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 potte d saplings. O querci was inoculated into the stems of 3-year-old sapli ngs, and a severe drought was imposed for about 40 d. Drought promoted an irreversible decline in total leaf specific conductance of all sap lings; direct measurement of losses of hydraulic conductivity in twigs and petioles revealed that a strong embolization occurred in the vess els 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 o f occlusions of embolised vessels in the rooted plants. The presence o f fungal spores in the stems did not induce any modification in these water relations on well-watered or stressed seedlings. The role of O q uerci in the oak decline symptoms as occurring in Europe may therefore be questioned.