Résumé
Quercus pubescens Willd. is a winter deciduous oak species which co-occurs 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 sotne trees experienced pre-dawn water potentials (fp^) as low as-4'5 MPa. Diurnal courses of net photosynthesis, stomatal conductance and I photochemical efficiency of photosystem 11 (PS II) were recorded on four trees in the field. Maximum assimilation rates (10-16-5 //mol m"^ s'\ depending on sampled trees) occurred in spring. Leaf carbon gain recovered in autumn even after water potential fell to-34 MPa. During progressive drought, Y^d ecreased substantially, leaf gas exchange was reduced and showed an important diurnal depression. At-2 MPa (f.,a), net photosynthesis and stomatal conductance were still about 5 /tmo! m"^ s"' and 50 mmol m"^ s"^, respectively. Pre-dawn maximal PS 11 photochemical efficiency was stabJe to-4 MPa. The diurnal decrease in photochemical efficiency of PS II (Fv/Fm), measured after 30 min of dark adaptation, was enhanced during drought but maxima] values always recovered at sunset, i.e. leaves only experienced short-term reversible PS II inhibition induced by high light and temperature. In conclusion, Q. pubescens displays a strategy of tolerance to drought and an efficient protection against high irradiance.