J. 1982. Field studies of water relations and photosynthesis in Scots pine. II. Influence of irrigation and fertiHzation on needle water potential of young pine trees. -Physiol. Plant. 54: 295-301.Needle water potential at noon and diurnal variation in needle water potential was measured with a pressure chamber during the growth seasons (1974, 1975 and 1976) in a 20-year-old stand of Scots pine (Pinus sylvestris L.), subjected to daily irrigation and nutrient treatments at Jadraiis, in central Sweden (60°50'N). In unstressed conditions there was little difference in water potential amongst the treatments. However, during a dry spell in 1976 the diurnal curves of water potential in the control and combined irrigated and fertilized treatment diverged by up to 6 bar in the middle of the day. Irrigation alone resulted in higher midday water potentials CPJ than in the controls only during this period. At other times in 1974, 1975 and early 1976, the values of W^ in the irrigated treatment were similar to the controls. Fertilization alone resulted in higher W^ than in the controls in both 1975 and 1976 during both wet and dry periods. However, the largest difference between treatment and control occurred in the combined, irrigated and fertilized treatment during the dry spell in 1976. Several possible explanations for the effect of fertilization on leaf water potential are discussed, including changes in hydraulic conductivity and water storage. The most likely explanation is a reduced transpiration rate as a result of more effective stomatal control of water loss.Additional key-words -Diumal variation, noon water potential, pressure chamber, water stress.K. Hillerdal-Hagstromer et al.