The Water Relations of Pinus sylvestris III. Diurnal and Seasonal Patterns of Water Potential

The water potential (ψ) in twigs from four clones of Scots pine (Pinus sylvestris L.) growing in close proximity to each other was measured in a pressure chamber during five selected periods between May 1972 and March 1973. Diurnal and seasonal patterns of ψ are presented in relation to irradiance, air temperature and vapour pressure deficit (VPD) for one cloudy and one clear day in each experimental period. Significant differences in ψ were found amongst most of the clones. Larger amplitudes and earlier day-time minima in ψ were found during the summer than during the winter. Noon values of ψ of less than ?12 bars were regularly found during the summer months. Plots of ψ against irradiance or VPD during a day, showed marked hysteresis as the result of the simultaneous influence on ψ of several environmental factors. Close linear relations resulted when ψ was plotted against potential evaporation rate (calculated from the Penman-Monteith formula). The slopes of these regression lines, essentially the flow resistance, showed marked seasonal variations. At both high and low evaporation rates, the clone that grew the fastest had the lowest values of ψ, and the clone that grew the slowest had the highest values of ψ, while the other two clones had intermediate values. Differences in solute potential or in stomatal, plant or soil resistance are discussed as possible explanations of the more or less constant differences between the clones during the year of observations.