TEMPERATURE-INDUCED CHANGES OF PHOTOSYSTEM-II ACTIVITY IN QUERCUS-ILEX AND PINUS-HALEPENSIS

The temperature dependence of chlorophyll fluorescence characteristics and their capacity to recover from heal stress were investigated in t wo Mediterranean evergreen tree species, a conifer, Pinus halepensis M ill., and an oak species, Quercus ilex L. Analyses were performed over the range 5-50 degrees C, in relation to the duration and the mode of heat exposure. Changes in initial fluorescence, indicating damage to the thylakoid membrane, were observed at about 48.5 degrees C in both species from slow heating F-o(T) curves (F-o is the initial (or minimu m) fluorescence in the dark and T is temperature). With increasing dur ation of heat exposure, the rise in F-o was observed from 40 degrees C in P. halepensis and from 45 degrees C in e. ilex. Pious halepensis s howed higher photosystem II photochemical efficiencies and photochemic al fluorescence quenching in the normal physiological range of tempera ture. However, at the highest temperatures, e. ilex showed a higher th ermal tolerance for all the measured fluorescence parameters. Quercus ilex leaves maintained photosynthetic capacity for up to 10 min at 50 degrees C. Based on the value of F-v/F-m, (where F-m is the maximum fl uorescence in the dark and F-v = F-m- F-o (variable fluorescence in th e dark)), a higher proportion of Q. ilex leaves recovered and recovere d faster, after heat exposure, than needles of P. halepensis. The cons equences for the ecology of both species are reviewed.