Short-term exposure to atmospheric ammonia does not affect frost hardeningof needles from three- and five-year-old Scots pine trees

The effect of atmospheric ammonia on frost hardening of needles from 3- and 5-year-old Scots pine trees was investigated. Trees were exposed to variou s concentrations of NH3 during different hardening stages under laboratory conditions and in experiments with open-top chambers under a natural harden ing regime during winter. Under laboratory conditions, exposure to 250 nL.L -1 NH3 did nor affect frost tolerance, whereas hardening was increased in n eedles of trees exposed to 500 nL.L-1 NH3. In trees that were hardened in o pen-top chambers, frost hardiness was not influenced by exposure to atmosph eric ammonia up to 106 nL.L-1 Needle nitrogen concentrations in the open-to p chambers increased with atmospheric NH3 concentration. Needle concentrati ons of potassium, calcium, magnesium and phosphorus were not affected by NH 3 and were within a normal range. The role of the nutrient status of the ne edles on the response of frost hardening of Scots pine needles towards expo sure to atmospheric ammonia is discussed.