The sorption isotherm for excised embryonic axes of recalcitrant (i.e. desiccation-sensitive) Quercus robur L. acorns was determined to find the relation between moisture content and water potential. Subsequently, physiological studies on the effect of desiccating the axes to a range of water potentials were undertaken. The respiratory capacity declined steeply after short exposure to water potentials from −5 to −30 MPa. The leachate conductivity increased significantly after exposure to −5 MPa and rose steeply after exposure to between −12 and −40 MPa. Axes were equilibrated at different relative humidities and the proline content showed a 15-fold increase with a peak value at −10 MPa. It was concluded that the critical water potential for initiation of damage was −5 MPa, and that axes accumulated proline as a response to desiccation stress. The embryonic axes from Q. robur behave more like typical vegetative tissue of angiosperms than like orthodox seeds.
