Field study of leaf photosynthetic performance by a Mediterranean deciduous oak tree (Quercus pubescens) during a severe summer drought

SUMMARY 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 some trees experienced pre‐dawn water potentials (Ψ pd ) as low as – 4·5 MPa. Diurnal courses of net photosynthesis, stomatal conductance and I photochemical efficiency of photosystem II (PS II) were recorded on four trees in the field. Maximum assimilation rates (10–16·5 μmol m −2 s −1 , depending on sampled trees) occurred in spring. Leaf carbon gain recovered in autumn even after water potential fell to – 3·4 MPa. During progressive drought, Ψ pd decreased substantially, leaf gas exchange was reduced and showed an important diurnal depression. At – 2 MPa (Ψ pd ), net photosynthesis and stomatal conductance were still about 5 μmol 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 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.