SEASONAL COURSES OF PHOTOSYNTHETIC ACTIVITY AND SUBLETHAL TEMPERATURETOLERANCE OF QUERCUS-ILEX LEAVES

The reaction of the photosynthetic system of a sclerophyllous tree of the Mediterranean region (Quercus ilex) to three temperature levels (o ne ambient and two sublethal) was analysed monthly for 1 year. Measure ments were carried out on leaves less than 3 months old as well as on leaves order than 3 months by laser-induced chlorophyll fluorescence. Treatments were: control (unheated), 50 degrees C for 30 min, and 60 d egrees C for 10 min. Both for unheated and treated leaves the vitality index shows a seasonal course depending on the treatments. At 50 degr ees C the photosynthetic apparatus of leaves of the two ages is modera tely affected. At 60 degrees C the apparatus is severely damaged whate ver the season and is unable to recover. The consequences for the mana gement of Q. ilex forests in relation to some thermal stresses are con sidered.