Discrepancies between water potential isotherm measurements on Pinus ponderosa seedling shoots: xylem hysteresis and apoplasmic osmotic potentials *

Abstract. Sap expression, air drying and a combined technique were used to measure the water potential isotherm of Pinus ponderosa Laws, seedling shoots with the pressure chamber. Discrepancies between water relations parameters derived from these techniques can be partially explained by air entry into air drying tissues, hysteresis in the xylem water potential isotherm and dilution of apoplasmic solutes during sap expression.