Summary Changes in photosystem II (PSII) thermo- tolerance during drought and

recovery were studied under con-trolled conditions in three Mediterranean cedar species (Cedrus brevifolia Henry, C. libani Loudon and C. atlantica Manetti). The temperature at which the quantum yield of PSII photochemistry was reduced by 15 % of its value at 25 °C was 3 to 4 °C higher in drought-treated plants than in well-watered plants. The drought-induced increase in PSII thermotolerance was already evident 8 days after water had been withheld from the seedlings, when net CO2 assimilation was still at 80 % of its initial value, and was visible for up to 12 days after re-water-ing. When seedlings of the three species were exposed to tem-peratures above 45 °C for 5 h, both maximal quantum yield of PSII photochemistry and net CO2 assimilation rate were signif-icantly reduced in unconditioned seedlings, whereas drought-preconditioned seedlings were almost unaffected by the heat treatment. Drought-preconditioned seedlings still exhibited a higher tolerance to heat stress than unconditioned seedlings 60 days after re-watering, although the transient, drought-in-duced osmotic adjustment had fully disappeared. Among spe-cies, C. atlantica was the most heat sensitive, whereas the heat treatment had no significant effect on the parameters measured in C. brevifolia