FIELD-STUDY OF LEAF PHOTOSYNTHETIC PERFORMANCE BY A MEDITERRANEAN DECIDUOUS OAK TREE (QUERCUS-PUBESCENS) DURING A SEVERE SUMMER DROUGHT

Quercus pubescens Willd. is a winter deciduous oak species which co-oc curs in the Mediterranean area with an evergreen oak, Q. ilex. As the knowledge of Q. pubescens assimilation patterns is poorly documented, we monitored leaf physiological parameters throughout a growing season . In summer, drought was particularly severe, and some trees experienc ed pre-dawn water potentials (Psi(pd)) as low as -4.5 MPa. Diurnal cou rses of net photosynthesis, stomatal conductance and I photochemical e fficiency of photosystem II (PS II) were recorded on four trees in the field. Maximum assimilation rates (10-16.5 mu mol m(-2) s(-1), depend ing on sampled trees) occurred in spring. Leaf carbon gain recovered i n autumn even after water potential fell to -3.4 MPa. During progressi ve drought, Psi(pd) decreased substantially, leaf gas exchange was red uced and showed an important diurnal depression. At -2 MPa (Psi(pd)), net photosynthesis and stomatal conductance were still about 5 mu mol m(-2) s(-1) and 50 mmol m(-2) s(-1) respectively. Pre-dawn maximal PS II photochemical efficiency was stable to -4 MPa. The diurnal decrease in photochemical efficiency of PS II (F-v/F-m), measured after 30 min of dark adaptation, was enhanced during drought but maximal values al ways recovered at sunset, i.e. leaves only experienced short-term reve rsible PS II inhibition induced by high light and temperature. In conc lusion, Q. pubescens displays a strategy of tolerance to drought and a n efficient protection against high irradiance.