VULNERABILITY OF YOUNG OAK SEEDLINGS (QUERCUS-ROBUR L) TO EMBOLISM - RESPONSES TO DROUGHT AND TO AN INOCULATION WITH OPHIOSTOMA-QUERCI (GEORGEVITCH) NANNF

Possible interactions between an infection with Ophiostoma querci and water stress on pedunculate oak (Quercus robur) were tested with potted saplings. O querci was inoculated into the stems of 3-year-old saplings, and a severe drought was imposed for about 40 d. Drought promoted an irreversible decline in total leaf specific conductance of all saplings; direct measurement of losses of hydraulic conductivity in twigs and petioles revealed that a strong embolization occurred in the vessels as soon as minimal leaf water potential decreased below -2.5 MPa. This vulnerability to cavitation on rooted seedlings was in agreement with earlier data obtained on cut branches from the same species left to freely dehydrate; a slight artifact was probably due to the onset of occlusions of embolised vessels in the rooted plants. The presence of fungal spores in the stems did not induce any modification in these water relations on well-watered or stressed seedlings. The role of O querci in the oak decline symptoms as occurring in Europe may therefore be questioned.