Physiological and growth responses of three Mediterranean oak species to different water availability regimes

The effects of summer drought on diurnal courses of gas exchange, photochemical efficiency of photosystem II and leaf growth characteristics in 2-year-old seedlings of three deciduous Mediterranean oak species, Quercus pubescens, Quercus ithaburensis and Quercus frainetto, were evaluated under field conditions in Greece during 2004 growing season. One-year-old seedlings were transplanted from nursery to field in February 2003. Half of the seedlings of each species were kept well-watered, while the other half were subject to drought except for natural rainfall. During spring and fall diurnal photosynthetic rates (A) increased with light intensity, and temperature in the morning peaks at midday and begins to decline in the afternoon. During hot summers, all species are exposed to substantial midday depression in A due to stomatal control. Despite midday decline which appears under severe drought, the photochemical efficiency of Q. ithaburensis and Q. pubescens had fully recovered during evenings, whereas the depression of photochemical efficiency in Q. frainetto was constant. Maximum photochemical efficiency (Fv/Fm) was negatively correlated with vapor pressure deficit. Q. ithaburensis presented significantly higher gas exchange rates than Q. frainetto and displayed rapid leaf appearance rate than the other two species. Q. ithaburensis appears to be more suitable for the restoration/reforestation of dryer environments, whereas Q. frainetto possesses the least drought tolerance characteristics.