C by the highest nitrogen treatments compared with the contr ol plants. Frost treatments to pot-grown C. vulgaris in November produ ced similar visible injury to attached and excised shoots from the sam e plants, both being significantly less damaged by a -18 degrees C fro st after a 7-month exposure to an NaNO3 mist solution (1.0 mM, pH 4.5) than were water-misted controls. Ammonium-mist treatments also margin ally reduced frost injury, but the effects were not statistically sign ificant. These results demonstrate that frost hardiness of C. vulgaris in the field can be assessed rapidly and accurately in the laboratory by analysis of electrolyte leakage from excised shoots.