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 two Mediterranean evergreen tree species, a conifer, Pinus halepensis Mill., 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 minimum) fluorescence in the dark and T is temperature). With increasing duration 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 showed higher photosystem II photochemical efficiencies and photochemical fluorescence quenching in the normal physiological range of temperature. However, at the highest temperatures, e. ilex showed a higher thermal 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 fluorescence in the dark and F-v = F-m- F-o (variable fluorescence in the dark)), a higher proportion of Q. ilex leaves recovered and recovered faster, after heat exposure, than needles of P. halepensis. The consequences for the ecology of both species are reviewed.