LEAF GAS-EXCHANGE AND FLUORESCENCE OF PHILLYREA-LATIFOLIA, PISTACIA-LENTISCUS AND QUERCUS-ILEX SAPLINGS IN SEVERE DROUGHT AND HIGH-TEMPERATURE CONDITIONS

Saplings of Phillyrea latifolia, Pistacia lentiscus and Quercus ilex w ere witheld watering for 7 days, followed by reirrigation. Incident ph otosynthetic photon flux density (PPFD), leaf temperature, net photosy nthetic rates, stomatal conductance, and photochemical efficiency of t he photosystem II (Delta F/F'm) were measured three times during the d ay. The watered plants had higher photosynthetic rates, stomatal condu ctances, Delta F/F'm and ETR than non-watered plants. However, watered plants were mildly water stressed as shown by low ratio of variable t o maximal fluorescence (Fv/Fm) and high non-photochemical fluorescence quenching (qN). Their Delta F/F'm was low in the morning and increase d in the evening, following the variations in PPFD. Watered plants of Q. ilex had lower photosynthetic activity, stomatal conductance and ph otosynthetic radiation use efficiency than Ph. latifolia and P. lentis cus, and, conversely, reached the highest Delta F/F'm and ETR. This se ems to indicate a different relationship between photosynthetic activi ty and electron transport rate in Q. ilex compared to the other two sp ecies. Ph. latifolia and P. lentiscus appeared to be better adapted to severe drought than Q. ilex. (C) 1998 Elsevier Science B.V. All right s reserved.