Physiological responses of Pinus sylvestris to atmospheric ammonia

Summary Young saplings of Pinus sylvestris were fumigated for 3 months with ammonia in concentrations ranging from 0 to 240 g m^-3. Despite the much higher concentrations than normal in the field, no visible damage occurred. Photosynthesis, dark respiration, transpiration and biomass production were stimulated. At 240 g m^-3 with high irradiance (PAR: 950 mol m^-2 s^-1), net photosynthesis was stimulated by 24% and dark respiration by 76%. Intitial light use efficiency was not significantly affected. Transpiration increased, both in the dark and at 950 mol m^-2 s^-1 by 40% and 57%, respectively. In the presence of ammonia, stomatal control was less efficient. Though growth of roots was not affected by NH₃, that of current year needles was stimulated, resulting in an increased mass ratio of needles to roots. The nitrogen content of the needles increased, but the contents of other mineral components did not change significantly. Due to increased transpiration per unit of needle area and increased mass of needles per tree, water loss per tree was about twice as high in the treatment with 240 g m^-3 as in the control. Towards the end of fumigation, a 10-day period without water supply followed and then the water potential of the shoots was measured as an indicator of water demand. This demand was higher with higher concentrations of NH₃, suggesting a higher risk of injury from drought.