There is no direct relationship between N-status and frost hardiness in needles of NH3-exposed Scots pine seedlings

The effect of short-term atmospheric ammonia deposition on frost hardening of needles of three-month-old seedlings of Scots pine (Pinus sylvestris L.) was studied. Plants were frost hardened under short day and moderate tempe rature conditions in the laboratory during exposure to gaseous NH3 concentr ations of 400 or 1000 nl l(-1) for 4 to 6 weeks. Exposure to NH3 resulted i n an increase of free ammonium and nitrogen content of the needles. Soluble sugar and starch content were not affected. Photosynthetic capacity and ch lorophyll a concentrations were increased as a consequence of NH3 exposure, but chlorophyll b and carotenoid were not influenced. NH3 exposure did not decrease frost tolerance of the needles. Exposure to 1000 nl l(-1) NH3 eve n resulted in an increase of frost hardiness. It was concluded that frost t olerance of Scots pine seedlings is not negatively affected by the alterati ons of N-status upon short-term NH3 exposure.