EFFECT OF NITROGEN STATUS ON FROST TOLERANCE OF OLIVE TREES

Trees with different nitrogen status: low (1.1-1.3%), adequate (1.4-1.6%) or high (1.7-1.8%) leaf N concentration of July samples, were selected from a long-term experiment in which mature ‘Picual’ olive trees were subjected for 13 consecutive years to different nitrogen fertilization regimes. Leaves were sampled from each group of trees from October to April. Discs of 7 mm diameter were collected from these leaves, placed in test tubes with an aqueous solution, and subjected to controlled freezing (0 to -26°C) in a glycol bath. After freezing, the electrolytic conductivity was measured, the leaf discs were then autoclaved to kill the tissues completely, and the conductivity was measured again. The relative conductivity was calculated as the percentage of the final reading. Lethal frost temperature (LT₅₀) was determined by plotting conductivity data against temperature, using a logistic regression model. In October, before the onset of dormancy, LT₅₀ decreased as leaf N concentration increased, those trees with an excess of nitrogen showing more frost tolerance. The contrary occurs in spring, where deficient trees were more tolerant than trees with an excess of nitrogen, since LT₅₀ increased with leaf N concentration. No differences were observed during winter, probably because the rest mechanism occurs regardless the nitrogen status of the tree.