DROUGHT AND PHOTOSYSTEM-II ACTIVITY IN 2 MEDITERRANEAN OAKS

The responses of photosystem II (PS II) to drought were analysed on tw o Mediterranean oak species, Quercus ilex and Q pubescens, using the c hlorophyll fluorescence pulse-amplitude-modulation technique. The maxi mal PS II photochemical efficiency (F-v/F-m) of the evergreen Q ilex a nd the deciduous Q pubescens oaks was only affected when leaf predawn water potential was lower than -4 MPa. This value is rarely observed o n mature trees growing in the field, but can be undergone by young see dlings during drought periods, hence confirming the stability of PS II . Whatever the irradiance, drought resulted, in both species, in lower values of PS II photochemical efficiency in a light-adapted state (De lta F/F-m'), due to stomatal closure and/or a direct inhibition of the dark reactions of photosynthesis. Diurnal decreases of F-v/F-m, of 30 min dark-adapted leaves were greater for lower predawn water potentia l, a recovery was observed in the late afternoon. The reversible decre ases in the diurnal time-courses of maximal fluorescence led us to ass ume the onset of protective mechanisms from permanent photodamages in Q ilex and, to a lesser extent, in Q pubescens.